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Re: Adding mirror copy to LVM Logical Volume uses 100% of allocated SAN storage

 
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Occasional Contributor

Adding mirror copy to LVM Logical Volume uses 100% of allocated SAN storage

Hello All,

I was hoping someone could tell me if the following behaviour is normal, and if there is a way to alter it.  For reference my testing has been on a BL860 i2 runngin 11iv3, using Netapp storage as the backend.  I'm relativley new to the whole LVM world, so please go easy!

When creating a new logical volume, the extents seem to be allocated thinly, i.e. no space is actually consumed on the SAN.  This is ideal.

When creating a new logical volume with the -m 1 option, extents seem to be allocated thinly on both PVs, again no consumption is seen on either SAN presenting luns.  Again ideal.

If the logical volume created in example 1 (or any other existing lv) has a mirror created with lvextend -m 1, all of the thinly allocated unused extents are sync'd to the new pv, consuming space on the second san, equal to the si\eo f the lv.  This seems rather wasteful, leaves consumed space unbalanced across the two sans, and when san side snapshots come into play, more space will be consumed protecting 'nothing'.  Is there anyway to change this behaviour?

If more info is required, please ask.

Thanks

1 REPLY 1
Honored Contributor

Re: Adding mirror copy to LVM Logical Volume uses 100% of allocated SAN storage

>> When creating a new logical volume, the extents seem to be allocated thinly, i.e. no space is actually consumed on the SAN.  This is ideal.

That is not normal and 'thinly' is never ideal. Logical volumes will use the space assigned by lvcreate. If you also add a filesystem with newfs, then directory structures are added in selected sectors. Mirroring performs the same operation so the lvol will use 2 PVs.

It will be helpful if you copy/paste the commands and results you used. Specifically, lvcreate,  lvextend, and pvdisplay -v.

If this NetApp has been configured for thin provisioning, ask your storage admin to turn this 'feature' off. It should never be used in a production environment.



Bill Hassell, sysadmin