OneFS Endurant Cache

The earlier blog post on multi-threaded I/O generated several questions on synchronous writes in OneFS. So this seemed like a useful topic to explore in a bit more detail.

OneFS natively provides a caching mechanism for synchronous writes – or writes that require a stable write acknowledgement to be returned to a client. This functionality is known as the Endurant Cache, or EC.

The EC operates in conjunction with the OneFS write cache, or coalescer, to ingest, protect and aggregate small, synchronous NFS writes. The incoming write blocks are staged to NVRAM, ensuring the integrity of the write, even during the unlikely event of a node’s power loss.  Furthermore, EC also creates multiple mirrored copies of the data, further guaranteeing protection from single node and, if desired, multiple node failures.

EC improves the latency associated with synchronous writes by reducing the time to acknowledgement back to the client. This process removes the Read-Modify-Write (R-M-W) operations from the acknowledgement latency path, while also leveraging the coalescer to optimize writes to disk. EC is also tightly coupled with OneFS’ multi-threaded I/O (Multi-writer) process, to support concurrent writes from multiple client writer threads to the same file. And the design of EC ensures that the cached writes do not impact snapshot performance.

The endurant cache uses write logging to combine and protect small writes at random offsets into 8KB linear writes. To achieve this, the writes go to special mirrored files, or ‘Logstores’. The response to a stable write request can be sent once the data is committed to the logstore. Logstores can be written to by several threads from the same node, and are highly optimized to enable low-latency concurrent writes.

Note that if a write uses the EC, the coalescer must also be used. If the coalescer is disabled on a file, but EC is enabled, the coalescer will still be active with all data backed by the EC.

So what exactly does an endurant cache write sequence look like?

Say an NFS client wishes to write a file to an Isilon cluster over NFS with the O_SYNC flag set, requiring a confirmed or synchronous write acknowledgement. Here is the sequence of events that occur to facilitate a stable write.

1)  A client, connected to node 3, begins the write process sending protocol level blocks. 4K is the optimal block size for the endurant cache.

2)  The NFS client’s writes are temporarily stored in the write coalescer portion of node 3’s RAM. The Write Coalescer aggregates uncommitted blocks so that the OneFS can, ideally, write out full protection groups where possible, reducing latency over protocols that allow “unstable” writes. Writing to RAM has far less latency that writing directly to disk.

3)  Once in the write coalescer, the endurant cache log-writer process writes mirrored copies of the data blocks in parallel to the EC Log Files.

The protection level of the mirrored EC log files is the same as that of the data being written by the NFS client.

4)  When the data copies are received into the EC Log Files, a stable write exists and a write acknowledgement (ACK) is returned to the NFS client confirming the stable write has occurred. The client assumes the write is completed and can close the write session.

5)  The write coalescer then processes the file just like a non-EC write at this point. The write coalescer fills and is routinely flushed as required as an asynchronous write via to the block allocation manager (BAM) and the BAM safe write (BSW) path processes.

6)  The file is split into 128K data stripe units (DSUs), parity protection (FEC) is calculated and FEC stripe units (FSUs) are created.

7)  The layout and write plan is then determined, and the stripe units are written to their corresponding nodes’ L2 Cache and NVRAM. The EC logfiles are cleared from NVRAM at this point. OneFS uses a Fast Invalid Path process to de-allocate the EC Log Files from NVRAM.

8)  Stripe Units are then flushed to physical disk.

9)  Once written to physical disk, the data stripe Unit (DSU) and FEC stripe unit (FSU) copies created during the write are cleared from NVRAM but remain in L2 cache until flushed to make room for more recently accessed data.

As far as protection goes, the number of logfile mirrors created by EC is always one more than the on-disk protection level of the file. For example:

File Protection Level Number of EC Mirrored Copies
+1n 2
2x 3
+2n 3
+2d:1n 3
+3n 4
+3d:1n 4
+4n 5

The EC mirrors are only used if the initiator node is lost. In the unlikely event that this occurs, the participant nodes replay their EC journals and complete the writes.

If the write is an EC candidate, the data remains in the coalescer, an EC write is constructed, and the appropriate coalescer region is marked as EC. The EC write is a write into a logstore (hidden mirrored file) and the data is placed into the journal.

Assuming the journal is sufficiently empty, the write is held there (cached) and only flushed to disk when the journal is full, thereby saving additional disk activity.

An optimal workload for EC involves small-block synchronous, sequential writes – something like an audit or redo log, for example. In that case, the coalescer will accumulate a full protection group’s worth of data and be able to perform an efficient FEC write.

The happy medium is a synchronous small block type load, particularly where the I/O rate is low and the client is latency-sensitive. In this case, the latency will be reduced and, if the I/O rate is low enough, it won’t create serious pressure.

The undesirable scenario is when the cluster is already spindle-bound and the workload is such that it generates a lot of journal pressure. In this case, EC is just going to aggravate things.

So how exactly do you configure the endurant cache?

