[linux-lvm] Sparse LVs, --virtualsize equal to --size

Marian Csontos mcsontos at redhat.com
Mon Jan 28 13:50:48 UTC 2013

On 01/25/2013 09:44 AM, Vangelis Koukis wrote:
> On Thu, Jan 24, 2013 at 11:42:35pm +0000, Alasdair G Kergon wrote:
>> So look at thin provisioning with its zeroing option.
>> External origin.  (support currently being added to lvm)
>> Or this not-yet-upstream target:
>> http://people.redhat.com/agk/patches/linux/editing/dm-add-zeroed-target.patch
>> Alasdair
> Thanks Alasdair,
> this seems to fit the bill perfectly, it's a shame it's
> not yet merged upstream.
> Until then, if we are to go with the "snapshot-over-the-zero-target"
> route, can you comment on quantifying the space overhead of tracking
> chunks in the snapshot?

Beware! Large old-style snapshots may take a very long time to 
activate[1] (reportedly up to few hours) and my guess is many smaller 
snapshots will behave the same[2], the total amount of chunks written to 
all snapshots being the key to slow start...

You should definitely try it with your workflow before going too far 
that direction.

[1]Just search for 'lvm large snapshot'. Someone here may be able to 
point you to a more complete data.

[2] Not even trying to think about many larger ones as your usecase 
suggests: just update kernel and java few times and with journal FS you 
will soon be at few gigabytes per VM.

-- Marian

> Thanks again,
> Vangelis.
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