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| author | Heinz Mauelshagen <heinzm@redhat.com> | 2017-03-22 17:44:38 +0100 |
|---|---|---|
| committer | Mike Snitzer <snitzer@redhat.com> | 2017-03-27 12:08:07 -0400 |
| commit | 6e53636fe81465d6810f4e0910e7238edf12a133 (patch) | |
| tree | 38be83f8fc4476c3fb4debce0b8135ed07854c1c /Documentation/device-mapper | |
| parent | 4464e36e06470e3d68dc26a874f0dbdffa09a6e8 (diff) | |
| download | lwn-6e53636fe81465d6810f4e0910e7238edf12a133.tar.gz lwn-6e53636fe81465d6810f4e0910e7238edf12a133.zip | |
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.txt | 11 |
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 |