[dm-devel] [PATCH v2] dm-raid.txt: document discard support
heinzm at redhat.com
heinzm at redhat.com
Tue Aug 25 15:15:07 UTC 2015
From: Heinz Mauelshagen <heinzm at redhat.com>
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 at redhat.com>
---
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
More information about the dm-devel
mailing list