Although on by default, setting the efs.bam.ec.mode sysctl to value ‘1’ will enable the Endurant Cache:

# isi_sysctl_cluster efs.bam.ec.mode=1

EC can also be enabled & disabled per directory:

# isi set -c [on|off|endurant_all|coal_only] <directory_name>

To enable the coalescer but switch of EC, run:

# isi set -c coal_only

And to disable the endurant cache completely:

# isi_for_array –s isi_sysctl_cluster efs.bam.ec.mode=0

A return value of zero on each node from the following command will verify that EC is disabled across the cluster:

# isi_for_array –s sysctl efs.bam.ec.stats.write_blocks efs.bam.ec.stats.write_blocks: 0

If the output to this command is incrementing, EC is delivering stable writes.

Be aware that if the Endurant Cache is disabled on a cluster the sysctl efs.bam.ec.stats.write_blocks output on each node will not return to zero, since this sysctl is a counter, not a rate. These counters won’t reset until the node is rebooted.

As mentioned previously, EC applies to stable writes. Namely:

  • Writes with O_SYNC and/or O_DIRECT flags set
  • Files on synchronous NFS mounts

When it comes to analyzing any performance issues involving EC workloads, consider the following:

  • What changed with the workload?
  • If upgrading OneFS, did the prior version also have EC enable?
  • If the workload has moved to new cluster hardware:
  • Does the performance issue occur during periods of high CPU utilization?
  • Which part of the workload is creating a deluge of stable writes?
  • Was there a large change in spindle or node count?
  • Has the OneFS protection level changed?
  • Is the SSD strategy the same?

Disabling EC is typically done cluster-wide and this can adversely impact certain workflow elements. If the EC load is localized to a subset of the files being written, an alternative way to reduce the EC heat might be to disable the coalescer buffers for some particular target directories, which would be a more targeted adjustment. This can be configured via the isi set –c off command.

One of the more likely causes of performance degradation is from applications aggressively flushing over-writes and, as a result, generating a flurry of ‘commit’ operations. This can generate heavy read/modify/write (r-m-w) cycles, inflating the average disk queue depth, and resulting in significantly slower random reads. The isi statistics protocol CLI command output will indicate whether the ‘commit’ rate is high.

It’s worth noting that synchronous writes do not require using the NFS ‘sync’ mount option. Any programmer who is concerned with write persistence can simply specify an O_FSYNC or O_DIRECT flag on the open() operation to force synchronous write semantics for that fie handle. With Linux, writes using O_DIRECT will be separately accounted-for in the Linux ‘mountstats’ output. Although it’s almost exclusively associated with NFS, the EC code is actually protocol-agnostic. If writes are synchronous (write-through) and are either misaligned or smaller than 8k, they have the potential to trigger EC, regardless of the protocol.

The endurant cache can provide a significant latency benefit for small (eg. 4K), random synchronous writes – albeit at a cost of some additional work for the system.

However, it’s worth bearing the following caveats in mind:

  • EC is not intended for more general purpose I/O.
  • There is a finite amount of EC available. As load increases, EC can potentially ‘fall behind’ and end up being a bottleneck.
  • Endurant Cache does not improve read performance, since it’s strictly part of the write process.
  • EC will not increase performance of asynchronous writes – only synchronous writes.

OneFS Writes

OneFS run equally across all the nodes in a cluster such that no one node controls the cluster and all nodes are true peers. Looking from a high-level at the components within each node, the I/O stack is split into a top layer, or initiator, and a bottom layer, or participant. This division is used as a logical model for the analysis of OneFS’ read and write paths.

At a physical-level, CPUs and memory cache in the nodes are simultaneously handling initiator and participant tasks for I/O taking place throughout the cluster. There are caches and a distributed lock manager that are excluded from the diagram below for simplicity’s sake.

When a client connects to a node to write a file, it is connecting to the top half or initiator of that node. Files are broken into smaller logical chunks called stripes before being written to the bottom half or participant of a node (disk). Failure-safe buffering using a write coalescer is used to ensure that writes are efficient and read-modify-write operations are avoided. The size of each file chunk is referred to as the stripe unit size. OneFS stripes data across all nodes and protects the files, directories and associated metadata via software erasure-code or mirroring.

OneFS determines the appropriate data layout to optimize for storage efficiency and performance. When a client connects to a node, that node’s initiator acts as the ‘captain’ for the write data layout of that file. Data, erasure code (FEC) protection, metadata and inodes are all distributed on multiple nodes, and spread across multiple drives within nodes. The following illustration shows a file write occurring across all nodes in a three node cluster.

OneFS uses a cluster’s Ethernet or Infiniband back-end network to allocate and automatically stripe data across all nodes . As data is written, it’s also protected at the specified level.

When writes take place, OneFS divides data out into atomic units called protection groups. Redundancy is built into protection groups, such that if every protection group is safe, then the entire file is safe. For files protected by FEC, a protection group consists of a series of data blocks as well as a set of parity blocks for reconstruction of the data blocks in the event of drive or node failure. For mirrored files, a protection group consists of all of the mirrors of a set of blocks.

