Re: XFS reflink vs ThinLVM

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

On Fri, Jan 17, 2020 at 10:58:15PM +0100, Gionatan Danti wrote:
> Il 15-01-2020 18:45 Gionatan Danti ha scritto:
> > Let me briefly describe the expected workload: thinly provisioned
> > virtual image storage. The problem with "plain" sparse file (ie:
> > without extsize hint) is that, after some time, the underlying vdisk
> > file will be very fragmented: consecutive physical blocks will be
> > assigned to very different logical blocks, leading to sub-par
> > performance when reading back the whole file (eg: for backup purpose).
> > 
> > I can easily simulate a worst-case scenario with fio, issuing random
> > write to a pre-created sparse file. While the random writes complete
> > very fast (because they are more-or-less sequentially written inside
> > the sparse file), reading back that file will have very low
> > performance: 10 MB/s vs 600+ MB/s for a preallocated file.
> 
> I would like to share some other observation/results, which I hope can be
> useful for other peoples.
> 
> Further testing shows that "cp --reflink" an highly fragmented files is a
> relatively long operation, easily in the range of 30s or more, during which
> the guest virtual machine is basically denied any access to the underlying
> virtual disk file.

How many fragments, and how big of a sparse file?

--D

> While the number of fragments required to reach reflink time of 30+ seconds
> is very high, this would be a quite common case when using thinly
> provisioned virtual disk files. With sparse file, any write done at guest OS
> level has a very good chance to create its own fragment (ie: allocating a
> discontiguous chunk as seen by logical/physical block mapping), leading to
> very fragmented files.
> 
> So, back to main topic: reflink is an invaluable tool, to be used *with*
> (rather than instead of) thin lvm:
> - thinlvm is the right tool for taking rolling volume snapshot;
> - reflink is extremely useful for "on-demand" snapshot of key files.
> 
> Thank you all for the very detailed and useful information you provided.
> Regards.
> 
> -- 
> Danti Gionatan
> Supporto Tecnico
> Assyoma S.r.l. - www.assyoma.it
> email: g.danti@assyoma.it - info@assyoma.it
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