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@xxxxxxxxxx - info@xxxxxxxxxx > GPG public key ID: FF5F32A8
