Re: [Qemu-devel] [PATCH 1/4] block: add enable_write_cache flag

[Christoph Hellwig]

On Wed, Sep 02, 2009 at 08:13:52AM -0500, Anthony Liguori wrote:
> Does barrier=0 make a performance difference?  IOW, would the typical 
> default ext3 deployment show worse behavior?

I'll give it a spin.

> >A kernel defconfig compile takes between 9m40s and 9m42s with
> >data=writeback and barrieres disabled, and with fdatasync barriers
> >enabled it actually is minimally faster,
> 
> If fdatasync different than fsync on ext3?  Does it result in a full 
> metadata commit?

ext3 honors the fdatasync flag and only does it's horrible job if the
metadata we care about is dirty, that is only if some non-timestamp
data is dirty.  Which means for a non-sparse image file it does
well, while for a sparse image file needing allocations it will
cause trouble.  And now that you mention it I've only tested the
non-sparse case which now is default for the management tools at
least on Fedora.