From: Heinz Mauelshagen <heinzm@xxxxxxxxxx> For raid4/5/6 data integrity reasons, 'discard_zeroes_data' needs to work properly. This version 2 of the patch ducuments the background of this critical requirement. Signed-off-by: Heinz Mauelshagen <heinzm@xxxxxxxxxx> --- Documentation/device-mapper/dm-raid.txt | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/Documentation/device-mapper/dm-raid.txt b/Documentation/device-mapper/dm-raid.txt index cb12af3..a2c645b 100644 --- a/Documentation/device-mapper/dm-raid.txt +++ b/Documentation/device-mapper/dm-raid.txt @@ -209,6 +209,36 @@ include: "repair" - Initiate a repair of the array. "reshape"- Currently unsupported (-EINVAL). +Discard Support +--------------- +The implementation of discard support amoung hardware vendors varies. When +a block is discarded, some storage devices will return zeros when it is +read. These devices set the 'discard_zeros_data' attribute. Other devices +will return random data. Confusingly, some devices will set +'discard_zeros_data', but will still not always return zeros 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_zeros_data' returning zeroes doing so, it is important that the +devices be consistent. Blocks may be discarded in the middle of a RAID 4/5/6 +stripe and if the 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 use RAID 4/5/6. + +Since the behavior of storage devices is unreliable in this respect - even +when reporting 'discard_zeros_data' - RAID 4/5/6 does not enable discard +support by default to ensure data integrity thus loosing some performance. + +Storage devices reporting 'discard_zeroes_data' which bahave properly are +increasingly whitelisted in the kernel and can thus be trusted. + +The following module parameter can be set to safely use them: + + 'devices_handle_discards_safely' + +Once set, RAID 4/5/6 will enable discards. + + Version History --------------- 1.0.0 Initial version. Support for RAID 4/5/6 -- 2.4.3 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
