From bd49784fd1e8f42c7600fbfa206361324857f373 Mon Sep 17 00:00:00 2001 From: Mikulas Patocka Date: Tue, 18 Aug 2015 16:26:16 -0400 Subject: dm stats: report precise_timestamps and histogram in @stats_list output If the user selected the precise_timestamps or histogram options, report it in the @stats_list message output. If the user didn't select these options, no extra tokens are reported, thus it is backward compatible with old software that doesn't know about precise timestamps and histogram. Signed-off-by: Mikulas Patocka Signed-off-by: Mike Snitzer Cc: stable@vger.kernel.org # 4.2 --- Documentation/device-mapper/statistics.txt | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'Documentation') diff --git a/Documentation/device-mapper/statistics.txt b/Documentation/device-mapper/statistics.txt index 4919b2dfd1b3..6f5ef944ca4c 100644 --- a/Documentation/device-mapper/statistics.txt +++ b/Documentation/device-mapper/statistics.txt @@ -121,6 +121,10 @@ Messages Output format: : + + precise_timestamps histogram:n1,n2,n3,... + + The strings "precise_timestamps" and "histogram" are printed only + if they were specified when creating the region. @stats_print [ ] -- cgit v1.2.3 From f15f4d720088c140cdf1fee6aeab3549dbdddc41 Mon Sep 17 00:00:00 2001 From: Heinz Mauelshagen Date: Tue, 25 Aug 2015 17:15:41 +0200 Subject: dm raid: document RAID 4/5/6 discard support For RAID 4/5/6 data integrity reasons 'discard_zeroes_data' must work properly. Signed-off-by: Heinz Mauelshagen Signed-off-by: Mike Snitzer --- Documentation/device-mapper/dm-raid.txt | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+) (limited to 'Documentation') diff --git a/Documentation/device-mapper/dm-raid.txt b/Documentation/device-mapper/dm-raid.txt index cb12af3b51c2..df2d636b6088 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 -- cgit v1.2.3