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

[Jan Kara]

  Hi,

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
  http://osdir.com/ml/linux-ext4/2009-01/msg00015.html
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...

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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