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| -rw-r--r-- | Documentation/device-mapper/dm-raid.txt | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/Documentation/device-mapper/dm-raid.txt b/Documentation/device-mapper/dm-raid.txt index cb12af3..df2d636 100644 --- a/Documentation/device-mapper/dm-raid.txt +++ b/Documentation/device-mapper/dm-raid.txt @@ -209,6 +209,37 @@ include: "repair" - Initiate a repair of the array. "reshape"- Currently unsupported (-EINVAL). + +Discard Support +--------------- +The implementation of discard support among hardware vendors varies. +When a block is discarded, some storage devices will return zeroes when +the block is read. These devices set the 'discard_zeroes_data' +attribute. Other devices will return random data. Confusingly, some +devices that advertise 'discard_zeroes_data' will not reliably return +zeroes when discarded blocks are read! Since RAID 4/5/6 uses blocks +from a number of devices to calculate parity blocks and (for performance +reasons) relies on 'discard_zeroes_data' being reliable, it is important +that the devices be consistent. Blocks may be discarded in the middle +of a RAID 4/5/6 stripe and if subsequent read results are not +consistent, the parity blocks may be calculated differently at any time; +making the parity blocks useless for redundancy. It is important to +understand how your hardware behaves with discards if you are going to +enable discards with RAID 4/5/6. + +Since the behavior of storage devices is unreliable in this respect, +even when reporting 'discard_zeroes_data', by default RAID 4/5/6 +discard support is disabled -- this ensures data integrity at the +expense of losing some performance. + +Storage devices that properly support 'discard_zeroes_data' are +increasingly whitelisted in the kernel and can thus be trusted. + +For trusted devices, the following dm-raid module parameter can be set +to safely enable discard support for RAID 4/5/6: + 'devices_handle_discards_safely' + + Version History --------------- 1.0.0 Initial version. Support for RAID 4/5/6 |