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https://issues.apache.org/jira/browse/KUDU-2102?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Adar Dembo resolved KUDU-2102.
------------------------------
Resolution: Fixed
Fix Version/s: 1.5.0
Fixed in commit 9a07b2fedcc0beacc76cb1c656be84c0c99c9da9.
> PosixRWFile::Sync doesn't guarantee durability when used with multiple threads
> ------------------------------------------------------------------------------
>
> Key: KUDU-2102
> URL: https://issues.apache.org/jira/browse/KUDU-2102
> Project: Kudu
> Issue Type: Bug
> Components: util
> Affects Versions: 1.5.0
> Reporter: Adar Dembo
> Assignee: Hao Hao
> Fix For: 1.5.0
>
>
> PosixRWFile uses an AtomicBool to "optimize" calls to Sync(). It works as
> follows:
> # The bool starts as false.
> # It is set to true in WriteV().
> # In Sync(), we CAS it from true to false. If the CAS succeeds, we actually
> do the fsync().
> The idea is that if two threads call Sync() at the same time, only one will
> actually do the fsync(). However, there's a problem here: the "losing" thread
> returns from Sync() early and operates under the assumption that the file's
> data has been made durable, even though it is still in the process of being
> synchronized to disk.
> We have two options:
> # Preserve the optimization but fix it so that the losing thread(s) wait for
> the "winning" thread to finish the fsync. This can be done with some more
> synchronization primitives (off the top of my head: a lock, a condition
> variable, and another boolean).
> # Remove the optimization and let the losing thread(s) perform additional
> fsyncs.
> To measure the effect of the optimization, I wrote a test program that opens
> a file and fsyncs it 1000 times. I ran it on an el6.6 box, on spinning disks
> mounted as xfs and ext4, and on files that were empty and 10G in size
> (dropping caches first). I measured the cost of an fsync to be around 200
> microseconds, suggesting that no I/O is being performed and that the overhead
> is purely syscall-related.
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