[linux-lvm] Snapshot behavior on classic LVM vs ThinLVM

Gionatan Danti g.danti at assyoma.it
Fri Apr 7 16:33:47 UTC 2017

Il 07-04-2017 15:50 L A Walsh ha scritto:
> Gionatan Danti wrote:
>> I more concerned about lenghtly snapshot activation due to a big, 
>> linear CoW table that must be read completely...
> ---
>    What is 'big'?  Are you just worried about the IO time?
> If that's the case, much will depend on your HW.  Are we talking
> using 8T hard disks concatenated into a single volume, or in a
> RAID1, or what?  W/a HW-RAID10 getting over 1GB/s isn't
> difficult for a contiguous read.  So how big is the CoW table
> and how fragmented is it?  Even w/fragments, with enough spindles
> you could still, likely, get enough I/O Ops where I/O speed shouldn't
> be a critical bottleneck...

For the logical volume itself, I target a 8+ TB size. However, what 
worries me is *not* LV size by itself (I know that LVM can be used on 
volume much bigger than that), rather the snapshot CoW table. In short, 
from reading this list and from first-hand testing, big snapshots (20+ 
GB) require lenghtly activation, due to inefficiency in how classic 
metadata (ie: non thinly-provided) are layed out/used. However, I read 
that this was somewhat addressed lately. Do you have any insight?

Danti Gionatan
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Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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