[linux-lvm] exposing snapshot block device

[Tomas Dalebjörk]

And the block size for thick snapshots can be set when using the lvcreate
command.
And the automatic growing of a snapshot can be configured too in the lvm
configuration.

Same issues with both thin and thick, if you run out of space.
//T

Den ons 23 okt. 2019 kl 13:24 skrev Tomas Dalebjörk <
tomas.dalebjork at gmail.com>:

> I have tested FusionIO together with old thick snapshots.
> I created the thick snapshot on a separate old traditional SATA drive,
> just to check if that could be used as a snapshot target for high
> performance disks; like a Fusion IO card.
> For those who doesn't know about FusionIO; they can deal with 150-250,000
> IOPS.
>
> And to be honest, I couldn't bottle neck the SATA disk I used as a thick
> snapshot target.
> The reason for why is simple:
> - thick snapshots uses sequential write techniques
>
> If I would have been using thin snapshots, than the writes would most
> likely be more randomized on disk, which would have required more spindles
> to coop with this.
>
> Anyhow;
> I am still eager to hear how to use an external device to import snapshots.
> And when I say "import"; I am not talking about copyback, more to use to
> read data from.
>
> Regards Tomas
>
> Den ons 23 okt. 2019 kl 13:08 skrev Gionatan Danti <g.danti at assyoma.it>:
>
>> On 23/10/19 12:46, Zdenek Kabelac wrote:
>> > Just few 'comments' - it's not really comparable - the efficiency of
>> > thin-pool metadata outperforms old snapshot in BIG way (there is no
>> > point to talk about snapshots that takes just couple of MiB)
>>
>> Yes, this matches my experience.
>>
>> > There is also BIG difference about the usage of old snapshot origin and
>> > snapshot.
>> >
>> > COW of old snapshot effectively cuts performance 1/2 if you write to
>> > origin.
>>
>> If used without non-volatile RAID controller, 1/2 is generous - I
>> measured performance as low as 1/5 (with fat snapshot).
>>
>> Talking about thin snapshot, an obvious performance optimization which
>> seems to not be implemented is to skip reading source data when
>> overwriting in larger-than-chunksize blocks.
>>
>> For example, consider a completely filled 64k chunk thin volume (with
>> thinpool having ample free space). Snapshotting it and writing a 4k
>> block on origin will obviously cause a read of the original 64k chunk,
>> an in-memory change of the 4k block and a write of the entire modified
>> 64k block to a new location. But writing, say, a 1 MB block should *not*
>> cause the same read on source: after all, the read data will be
>> immediately discarded, overwritten by the changed 1 MB block.
>>
>> However, my testing shows that source chunks are always read, even when
>> completely overwritten.
>>
>> Am I missing something?
>>
>> --
>> Danti Gionatan
>> Supporto Tecnico
>> Assyoma S.r.l. - www.assyoma.it
>> email: g.danti at assyoma.it - info at assyoma.it
>> GPG public key ID: FF5F32A8
>>
>
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