[linux-lvm] Time needed for take a snapshot

[Zdenek Kabelac]

Dne 20. 01. 20 v 11:36 Gionatan Danti napsal(a):
> Il 20-01-2020 10:22 Zdenek Kabelac ha scritto:
>> So having  thousands of LVs in a single VG will become probably your 
>> bottleneck.
> 
> Hi Zdenek, I was thinking more about having few LVs, but with different amount 
> of data/mapping.
> 
> For example, is a very fragmented volume (ie: one written randomically) 
> significantly slower to snapshot than an almost empty volume? I fully expect 
> some small difference; however, if an empty volume take 0.2s and a fragmented 
> one 20s, this would surely be significant.
> 
> Note that I never had such a slow snapshot, rather, even on aged and big 
> volumes, it always take <1s. However, other experiences are welcome.

Yep - kernel metadata 'per thin LV' are reasonably small - so even for big 
thin devices it still should fit within your time boundaries.
(effectively thin snapshot just increases 'mapping' sharing between origin and 
its snapshot - so the time needed depends on how many bTree nodes needs to be 
updated - so if you would manage to create heavily fragmented multi TiB 
thinLV, the time depends on speed of you metadata device - as long as device 
is fast (i.e. >= SSD) - operation should be quick.
But ATM there is no scientific proof for the worst case scenario.

Regards

Zdenek