On 2/8/22 15:37, Zdenek Kabelac wrote: > Dne 08. 02. 22 v 20:00 Demi Marie Obenour napsal(a): >> Are thin volumes (which start as snapshots of a blank volume) efficient >> for building virtual machine images? Given the nature of this workload >> (writing to lots of new, possibly-small files, then copying data from >> them to a huge disk image), I expect that this will cause sharing to be >> broken many, many times, and the kernel code that breaks sharing appears >> to be rather heavyweight. Furthermore, since zeroing is enabled, this >> might cause substantial write amplification. Turning zeroing off is not >> an option for security reasons. >> >> Is there a way to determine if breaking sharing is the cause of >> performance problems? If it is, are there any better solutions? > > Hi > > Usually the smaller the thin chunks size is the smaller the problem gets. > With current released version of thin-provisioning minimal chunk size is > 64KiB. So you can't use smaller value to further reduce this impact. > > Note - even if you do a lot of tiny 4KiB writes - only the 'first' such write > into 64K area breaks sharing all following writes to same location no longer > have this penalty (also zeroing with 64K is less impactful...) > > But it's clear thin-provisioning comes with some price - so if it's not good > enough from time constrains some other solutions might need to be explored. > (i.e. caching, better hw, splitting FS into multiple partitions with > 'read-only sections,....) Are the code paths that break sharing as heavyweight as I was worried about? Would a hypothetical dm-thin2 that used dm-bio-prison-v2 be faster? -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab
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