Re: [linux-lvm] about the lying nature of thin

[Gionatan Danti <g.danti@assyoma.it>]

On 29/04/2016 10:44, Marek Podmaka wrote:
> Hello Xen,
> Now I'm not sure what your use-case for thin pools is.
>
> I don't see it much useful if the presented space is smaller than
> available physical space. In that case I can just use plain LVM with
> PV/VG/LV. For snaphosts you don't care much as if the snapshot
> overfills, it just becomes invalid, but won't influence the original
> LV.
>

Let me add one important use case: have fast, flexible snapshots.

In the past I used classic LVM to build our virtualization servers, but 
this means I was basically forced to use a separate volume for each VM: 
using a single big volume and filesystem for all the VMs means that, 
while snapshotting it for backup purpose, I/O become VERY slow on ALL 
virtual machines.

On the other hand, thin pools provide much faster snapshots. On the 
latest builds, I begin using a single large thin volume, on top of a 
single large thin pool, to host a single filesystem that can be 
snapshotted with no big slowdown on the I/O part.

I understand that it is a tradeoff - classic LVM mostly provides 
contiguous blocks, so fragmentation remain quite low, while thin 
pools/volumes are much more prone to fragament, but with large enough 
chunks it is not such a big problem.

Regards.

-- 
Danti Gionatan
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