Re: [PATCH] notes on volatile write caches vs fdatasync

From: Jan Kara
Date: Thu Aug 27 2009 - 09:03:05 EST


On Thu 27-08-09 03:16:24, Christoph Hellwig wrote:
> There are two related issues when dealing with volatile write caches,
> the popular and beaten to death one are write barriers to guarantee
> write ordering and stable storage for log writes. For this post
> I assume naively this works perfectly for all filesystems supporting it.
> The second issue are plain cache flush. Yes, they happen to be the
> base for the barrier implementation on all common disks in Linux, but
> there are cases where we need to issue them even without a log barrier.
> Think about a plain write into a file that is already fully allocated.
> Or the O_DIRECT version of them same. If we do an fdatasync after these
> we really do expect the write to really be on disk, not just in the disk
> cache, right? The same is true for O_SYNC, but I ignore it for this
> write out as with Jan's patch series O_SYNC writes will be implemented
> by a range-fdatasync after the actual write, so after that this sync
> section covers it, too.
I've noticed this as well when we were tracking some problems Pavel
Machek found with his USB stick. I even wrote a patch at the time
but it somehow died out. Now, the situation should be simpler with
fsync paths cleaned up... BTW: People wanted this to be configurable per
block device which probably makes sence...

Jan Kara <jack@xxxxxxx>
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