OneFS is capable of switching the type of protection group used in a file dynamically, as it is writing. This allows the cluster to continue without blocking in situations when temporary node failure prevents the desired level of parity protection from being applied. In this case, mirroring can be used temporarily to allow writes to continue. When nodes are restored to the cluster, these mirrored protection groups are automatically converted back to FEC protected.

During a write, data is broken into stripe units and these are spread across multiple nodes as a protection group. As data is being laid out across the cluster, erasure codes or mirrors, as required, are distributed within each protection group to ensure that files are protected at all times.

One of the key functions of the OneFS AutoBalance job is to reallocate and balance data and, where possible, make storage space more usable and efficient. In most cases, the stripe width of larger files can be increased to take advantage of new free space, as nodes are added, and to make the on-disk layout more efficient.

The initiator top half of the ‘captain’ node uses a modified two-phase commit (2PC) transaction to safely distribute writes across the cluster, as follows:

Every node that owns blocks in a particular write operation is involved in a two-phase commit mechanism, which relies on NVRAM for journaling all the transactions that are occurring across every node in the storage cluster. Using multiple nodes’ NVRAM in parallel allows for high-throughput writes, while maintaining data safety against all manner of failure conditions, including power failures. In the event that a node should fail mid-transaction, the transaction is restarted instantly without that node involved. When the node returns, it simply replays its journal from NVRAM.

In a write operation, the initiator also orchestrates the layout of data and metadata, the creation of erasure codes, and lock management and permissions control. OneFS can also optimize layout decisions made by to better suit the workflow. These access patterns, which can be configured at a per-file or directory level, include:

  • Concurrency: Optimizes for current load on the cluster, featuring many simultaneous clients.
  • Streaming:  Optimizes for high-speed streaming of a single file, for example to enable very fast reading with a single client.
  • Random:  Optimizes for unpredictable access to the file, by adjusting striping and disabling the use of prefetch.

OneFS Multi-writer

In the previous blog article we took a look at write locking and shared access in OneFS. Next, we’ll delve another layer deeper into OneFS concurrent file access.

The OneFS locking hierarchy also provides a mechanism called Multi-writer, which allows a cluster to support concurrent writes from multiple client writer threads to the same file. This granular write locking is achieved by sub-diving the file into separate regions and granting exclusive data write locks to these individual ranges, as opposed to the entire file. This process allows multiple clients, or write threads, attached to a node to simultaneously write to different regions of the same file.

Concurrent writes to a single file need more than just supporting data locks for ranges. Each writer also needs to update a file’s metadata attributes such as timestamps, block count, etc. A mechanism for managing inode consistency is also needed, since OneFS is based on the concept of a single inode lock per file.

In addition to the standard shared read and exclusive write locks, OneFS also provides the following locking primitives, via journal deltas, to allow multiple threads to simultaneously read or write a file’s metadata attributes:

OneFS Lock Types include:

Exclusive: A thread can read or modify any field in the inode. When the transaction is committed, the entire inode block is written to disk, along with any extended attribute blocks.

Shared: A thread can read, but not modify, any inode field.

DeltaWrite: A thread can modify any inode fields which support delta-writes. These operations are sent to the journal as a set of deltas when the transaction is committed.

DeltaRead: A thread can read any field which cannot be modified by inode deltas.

These locks allow separate threads to have a Shared lock on the same LIN, or for different threads to have a DeltaWrite lock on the same LIN. However, it is not possible for one thread to have a Shared lock and another to have a DeltaWrite. This is because the Shared thread cannot perform a coherent read of a field which is in the process of being modified by the DeltaWrite thread.

The DeltaRead lock is compatible with both the Shared and DeltaWrite lock. Typically the file system will attempt to take a DeltaRead lock for a read operation, and a DeltaWrite lock for a write, since this allows maximum concurrency, as all these locks are compatible.

Here’s what the write lock incompatibilities looks like:

OneFS protects data by writing file blocks (restriping) across multiple drives on different nodes. The Job Engine defines a ‘restripe set’ comprising jobs which involve file system management, protection and on-disk layout. The restripe set contains the following jobs:

  • AutoBalance & AutoBalanceLin
  • FlexProtect & FlexProtectLin
  • MediaScan
  • MultiScan
  • SetProtectPlus
  • SmartPools
  • Upgrade

Multi-writer for restripe allows multiple restripe worker threads to operate on a single file concurrently. This in turn improves read/write performance during file re-protection operations, plus helps reduce the window of risk (MTTDL) during drive Smartfails, etc. This is particularly true for workflows consisting of large files, while one of the above restripe jobs is running. Typically, the larger the files on the cluster, the more benefit multi-writer for restripe will offer.

With multi-writer for restripe, an exclusive lock is no longer required on the LIN during the actual restripe of data. Instead, OneFS tries to use a delta write lock to update the cursors used to track which parts of the file need restriping. This means that a client application or program should be able to continue to write to the file while the restripe operation is underway.

An exclusive lock is only required for a very short period of time while a file is set up to be restriped. A file will have fixed widths for each restripe lock, and the number of range locks will depend on the quantity of threads and nodes which are actively restriping a single file.

