[dm-devel] [PATCH] dm-raid.txt: document discard support
heinzm at redhat.com
heinzm at redhat.com
Thu Aug 6 14:44:39 UTC 2015
From: Heinz Mauelshagen <heinzm at redhat.com>
For raid4/5/6 data integrity reasons, 'discard_zeroes_data' needs to work properly.
This patch ducuments the background of this critical requirement.
Signed-off-by: Heinz Mauelshagen <heinzm at redhat.com>
---
Documentation/device-mapper/dm-raid.txt | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/Documentation/device-mapper/dm-raid.txt b/Documentation/device-mapper/dm-raid.txt
index cb12af3..597ba0a 100644
--- a/Documentation/device-mapper/dm-raid.txt
+++ b/Documentation/device-mapper/dm-raid.txt
@@ -209,6 +209,32 @@ 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 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. If the storage has been verified to return zeros when
+reading a discarded block, the following module parameter can be set:
+
+ 'devices_handle_discards_safely'
+
+Once set, RAID 4/5/6 will enable discards and can safely use them.
+
+
Version History
---------------
1.0.0 Initial version. Support for RAID 4/5/6
--
2.1.0
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