diff options
Diffstat (limited to 'md.4')
| -rw-r--r-- | md.4 | 54 |
1 files changed, 54 insertions, 0 deletions
@@ -916,6 +916,60 @@ slow). The extra latency of the remote link will not slow down normal operations, but the remote system will still have a reasonably up-to-date copy of all data. +.SS FAILFAST + +From Linux 4.10, +.I +md +supports FAILFAST for RAID1 and RAID10 arrays. This is a flag that +can be set on individual drives, though it is usually set on all +drives, or no drives. + +When +.I md +sends an I/O request to a drive that is marked as FAILFAST, and when +the array could survive the loss of that drive without losing data, +.I md +will request that the underlying device does not perform any retries. +This means that a failure will be reported to +.I md +promptly, and it can mark the device as faulty and continue using the +other device(s). +.I md +cannot control the timeout that the underlying devices use to +determine failure. Any changes desired to that timeout must be set +explictly on the underlying device, separately from using +.IR mdadm . + +If a FAILFAST request does fail, and if it is still safe to mark the +device as faulty without data loss, that will be done and the array +will continue functioning on a reduced number of devices. If it is not +possible to safely mark the device as faulty, +.I md +will retry the request without disabling retries in the underlying +device. In any case, +.I md +will not attempt to repair read errors on a device marked as FAILFAST +by writing out the correct. It will just mark the device as faulty. + +FAILFAST is appropriate for storage arrays that have a low probability +of true failure, but will sometimes introduce unacceptable delays to +I/O requests while performing internal maintenance. The value of +setting FAILFAST involves a trade-off. The gain is that the chance of +unacceptable delays is substantially reduced. The cost is that the +unlikely event of data-loss on one device is slightly more likely to +result in data-loss for the array. + +When a device in an array using FAILFAST is marked as faulty, it will +usually become usable again in a short while. +.I mdadm +makes no attempt to detect that possibility. Some separate +mechanism, tuned to the specific details of the expected failure modes, +needs to be created to monitor devices to see when they return to full +functionality, and to then re-add them to the array. In order of +this "re-add" functionality to be effective, an array using FAILFAST +should always have a write-intent bitmap. + .SS RESTRIPING .IR Restriping , |