[linux-lvm] faster snapshot creation?

[Eric Toombs]

I tried thin pools and got the same snapshot creation time, about .45s.

I also tried creating the entire vg in memory with losetup and that
didn't change the creation time either.

On 2020-02-25 04:38, Zdenek Kabelac wrote:
> Dne 22. 02. 20 v 12:58 Eric Toombs napsal(a):
>> Snapshot creation is already pretty fast:
>>
>>> $ time sudo lvcreate --size 512M --snapshot --name snap
>>> /dev/testdbs/template
>>>    Logical volume "snap" created.
>>> 0.03user 0.05system 0:00.46elapsed 18%CPU (0avgtext+0avgdata
>>> 28916maxresident)k
>>> 768inputs+9828outputs (0major+6315minor)pagefaults 0swaps
>>
>> That's about half a second in real time. But I have a scenario that
>> would benefit from it being even faster. I'm doing many small unit tests
>> starting from a template filesystem. I do the snapshot, run the unit
>> test on the snapshot, then delete the snapshot afterwards using
>> lvremove. Each unit test, though, takes much less than a second to run
>> (often on the order of 10ms), so most of the time is being spent making
>> these snapshots.
>>
>> So, is there a sort of "dumber" way of making these snapshots, maybe by
>> changing the allocation algorithm or something?
> 
> Hi
> 
> IMHO - what takes most of the time are these couple things:
> 
> Each command has 'non-trivial' time overhead on its startup (scanning
> you system with devices and validating everything)
> 
> For old snapshots -  COW are needs to be 'created' & 'zeroed' as
> separate LV.
> 
> Then you need to 'flush' all existing IO on origin device (so it's in
> the consistent states - i.e. the filesystem synchronizes all it's
> content in its metadata) - this all takes some measurable amount of time.
> 
> You can 'prepare' empty zeroed LV ahead of time and then use
> 'lvconvert' to attach snapshot (with -Zn)  - this should speed-up
> attachment
> of snapshot. For the 'second' point you could likely issue 'sync' ahead
> of time so most buffers will be flushed (if there is no big IO traffic).
> 
> Saying all this - why are you using old snapshot when you are targeting
> for performance ??
> 
> You really should consider usage of thin-pool - where you could chain a
> long series of snapshot without having a dramatic performance
> degradation of the whole IO throughput  - old snapshot are really meant
> to be used only if you want to take i.e. backup of a filesystem and you
> need some 'consistent' point in time - for everything else you should be
> using thin-pools nowdays...
> 
> Regards
> 
> Zdenek
>