OneFS File Locking and Concurrent Access

There has been a bevy of recent questions around how OneFS allows various clients attached to different nodes of a cluster to simultaneously read from and write to the same file. So it seemed like a good time for a quick refresher on some of the concepts and mechanics behind OneFS concurrency and distributed locking.

File locking is the mechanism that allows multiple users or processes to access data concurrently and safely. For reading data, this is a fairly straightforward process involving shared locks. With writes, however, things become more complex and require exclusive locking, since data must be kept consistent.

OneFS has a fully distributed lock manager that marshals locks on data across all the nodes in a storage cluster. This locking manager is highly extensible and allows for multiple lock types to support both file system locks as well as cluster-coherent protocol-level locks, such as SMB share mode locks or NFS advisory-mode locks. OneFS also has support for delegated locks such as SMB oplocks and NFSv4 delegations.

Every node in a cluster is able to act as coordinator for locking resources, and a coordinator is assigned to lockable resources based upon a hashing algorithm. This selection algorithm is designed so that the coordinator almost always ends up on a different node than the initiator of the request. When a lock is requested for a file, it can either be a shared lock or an exclusive lock. A shared lock is primarily used for reads and allows multiple users to share the lock simultaneously. An exclusive lock, on the other hand, allows only one user access to the resource at any given moment, and is typically used for writes. Exclusive lock types include:

Mark Lock:  An exclusive lock resource used to synchronize the marking and sweeping processes for the Collect job engine job.

Snapshot Lock:  As the name suggests, the exclusive snapshot lock which synchronizes the process of creating and deleting snapshots.

Write Lock:  An exclusive lock that’s used to quiesce writes for particular operations, including snapshot creates, non-empty directory renames, and marks.

The OneFS locking infrastructure has its own terminology, and includes the following definitions:

Domain: Refers to the specific lock attributes (recursion, deadlock detection, memory use limits, etc) and context for a particular lock application. There is one definition of owner, resource, and lock types, and only locks within a particular domain may conflict.

Lock Type: Determines the contention among lockers. A shared or read lock does not contend with other types of shared or read locks, while an exclusive or write lock contends with all other types. Lock types include:
• Advisory
• Anti-virus
• Data
• Delete
• LIN
• Mark
• Oplocks
• Quota
• Read
• Share Mode
• SMB byte-range
• Snapshot
• Write

Locker: Identifies the entity which acquires a lock.

Owner: A locker which has successfully acquired a particular lock. A locker may own multiple locks of the same or different type as a result of recursive locking.

Resource: Identifies a particular lock. Lock acquisition only contends on the same resource. The resource ID is typically a LIN to associate locks with files.

Waiter: Has requested a lock, but has not yet been granted or acquired it.

Here’s an example of how threads from different nodes could request a lock from the coordinator:

1. Node 2 is selected as the lock coordinator of these resources.
2. Thread 1 from Node 4 and thread 2 from Node 3 request a shared lock on a file from Node 2 at the same time.
3. Node 2 checks if an exclusive lock exists for the requested file.
4. If no exclusive locks exist, Node 2 grants thread 1 from Node 4 and thread 2 from Node 3 shared locks on the requested file.
5. Node 3 and Node 4 are now performing a read on the requested file.
6. Thread 3 from Node 1 requests an exclusive lock for the same file as being read by Node 3 and Node 4.
7. Node 2 checks with Node 3 and Node 4 if the shared locks can be reclaimed.
8. Node 3 and Node 4 are still reading so Node 2 asks thread 3 from Node 1 to wait for a brief instant.
9. Thread 3 from Node 1 blocks until the exclusive lock is granted by Node 2 and then completes the write operation.

OneFS Drive Performance Statistics

****************************************************************************************************

Warning:  We’re moving, and are currently in the process of blog site migration.

These articles are also being posted to our new location – www.unstructureddatatips.com – while the Dell EMC Community Network is being decommissioned.

Many thanks in advance for your patience during this transition period!

****************************************************************************************************

The previous post examined some of the general cluster performance metrics. In this article we’ll focus in on the disk subsystem and take a quick look at some of the drive statistics counters. As we’ll see, OneFS offers several tools to inspect and report on both drive health and performance.

Let’s start with some drive failure and wear reporting tools….

The following cluster-wide command will indicate any drives that are marked as smartfail, empty, stalled, or down:

# isi_for_array -sX 'isi devices list | egrep -vi “healthy|L3”'

Usually, any node that requires a drive replacement will have an amber warning light on the front display panel. Also, the drive that needs swapping out will typically be marked by a red LED.

Alternatively, isi_drivenum will also show the drive bay location of each drive, plus a variety of other disk related info, etc.

