[linux-lvm] Fast thin volume preallocation?

[Gionatan Danti]

Il 03-06-2019 15:23 Joe Thornber ha scritto:
> On Fri, May 31, 2019 at 03:13:41PM +0200, Gionatan Danti wrote:
> 
>> - does standard lvmthin support something similar? If not, how do you 
>> see a
>> zero coalesce/compression/trim/whatever feature?
> 
> There isn't such a feature as yet.

Ok, so the NAS vendor did something custom/proprietary. This is a reason 
why I do *not* like such devices: how much testing and verification are 
done before and after such customization? Not a lot, I would say.

> With the next iteration of thin I'd like to
> get away from the fixed allocation block sizes that we're using which
> should greatly
> reduce the number of mappings that we need to create to provision a
> thick volume and
> so speed it up a lot.

Interesting. Do you plan to have an dynamic chunk size bound to some 
minumum/maximum? If so, how do you plan to tackle situations where a 
single big chunk should be split due to a small CoW write happening 
inside the big chunk? Example: a 128MB chunk-sized thin LV which is 
snapshotted and a small 4K writes happening at offset 32M from chunk 
start.

> Presumably you want a thick volume but inside a thin pool so that you
> can used snapshots?
> If so have you considered the 'external snapshot' feature?

Yes, in some cases they are quite useful. Still, a fast volume 
allocation can be an handy addition.

-- 
Danti Gionatan
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