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https://issues.apache.org/jira/browse/OAK-6888?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16235631#comment-16235631
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Andrei Dulceanu commented on OAK-6888:
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[~frm], I was wondering how the current version of the patch solves the general
case, since currently the {{TarMK flush}} thread still uses {{tryFlush}}. In
{{DataStoreTestBase#testSync}} and {{DataStoreTestBase#testSyncBigBlob}} there
are calls to {{primary.flush()}} before the sync happens. This forces the flush
on the primary, before that single client sync run.
My question is: how are we protecting ourselves from scenarios like the one you
already described, if we still use {{FileStore#tryFlush}} in the {{TarMK
flush}} thread? Doesn't this defeat the whole purpose of this fix? OTOH,
waiting for every flush to succeed (w, w/o a cold standby attached) will have a
tremendous impact on performance, right?
/cc [~mduerig]
> Flushing the FileStore might return before data is persisted
> ------------------------------------------------------------
>
> Key: OAK-6888
> URL: https://issues.apache.org/jira/browse/OAK-6888
> Project: Jackrabbit Oak
> Issue Type: Bug
> Components: segment-tar
> Reporter: Francesco Mari
> Assignee: Francesco Mari
> Priority: Major
> Fix For: 1.8, 1.7.11
>
> Attachments: failure.txt
>
>
> The implementation of {{FileStore#flush}} might return before all the
> expected data is persisted on disk.
> The root cause of this behaviour is the implementation of
> {{TarRevisions#flush}}, which is too lenient when acquiring the lock for the
> journal file. If a background flush operation is in progress and a user calls
> {{FileStore#flush}}, that method will immediately return because the lock of
> the journal file is already owned by the background flush operation. The
> caller doesn't have the guarantee that everything committed before
> {{FileStore#flush}} is persisted to disk when the method returns.
> A fix for this problem might be to create an additional implementation of
> flush. The current implementation, needed for the background flush thread,
> will not be exposed to the users of {{FileStore}}. The new implementation of
> {{TarRevisions#flush}} should have stricter semantics and always guarantee
> that the persisted head contains everything visible to the user of
> {{FileStore}} before the flush operation was started.
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