# isi_for_array -sX ‘isi_drivenum –A’

This next command provides drive wear information for each node’s flash (SSD) boot drives:

# isi_for_array -sSX "isi_radish -a /dev/da* | grep -e FW: -e 'Percent Life' | grep -v Used”

However, the output is in hex. This can be converted to a decimal percent value using the following shell command, where <value> is the raw hex output:

# echo "ibase=16; <value>" | bc

Alternatively, the following perl script will also translate the isi_radish command output from hex into comprehensible ‘life remaining’ percentages:

#!/usr/bin/perl

use strict;

use warnings;

my @drives = ('ada0', 'ada1');

foreach my $drive (@drives)

{

print "$drive:\n";

open CMD,'-|',"isi_for_array -s isi_radish -vt /dev/$drive" or

die "Failed to open pipe!\n";

while (<CMD>)

{

if (m/^(\S+).*(Life Remaining|Lifetime Left).*\(raw\s+([^)]+)/i)

{

print "$1 ".hex($3)."%\n";

}

}

}

The following drive statistics can be useful for both performance analysis and troubleshooting purposes.

General disk activity stats are available via the isi statistics command.

For example:

# isi statistics system –-nodes=all --oprates --nohumanize

This output will give you the per-node OPS over protocol, network and disk. On the disk side, the sum of DiskIn (writes)  and DIskOut (reads) gives the total IOPS for all the drives per node.

For the next level of granularity, the following drive statistics command provides individual SSSD drive info. The sum of OpsIn and OpsOut is the total IOPS per drive in the cluster.

# isi statistics drive -nall -–long --type=sata --sort=busy | head -20

And the same info for SSDs:

# isi statistics drive -nall --long --type=ssd --sort=busy | head -20

The primary counters of interest in drive stats data are often the ‘TimeInQ’, ‘Queued’, OpsIn, OpsOut, and IO and the ’Busy’ percentage of each disk. If most or all the drives have high busy percentages, this indicates a uniform resource constraint, and there is a strong likelihood that the cluster is spindle bound. If, say, the top five drives are much busier than the rest, this suggests a workflow hot-spot.

# isi statistics pstat

The read and write mix, plus metadata operations, for a particular protocol can be gleaned from the output of the isi statistics pstat command. In addition to disk statistics, CPU and network stats are also provided. The –protocol parameter is used to specify the core NAS protocols such as NFSv3, NFSv4, SMB1, SMB2, HDFS, etc. Additionally, OneFS specific protocol stats, including job engine (jobd), platform API (papi), IRP, etc, are also available.

For example, the following will show NFSv3 stats in a ‘top’ format, refreshed every 6 seconds by default:

# isi statistics pstat --protocol nfs3 --format top

The uptime command provides system load average for 1, 5, and 15 minute intervals, and is comprised of both CPU queues and disk queues stats.

# isi_for_array -s 'uptime'

It’s worth noting that this command’s output does not take CPU quantity into account. As such, a load average of 1 on a single CPU means the node is pegged. However, that load average of 1 on a dual CPU system means the CPU is 50% idle.

The following command will give the CPU count:

# isi statistics query current --nodes all --degraded --stats node.cpu.count

The sum of disk ops across a cluster per node is available via the following syntax:

# isi statistics query current --nodes=all --stats=node.disk.xfers.rate.sum

There are a whole slew of more detailed drive metrics that OneFS makes available for query.

Disk time in queue provides an indication as to how long an operation is queued on a drive. This indicator is key if a cluster is disk-bound. A time in queue value of 10 to 50 milliseconds is concerning, whereas a value of 50 to 100 milliseconds indicates a potential problem.

The following CLI syntax can be used to obtain the maximum, minimum, and average values for disk time in queue for SATA drives in this case:

# isi statistics drive --nodes=all --degraded --no-header --no-footer | awk ' /SATA/ {sum+=$8; max=0; min=1000} {if ($8>max) max=$8; if ($8<min) min=$8} END {print “Min = “,min; print “Max = “,max; print “Average = “,sum/NR}’

The following command displays the time in queue for 30 drives sorted highest-to-lowest:

# isi statistics drive list -n all --sort=timeinq | head -n 30

Queue depth indicates how many operations are queued on drives. A queue depth of 5 to 10 is considered heavy queuing.

The following CLI command can be used to obtain the maximum, minimum, and average values for disk queue depth of SATA drives. If there’s a big delta between the maximum number and average number in the queue, it’s worth investigating further to determine whether an individual drive is working excessively.

# isi statistics drive --nodes=all --degraded --no-header --no-footer | awk ' /SATA/ {sum+=$9; max=0; min=1000} {if ($9>max) max=$9; if ($9<min) min=$9} END {print “Min = “,min; print “Max = “,max; print “Average = “,sum/NR}’

For information on SAS or SSD drives, you can substitute SAS or SSD for SATA in the above syntax.

To display queue depth for twenty drives sorted highest-to-lowest, run the following command:

# isi statistics drive list -n all --sort=queued | head -n 20

Note that the TimeAvg metric, as reported by isi statistics drive command, represents all the latency at the disk that doesn’t include the scheduler wait time (TimeInQ). So this is a measure of disk access time (ie. send the op, wait, receive response). The Total Time at the disk is a sum of the access time (TimeAvg) and the scheduler time (TimeInQ).

The disk percent busy metric can he useful to determine if a drive is getting pegged. However, it does not indicate how much extra work may be in the queue. To obtain the maximum, minimum, and average disk busy values for SATA drives, run the following command. For information on SAS or SSD drives, you can include SAS or SSD respectively, instead of SATA.

