Re: XFS reflink vs ThinLVM

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Sat, Jan 18, 2020 at 12:08:48PM +0100, Gionatan Danti wrote:
> Il 18-01-2020 00:42 Darrick J. Wong ha scritto:
> > How many fragments, and how big of a sparse file?
> 
> A just installed CentOS 8 guest using a 20 GB sparse file vdisk had about
> 2000 fragments.
> 
> After running "fio --name=test --filename=test.img --rw=randwrite --size=4G"

4GB / 1M extents == 4096, which is probably the fs blocksize :)

I wonder, do you get different results if you set an extent size hint
on the dir before running fio?

I forgot(?) to mention that if you're mostly dealing with sparse VM
images then you might as well set a extent size hint and forego delayed
allocation because it won't help you much.

--D

> for about 30 mins, it ended with over 1M fragments/extents. At that point,
> reflinking that file took over 2 mins, and unlinking it about 4 mins.
> 
> I understand fio randwrite pattern is a worst case scenario; still, I think
> the results are interesting and telling for "aged" virtual machines.
> 
> As a side note, a just installed Win2019 guest backed with an 80 GB sparse
> file had about 18000 fragments.
> Thanks.
> 
> -- 
> Danti Gionatan
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> Assyoma S.r.l. - www.assyoma.it
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