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authorHeinz Mauelshagen <heinzm@redhat.com>2017-03-22 17:44:38 +0100
committerMike Snitzer <snitzer@redhat.com>2017-03-27 12:08:07 -0400
commit6e53636fe81465d6810f4e0910e7238edf12a133 (patch)
tree38be83f8fc4476c3fb4debce0b8135ed07854c1c /Documentation/device-mapper
parent4464e36e06470e3d68dc26a874f0dbdffa09a6e8 (diff)
dm raid: add raid4/5/6 journal write-back support via journal_mode option
Commit 63c32ed4afc ("dm raid: add raid4/5/6 journaling support") added journal support to close the raid4/5/6 "write hole" -- in terms of writethrough caching. Introduce a "journal_mode" feature and use the new r5c_journal_mode_set() API to add support for switching the journal device's cache mode between write-through (the current default) and write-back. NOTE: If the journal device is not layered on resilent storage and it fails, write-through mode will cause the "write hole" to reoccur. But if the journal fails while in write-back mode it will cause data loss for any dirty cache entries unless resilent storage is used for the journal. Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Diffstat (limited to 'Documentation/device-mapper')
-rw-r--r--Documentation/device-mapper/dm-raid.txt11
1 files changed, 10 insertions, 1 deletions
diff --git a/Documentation/device-mapper/dm-raid.txt b/Documentation/device-mapper/dm-raid.txt
index 95c4c8dd6dd1..7e06e65586d4 100644
--- a/Documentation/device-mapper/dm-raid.txt
+++ b/Documentation/device-mapper/dm-raid.txt
@@ -170,6 +170,13 @@ The target is named "raid" and it accepts the following parameters:
Takeover/reshape is not possible with a raid4/5/6 journal device;
it has to be deconfigured before requesting these.
+ [journal_mode <mode>]
+ This option sets the caching mode on journaled raid4/5/6 raid sets
+ (see 'journal_dev <dev>' above) to 'writethrough' or 'writeback'.
+ If 'writeback' is selected the journal device has to be resilient
+ and must not suffer from the 'write hole' problem itself (e.g. use
+ raid1 or raid10) to avoid a single point of failure.
+
<#raid_devs>: The number of devices composing the array.
Each device consists of two entries. The first is the device
containing the metadata (if any); the second is the one containing the
@@ -254,7 +261,8 @@ recovery. Here is a fuller description of the individual fields:
<data_offset> The current data offset to the start of the user data on
each component device of a raid set (see the respective
raid parameter to support out-of-place reshaping).
- <journal_char> 'A' - active raid4/5/6 journal device.
+ <journal_char> 'A' - active write-through journal device.
+ 'a' - active write-back journal device.
'D' - dead journal device.
'-' - no journal device.
@@ -334,3 +342,4 @@ Version History
1.10.1 Fix data corruption on reshape request
1.11.0 Fix table line argument order
(wrong raid10_copies/raid10_format sequence)
+1.11.1 Add raid4/5/6 journal write-back support via journal_mode option