Re: [linux-lvm] Fast thin volume preallocation?

[Zdenek Kabelac <zkabelac@redhat.com>]

Dne 03. 06. 19 v 23:12 Ilia Zykov napsal(a):
>>
>>> 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.
>>
> 
> Hello.
> Can I use external snapshot for fast zero allocation?
>   "thpool" - is lvmthin with lvm zeroing disabled
> 
> # lvcreate -n ext2T -V 2TiB --thinpool thpool VG
> # lvchange --permission r VG/ext2T
> 
> # lvcreate -n zeroed_lve -s VG/ext2T --thinpool VG/thpool
> 
> Or it will be the same as zeroing enabled?


Hi

It's not clear what do you want to achieve.

External origin for thin-volume is always a read-only LV.

External origin can be of any type - i.e. even thinLV from another thin-pool
and single origin can be used for number of thin volumes with external origin.

For now you CANNOT 'merge' such thinLV with external origin - so the only way
to get 'writable' origin back is to drop all thins that are using it as 
external origin.

Now - how the 'zeroing' plays the role here ?

Zeroing is property of thin-pool that serves thinLV.
Unprovisioned blocks always do return 'zeroes'.

When block is provisioned - unwritten portions needs to be zeroed (depending 
on zeroing setting)  - the bigger the block the bigger the chance the zeroing 
will take more time - lvm2 does not recommend using thin-pool with zeroing and 
chunksize bigger then 512KB.

Most modern filesystem keep track of written pieces of a block device 
themselves, so zeroing has almost no practical use.

Regards

Zdenek