[jira] [Commented] (KUDU-2195) Enforce durability happened before relationships on multiple disks

["ASF subversion and git services (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KUDU-2195?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17461131#comment-17461131 ] 

ASF subversion and git services commented on KUDU-2195:
-------------------------------------------------------

Commit 6908620821ab93cb964f0f7a3674abf9843a9b5f in kudu's branch refs/heads/master from Andrew Wong
[ https://gitbox.apache.org/repos/asf?p=kudu.git;h=6908620 ]

[consensus] force fsync metadata on XFS

We've seen issues like KUDU-2195 rear their heads more prominently on
XFS, sometimes requiring costly surgery to fix by restoring metadata if
all replicas lose their consensus metadata.

This patch adds to the band-aid flag --cmeta_force_fsync, checking if
the metadata is on an XFS mount, and if so, always fsyncing. This new
behavior is enabled by default with the --cmeta_fsync_override_on_xfs
flag.

Change-Id: Ie97138e3522f20325be104d3665d887693b79a10
Reviewed-on: http://gerrit.cloudera.org:8080/18101
Reviewed-by: Alexey Serbin <as...@cloudera.com>
Tested-by: Kudu Jenkins
Reviewed-by: Attila Bukor <ab...@apache.org>


> Enforce durability happened before relationships on multiple disks
> ------------------------------------------------------------------
>
>                 Key: KUDU-2195
>                 URL: https://issues.apache.org/jira/browse/KUDU-2195
>             Project: Kudu
>          Issue Type: Bug
>          Components: consensus, tablet
>    Affects Versions: 1.9.0
>            Reporter: David Alves
>            Priority: Major
>
> When using weaker durability semantics (e.g. when log_force_fsync is off) we should still enforce certain happened before relationships which are not currently being enforced when using different disks for the wal and data.
> The two cases that come to mind where this is relevant are:
> 1) cmeta (c) -> wal (w) : We flush cmeta before flushing the wal (for instance on term change) with the intention that either {}, \{c} or \{c, w} were made durable.
> 2) wal (w) -> tablet meta (t): We flush the wal before tablet metadata to make sure that that all commit messages that refer to on disk row sets (and deltas) are on disk before the row sets they point to, i.e. with the intention that either {}, \{w} or \{w, t} were made durable.
> With strong durability semantics these are always made durable in the right order. With weaker semantics that is not the case though. If using the same disk for both the wal and data then the invariants are  still preserved, as buffers will be flushed in the right order but if using different disks for the wal and data (and because cmeta is stored with the data) that is not always the case.
> 1) in ext4 is actually safe, because we perform an fsync (indirect, rename() implies fsync in ext4) when flushing cmeta. But it is not for xfs.
> 2) Is not safe in either filesystem.
> --- Possible solutions --
> For 1): Store cmeta with the wal; actually always fsync cmeta.
> For 2): Store tablet meta with the wal; always fsync the wal before flushing tablet meta.



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