# isi statistics drive --nodes=all --degraded --no-header --no-footer | awk ' /SATA/ {sum+=$10; max=0; min=1000} {if ($10>max) max=$10; if ($10min) min=$10} END {print “Min = “,min; print “Max = “,max; print “Average = “,sum/NR}’

To display disk percent busy for twenty drives sorted highest-to-lowest issue, run the following command.

# isi statistics drive -nall --output=busy | head -n 20

OneFS Performance Statistics

*********************************************************

Warning:  We’re moving, and are currently in the process of blog site migration.

These articles are also being posted to our new location – www.unstructureddatatips.com – while the Dell EMC Community Network is being decommissioned.

Many thanks in advance for your patience during this transition period!

*********************************************************

There have been several recent inquiries on how to effectively gather performance statistics on a cluster, so it seemed like a useful topic to dig into further in a blog article:

First, two cardinal rules… Before planning or undertaking any performance tuning on a cluster, or its attached clients:

1)  Record the original cluster settings before making any configuration changes to OneFS or its data services.

2)  Measure and analyze how the various workloads in your environment interact with and consume storage resources.

Performance measurement is done by gathering statistics about the common file sizes and I/O operations, including CPU and memory load, network traffic utilization, and latency. To obtain key metrics and wall-clock timing data for delete, renew lease, create, remove, set userdata, get entry, and other file system operations, connect to a node via SSH and run the following command as root to enable the vopstat system control:

# sysctl efs.util.vopstats.record_timings=1

After enabling vopstats, they can be viewed by running the ‘sysctl efs.util.vopstats’ command as root: Here is an example of the command’s output:

# sysctl efs.util.vopstats.

efs.util.vopstats.ifs_snap_set_userdata.initiated: 26 

efs.util.vopstats.ifs_snap_set_userdata.fast_path: 0 

efs.util.vopstats.ifs_snap_set_userdata.read_bytes: 0

efs.util.vopstats.ifs_snap_set_userdata.read_ops: 0

efs.util.vopstats.ifs_snap_set_userdata.raw_read_bytes: 0

efs.util.vopstats.ifs_snap_set_userdata.raw_read_ops: 0

efs.util.vopstats.ifs_snap_set_userdata.raw_write_bytes: 6432768

efs.util.vopstats.ifs_snap_set_userdata.raw_write_ops: 2094

efs.util.vopstats.ifs_snap_set_userdata.timed: 0

efs.util.vopstats.ifs_snap_set_userdata.total_time: 0

efs.util.vopstats.ifs_snap_set_userdata.total_sqr_time: 0

efs.util.vopstats.ifs_snap_set_userdata.fast_path_timed: 0

efs.util.vopstats.ifs_snap_set_userdata.fast_path_total_time: 0   

efs.util.vopstats.ifs_snap_set_userdata.fast_path_total_sqr_time: 0

The time data captures the number of operations that cross the OneFS clock tick, which is 10 milliseconds. Independent of the number of events, the total_sq_time provides no actionable information because of the granularity of events.

To analyze the operations, use the total_time value instead. The following example shows only the total time records in the vopstats:

# sysctl efs.util.vopstats | grep –e "total_ time: [ ^0]"

efs.util.vopstats.access_rights.total_time: 40000

efs.util.vopstats.lookup.total_time:  30001

efs.util.vopstats.unlocked_ write_mbuf.total_time:  340006

efs.util.vopstats.unlocked_write_mbuf.fast_path_total_time: 340006

efs.util.vopstats.commit.total_time: 3940137

efs.util.vopstats.unlocked_getattr.total_time: 280006

efs.util.vopstats.unlocked_getattr.fast_path_total_time:   50001 efs.util.vopstats.inactive.total_time: 100004

efs.util.vopstats.islocked.total_time:   30001 efs.util.vopstats.lock1.total_time:   280005

efs.util.vopstats.unlocked_read_mbuf.total_time: 11720146 efs.util.vopstats.readdir.total_time: 20000

efs.util.vopstats.setattr.total_time: 220010 efs.util.vopstats.unlock.total_time: 20001

efs.util.vopstats.ifs_snap_delete_resume.timed: 77350

efs.util.vopstats.ifs_snap_delete_resume.total_time: 720014

efs.util.vopstats.ifs_snap_delete_resume.total_sqr_time: 7200280042

The ‘isi statistics’ CLI command is a great tool for the task here, plus its output is current (ie. real time). It’s a versatile utility, providing real time stats with the following subcommand-level syntax

Statistics

Category

Details
Client Display cluster usage statistics organized according to cluster hosts and users
Drive Show performance by drive
Heat Identify the most accessed files/directories
List List valid arguments to given option
Protocol Display cluster usage statistics organized by communication protocol
Pstat Generate detailed protocol statistics along with CPU, OneFS, network & disk stats
Query Query for specific statistics. There are current and history modes
System Display general cluster statistics (Op rates for protocols, network & disk traffic (kB/s)

Full command syntax and a description of the options can be accessed via isi statistics –help or via the man page (man isi-statistics).

