Re: XFS reflink vs ThinLVM

[Gionatan Danti <g.danti@assyoma.it>]

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.

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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