[linux-lvm] Thin pool performance when allocating lots of blocks

Demi Marie Obenour demi at invisiblethingslab.com
Tue Feb 8 21:02:40 UTC 2022

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

Demi Marie Obenour (she/her/hers)
Invisible Things Lab
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