The ‘isi statistics pstat’ command provides statistics per protocol operation, client connections, and the file system. For example, for  NFSv3:

# isi statistics pstat --protocol=nfs3

The ‘isi statistics client’ CLI command  provides I/O and timing data by client name and/or IP address, depending on options – plus the username if it can be determined. For example, to generate a list of the top NFSv3 clients on a cluster, the following command can be used:

# isi statistics client --protocols=nfs3 –-format=top

Or, for SMB clients:

# isi statistics client --protocols=smb2 –-format=top

The extensive list of protocols for which client data can be displayed includes:

nfs3 | smb1 | nlm | ftp | http | siq | smb2 | nfs4 | papi | jobd | irp | lsass_in | lsass_out | hdfs | all | internal | external

SMB2 and SMB3 current connections are both displayed in the following stats command:

# isi statistics query current --stats node.clientstats.active.smb2

Or for SMB2 + 3 historical connection data:

# isi statistics query history --stats node.clientstats.active.smb2

The following command will total by users, as opposed to node. This can be helpful when investigating HPC workloads, or other workflows involving compute clusters:

# isi statistics client --protocols=nfs3 –-format=top --numeric --totalby=username --sort=ops,timemax

This ‘heat’ command option can be useful for viewing the files that are being utilized:

# isi statistics heat --long --classes=read,write,namespace_read,namespace_write | head -10

This shows the amount of contention where parallel user(s) operations are targeting the same object.

# isi statistics heat --long --classes=read,write,namespace_read,namespace_write --events=blocked,contended,deadlocked | head -10

It can also be useful to constrain statistics reporting to a single node. For example, the following command will show the fifteen hottest files on node 4.

# isi statistics heat --limit=15 --nodes=4

It’s worth noting that isi statistics doesn’t directly tie a client to a file or directory path. Both isi statistics heat and isi statistics client provide some of this information, but not together. The only directory/file related statistics come from the ‘heat’ stats, which track the hottest accesses in the filesystem.

The system and drive statistics can also be useful for performance analysis and troubleshooting purposes. For example:

# isi statistics system -–nodes=all --oprates --nohumanize

This output will give you the per-node OPS over protocol, network and disk. On the disk side, the sum of DiskIn (writes)  and DIskOut (reads) gives the total IOPS for all the drives per node.

For the next level of granularity, the drive statistics command provides individual disk info.

# isi statistics drive -nall -–long --type=sata --sort=busy | head -20

If most or all the drives have high busy percentages, this indicates a uniform resource constraint, and there is a strong likelihood that the cluster is spindle bound. If, say, the top five drives are much busier than the rest, this would suggest a workflow hot-spot.

OneFS Time Synchronization & NTP

We’re in the process of blog site migration. Currently, these articles are also being posted to the former Dell EMC Community Network Isilon blog, until the DECN site is finally decommissioned:

https://community.emc.com/community/products/isilon/blog

Thank you in advance for your patience during this transition period.

——————————————————

That said, there have been several recent inquiries from the field around cluster time synchronization recently. So it seemed like an article on managing time and date on Isilon would be useful – and timely.

So, OneFS provides a network time protocol (NTP) service to ensure that all nodes in a cluster can easily be synchronized to the same time source. This service automatically adjusts a cluster’s date and time settings to that of one or more external NTP servers.

NTP configuration on a cluster is performed by using the ‘isi ntp’ command line (CLI) utility, rather than modifying the nodes’ /etc/ntp.conf files manually. The syntax for this command is divided into two parts: servers and settings. For example:

# isi ntp settings

Description:

    View and modify cluster NTP configuration.

Required Privileges:

    ISI_PRIV_NTP

Usage:

    isi ntp settings <action>

        [--timeout <integer>]

        [{--help | -h}]

Actions:

    modify    Modify cluster NTP configuration.

    view      View cluster NTP configuration.

Options:

  Display Options:

    --timeout <integer>

        Number of seconds for a command timeout (specified as 'isi --timeout NNN

        <command>').

    --help | -h

        Display help for this command.


There is also an isi_ntp_config CLI command available in OneFS that provides a richer configuration set and combines the server and settings functionality :

Usage: isi_ntp_config COMMAND [ARGUMENTS ...]

Commands:

    help

      Print this help and exit.

    list

      List all configured info.

    add server SERVER [OPTION]

      Add SERVER to ntp.conf.  If ntp.conf is already

      configured for SERVER, the configuration will be replaced.

      You can specify any server option. See NTP.CONF(5)


    delete server SERVER

      Remove server configuration for SERVER if it exists.


    add exclude NODE [NODE...]

      Add NODE (or space separated nodes) to NTP excluded entry.

      Excluded nodes are not used for NTP communication with external

      NTP servers.


    delete exclude NODE [NODE...]

      Delete NODE (or space separated Nodes) from NTP excluded entry.


    keyfile KEYFILE_PATH

      Specify keyfile path for NTP auth. Specify "" to clear value.

