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Re: LVM negates benefits of jounaling filesystems? [was RFE: autofsck]



Alan Cox wrote:
> On Tue, Jun 10, 2008 at 09:05:34AM -0500, Eric Sandeen wrote:
>> Barriers do have performance impact, which is probably historically why
>> they're off by default on ext3.  I'm not totally convinced that it's a
> 
> The cost is dropping rapidly with SATA devices. With a modern SATA disk and
> NCQ we really really want them on - you've got 31 commands in flight and 16MB
> or so of drive cache that will be written back in *arbitary* order. Thats
> a recipe for disaster. A few years ago with one command at a time wind up
> disks and at most 2MB it wasn't so bad. It really should be getting set on by
> either our kernel or the installer in the SATA cases.
> 
> Alan
> 

I think the problem is, barriers are really implemented as drive cache
flushes.  So under certain workloads the performance really does hurt.
But if the alternative is a good chance of filesystem corruption on
power loss, remind me again why we run a journaling fileystem at all?  :)

If ext3 doesn't get barriers on by default upstream then I would suggest
that yes, we should patch the kernel ourselves to do so, or set the
installer to create fstabs which specify it for filesystems that don't
have barriers on by default.  ... after lvm stops filtering it out, anyway.

-Eric



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