There have been several inquiries recently around PowerScale clusters and hardware fault tolerance, above and beyond file level data protection via erasure coding. So it seemed like a useful topic for a blog article, and here are some of the techniques which OneFS employs to help protect data against the threat of hardware errors:
File system journal
Every PowerScale node is equipped with a battery backed NVRAM file system journal. Each journal is used by OneFS as stable storage, and guards write transactions against sudden power loss or other catastrophic events. The journal protects the consistency of the file system and the battery charge lasts up to three days. Since each member node of a cluster contains an NVRAM controller, the entire OneFS file system is therefore fully journaled.
Proactive device failure
OneFS will proactively remove, or SmartFail, any drive that reaches a particular threshold of detected Error Correction Code (ECC) errors, and automatically reconstruct the data from that drive and locate it elsewhere on the cluster. Both SmartFail and the subsequent repair process are fully automated and hence require no administrator intervention.
ISI Data Integrity (IDI) is the OneFS process that protects file system structures against corruption via 32-bit CRC checksums. All OneFS blocks, both for file and metadata, utilize checksum verification. Metadata checksums are housed in the metadata blocks themselves, whereas file data checksums are stored as metadata, thereby providing referential integrity. All checksums are recomputed by the initiator, the node servicing a particular read, on every request.
In the event that the recomputed checksum does not match the stored checksum, OneFS will generate a system alert, log the event, retrieve and return the corresponding error correcting code (ECC) block to the client and attempt to repair the suspect data block.
In addition to blocks and metadata, OneFS also provides checksum verification for Remote Block Management (RBM) protocol data. As mentioned above, the RBM is a unicast, RPC-based protocol used over the back-end cluster interconnect. Checksums on the RBM protocol are in addition to the InfiniBand hardware checksums provided at the network layer, and are used to detect and isolate machines with certain faulty hardware components and exhibiting other failure states.
Dynamic sector repair
OneFS includes a Dynamic Sector Repair (DSR) feature whereby bad disk sectors can be forced by the file system to be rewritten elsewhere. When OneFS fails to read a block during normal operation, DSR is invoked to reconstruct the missing data and write it to either a different location on the drive or to another drive on the node. This is done to ensure that subsequent reads of the block do not fail. DSR is fully automated and completely transparent to the end-user. Disk sector errors and Cyclic Redundancy Check (CRC) mismatches use almost the same mechanism as the drive rebuild process.
MediaScan’s role within OneFS is to check disk sectors and deploy the above DSR mechanism in order to force disk drives to fix any sector ECC errors they may encounter. Implemented as one of the phases of the OneFS job engine, MediaScan is run automatically based on a predefined schedule. Designed as a low-impact, background process, MediaScan is fully distributed and can thereby leverage the benefits of a cluster’s parallel architecture.
IntegrityScan, another component of the OneFS job engine, is responsible for examining the entire file system for inconsistencies. It does this by systematically reading every block and verifying its associated checksum. Unlike traditional ‘fsck’ style file system integrity checking tools, IntegrityScan is designed to run while the cluster is fully operational, thereby removing the need for any downtime. In the event that IntegrityScan detects a checksum mismatch, a system alert is generated and written to the syslog and OneFS automatically attempts to repair the suspect block.
The IntegrityScan phase is run manually if the integrity of the file system is ever in doubt. Although this process may take several days to complete, the file system is online and completely available during this time. Additionally, like all phases of the OneFS job engine, IntegrityScan can be prioritized, paused or stopped, depending on the impact to cluster operations and other jobs.
Because OneFS protects its data at the file-level, any inconsistencies or data loss is isolated to the unavailable or failing device—the rest of the file system remains intact and available.
For example, a ten node, S210 cluster, protected at +2d:1n, sustains three simultaneous drive failures—one in each of three nodes. Even in this degraded state, I/O errors would only occur on the very small subset of data housed on all three of these drives. The remainder of the data striped across the other two hundred and thirty-seven drives would be totally unaffected. Contrast this behavior with a traditional RAID6 system, where losing more than two drives in a RAID-set will render it unusable and necessitate a full restore from backups.
Similarly, in the unlikely event that a portion of the file system does become corrupt (whether as a result of a software or firmware bug, etc) or a media error occurs where a section of the disk has failed, only the portion of the file system associated with this area on disk will be affected. All healthy areas will still be available and protected.
As mentioned above, referential checksums of both data and meta-data are used to catch silent data corruption (data corruption not associated with hardware failures).The checksums for file data blocks are stored as metadata, outside the actual blocks they reference, and thus provide referential integrity.
Accelerated drive rebuilds
The time that it takes a storage system to rebuild data from a failed disk drive is crucial to the data reliability of that system. With the advent of four terabyte drives, and the creation of increasingly larger single volumes and file systems, typical recovery times for multi-terabyte drive failures are becoming multiple days or even weeks. During this MTTDL period, storage systems are vulnerable to additional drive failures and the resulting data loss and downtime.
Since OneFS is built upon a highly distributed architecture, it’s able to leverage the CPU, memory and spindles from multiple nodes to reconstruct data from failed drives in a highly parallel and efficient manner. Because a PowerScale cluster is not bound by the speed of any particular drive, OneFS is able to recover from drive failures extremely quickly and this efficiency grows relative to cluster size. As such, a failed drive within a cluster will be rebuilt an order of magnitude faster than hardware RAID-based storage devices. Additionally, OneFS has no requirement for dedicated ‘hot-spare’ drives.
Automatic drive firmware updates
Clusters support automatic drive firmware updates for new and replacement drives, as part of the non-disruptive firmware update process. Firmware updates are delivered via drive support packages, which both simplify and streamline the management of existing and new drives across the cluster. This ensures that drive firmware is up to date and mitigates the likelihood of failures due to known drive issues. As such, automatic drive firmware updates are an important component of OneFS’ high availability and non-disruptive operations strategy.