Ariel Weisberg commented on CASSANDRA-14383:

There is one usage of trySync on a directory in SequentialWriter which we use 
for data we care about. There is also usage in LogReplica. There are sync calls 
that go to NativeLibrary instead of SyncUtil and we shouldn't be doing that.

Generally things are a little confused with some duplication that could be 
cleaned up. There is a helper for opening the directory and getting an FD on it 
so other code shouldn't be doing it directly.

The bigger question of what we should do when fsync generates an error I am 
still undecided on.

> If fsync fails it's always an issue and continuing execution is suspect
> -----------------------------------------------------------------------
>                 Key: CASSANDRA-14383
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14383
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Ariel Weisberg
>            Assignee: Ariel Weisberg
>            Priority: Major
>             Fix For: 2.1.x, 3.0.x, 3.11.x, 4.0.x
> We can't catch fsync errors and continue so we shouldn't have code that does 
> that in C*. There was a Postgres bug where fsync returned an error and the FS 
> lost data, but subsequent fsyncs succeeded.
> The [LastErrorException code in 
> NativeLibrary.trySync|https://github.com/apache/cassandra/commit/be313935e54be450d9aaabda7965a2f266e922c9#diff-4258621cdf765f0fea6770db5d40038fR307]
>  looks a little janky. What's up with that? When would trySync be something 
> we would merely try? If try is good enough why do it at all considering try 
> is the default behavior of a series of unsynced filesystem operations.
> -Also when we fsync in FD it's not just fsyncing that file the FS is 
> potentially fsyncing other data and the error code we get could be related to 
> that other data so we can't safely ignore it. The filesystem could be 
> internally inconsistent as well. This happens because the FS journaling may 
> force the FS to flush other data as well to preserve the ordering 
> requirements of journaled metadata.- I'm actually not 100% sure when/if this 
> is the case.
> If we ignore fsync errors it needs to be for whitelisted reasons such as a 
> bad FD.
> I know we have FSErrorHandler and it makes sense for reads, but I'm not sold 
> on it being the right answer for writes. We don't retry flushing a memtable 
> or writing to the commit log to my knowledge. We could go read only and I 
> need to check if that is what we do in practice.

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