      KEYFILE_PATH has to be a path under /ifs.


    chimers [COUNT | "default"]

      Display or modify the number of chimers NTP uses.

      Specify "default" to clear the value.

By default, if the cluster has more than three nodes, three of the nodes are selected as ‘chimers’. Chimers are nodes which can contact the external NTP servers. If the cluster comprises three nodes or less, only one node will be selected as a chimer. If no external NTP server is set, they will use the local clock instead. The other non-chimer nodes will use the chimer nodes as their NTP servers. The chimer nodes are selected by the lowest node number which is not excluded from chimer duty.

If a node is configured as a chimer. its /etc/ntp.conf entry will resemble:

# This node is one of the 3 chimer nodes that can contact external NTP

# servers. The non-chimer nodes will use this node as well as the other

# chimers as their NTP servers.

server time.isilon.com

# The other chimer nodes on this cluster:

server 192.168.10.150 iburst

server 192.168.10.151 iburst

# If none or bad connection to external servers this node may become

# the time server for this cluster. The system clock will be a time

# source and run at a high stratum

In addition to managing NTP servers and authentication, individual nodes can also be excluded from communicating with external NTP servers.

The local clock of the node is set as an NTP server at a high stratum level. In NTP, a server with lower stratum number is preferred, so if an external NTP server is set the system will prefer an external time server if configured. The stratum level for the chimer is determined by the chimer number. The first chimer is set to stratum 9, the second to stratum 11, and the others continue to increment the stratum number by 2. This is so the non-chimer nodes will prefer to get the time from the first chimer if available.

For a non-chimer node, its /etc/ntp.conf entry will resemble:

# This node is _not_ one of the 3 chimer nodes that can contact external

# NTP servers. These are the cluster's chimer nodes:

server 192.168.10.149 iburst true

server 192.168.10.150 iburst true

server 192.168.10.151 iburst true


When configuring NTP on a cluster, more than one NTP server can be specified to synchronize the system time from. This allows for full redundancy of ysnc targets. The cluster periodically contacts these server(s) and adjusts the time and/or date as necessary, based on the information it receives.

The ‘isi_ntp_config’ CLI command can be used to configure which NTP servers a cluster will reference. For example, the following syntax will add the server ‘time.isilon.com’:

# isi_ntp_config add server time.isilon.com

Alternatively, the NTP configuration can also be managed from the WebUI by browsing to Cluster Management > General Settings > NTP.

NTP also provides basic authentication-based security via symmetrical keys, if desired.

If no NTP servers are available, Windows Active Directory (AD) can synchronize domain members to a primary clock running on the domain controller(s). If there are no external NTP servers configured and the cluster is joined to AD, OneFS will use the Windows domain controller as the NTP time server. If the cluster and domain time become out of sync by more than 4 minutes, OneFS generates an event notification.

Be aware though, that if the cluster and Active Directory drift out of time sync by more than 5 minutes, AD authentication will cease to function.

If neither an NTP server or domain controller are available, the cluster’s time, date, and time zone can also be set manually using the ‘isi config’ CLI command.  For example:

1.     Run the ‘isi config’ command. The command-line prompt changes to indicate that you are in the isi config subsystem:

# isi config

Welcome to the Isilon IQ configuration console.

Copyright (c) 2001-2017 EMC Corporation. All Rights Reserved.

Enter 'help' to see list of available commands.

Enter 'help <command>' to see help for a specific command.

Enter 'quit' at any prompt to discard changes and exit.

        Node build: Isilon OneFS v8.2.2 B_8_2_2(RELEASE)Node serial number: JWXER170300301

>>>


2.     Specify the current date and time by running the date command. For example, the following command sets the cluster time to 9:20 AM on April 23, 2020:

>>> date 2020/04/23 09:20:00

Date is set to 2020/04/23 09:20:00

3.     The ‘help timezone’ command will list the available timezones. For example:

>>> help timezone


timezone [<timezone identifier>]


Sets the time zone on the cluster to the specified time zone.

Valid time zone identifiers are:

        Greenwich Mean Time

        Eastern Time Zone

        Central Time Zone

        Mountain Time Zone

        Pacific Time Zone

        Arizona

        Alaska

        Hawaii

        Japan

        Advanced

4.     To verify the currently configured time zone, run the ‘timezone’ command. For example:

>>> timezone

The current time zone is: Greenwich Mean Time

5.     To change the time zone, enter the timezone command followed by one of the displayed options. For example, the following command changes the time zone to Alaska:

>>> timezone Alaska

Time zone is set to Alaska

A message confirming the new time zone setting displays. If your desired time zone did not display when you ran the help timezone command, enter ‘timezone Advanced’. After a warning screen, you will proceed to a list of regions. When you select a region, a list of specific time zones for that region appears. Select the desired time zone (you may need to scroll), then enter OK or Cancel until you return to the isi config prompt.

6.     When done, run the commit command to save your changes and exit isi config.

>>> commit

Commit succeeded.

Alternatively, these time and date parameters can also be managed from the WebUI by browsing to Cluster Management > General Settings > Date and Time.