[linux-lvm] Why does thinpool take 2*poolmetadatasize space?

[linux-lvm] Why does thinpool take 2*poolmetadatasize space?

[Scott Moser]

When I create an lvmthinpool with size S and poolmetadatasize P,
it reduces the available freespace by S+2P. I expected that to
be S+P. Where did the extra poolmetadatasize get used?

See below for example.
before lvcreate we had 255868 free, after we had 254588.
The difference is 1280.  (1024 + 2*128).

# lvm vgcreate --metadatasize=128m myvg0 /dev/vda
  Physical volume "/dev/vda" successfully created.
  Volume group "myvg0" successfully created

# pvs --unit=m
  PV         VG    Fmt  Attr PSize      PFree
  /dev/vda   myvg0 lvm2 a--  255868.00m 255868.00m

# vgs --unit=m
  VG    #PV #LV #SN Attr   VSize      VFree
  myvg0   1   0   0 wz--n- 255868.00m 255868.00m

# lvm lvcreate --ignoremonitoring --yes --activate=y \
   --setactivationskip=n --size=1024m --poolmetadatasize=128m \
   --thinpool=mythinpool myvg0
  Thin pool volume with chunk size 64.00 KiB can address at most 15.81 TiB of da
ta.
  Logical volume "mythinpool" created.

# vgs --unit=m
  VG    #PV #LV #SN Attr   VSize      VFree
  myvg0   1   1   0 wz--n- 255868.00m 254588.00m

# lvs --all --unit=m
  LV                 VG    Attr       LSize    Pool Origin Data%
Meta%  Move Log Cpy%Sync Convert
  [lvol0_pmspare]    myvg0 ewi-------  128.00m
  mythinpool         myvg0 twi-a-tz-- 1024.00m             0.00   10.03
  [mythinpool_tdata] myvg0 Twi-ao---- 1024.00m
  [mythinpool_tmeta] myvg0 ewi-ao----  128.00m

Re: [linux-lvm] Why does thinpool take 2*poolmetadatasize space?

[Alasdair G Kergon]

On Tue, Oct 20, 2020 at 12:33:41PM -0400, Scott Moser wrote:
> When I create an lvmthinpool with size S and poolmetadatasize P,
> it reduces the available freespace by S+2P. I expected that to
> be S+P. Where did the extra poolmetadatasize get used?

So that you have sufficient disk space to store a second copy of it
if you need to perform any work on the metadata in future.

Recovery operations, for example, create a recovered copy in that
'spare' space then swap to use it leaving the space used by the
original metadata as the new 'spare'.

Alasdair

Re: [linux-lvm] Why does thinpool take 2*poolmetadatasize space?

[Zdenek Kabelac]

Dne 20. 10. 20 v 18:33 Scott Moser napsal(a):
> When I create an lvmthinpool with size S and poolmetadatasize P,
> it reduces the available freespace by S+2P. I expected that to
> be S+P. Where did the extra poolmetadatasize get used?
> 
> See below for example.
> before lvcreate we had 255868 free, after we had 254588.
> The difference is 1280.  (1024 + 2*128).
> 


lvm2 preallocated hidden _pmspare volume which has the size of
biggest metadata LV in a VG.

Such LV is used for automated 'lvconvert --repair'.

If you don't want this LV to be created you can add

--poolmetadataspare y|n

but in this case you are fully in your own to figure out,
where to take the space for repaired metadata.

You can also at any time lvremove _pmspare LV if necessary.

Regards

Zdenek