Linux – How to find the physical volume(s) that hold a logical volume in LVM

It is also possible to build a mirror first using another single PV, and split of the striped volume afterwards. This requires a free PV but then again I assume pvmove needs this as well.

If you have a volume lvsplit using PV's sda1 and sdb1 for example, and sdc1 is a (temporary) PV with enough free extents, you can do this:

lvconvert -m 1 /dev/volumegroupname/lvsplit

This will create a mirror from your striped volume, using sdc1 to build the mirror

...let the mirror build...

lvconvert -m 0 /dev/volumegroupname/lvsplit /dev/sda1 /dev/sdb1

This will convert the mirror back to a single volume taking out the extents on PV /dev/sda1 and /dev/sdb1, leaving sdc1 as the only PV for your now lineair LV.

You can then pvmove from sdc1 to another PV, or use the mirror technique instead of pvmove to migrate back to sda1 or sdb1.

It seems this is not possible from the commandline. striping is quite different from the default linear allocation scheme. So while you can grow a file system across multiple physical volumes (PV), you do not get the "load balancing" advantages of striping.

To grow the filesystem, you would do something like this:

pvcreate /dev/mapper/md2_crypt
vgextend crypto /dev/mapper/md2_crypt
umount /dev/mapper/crypto-srv
fsck.ext4 -C0 -f /dev/mapper/crypto-srv
lvresize -l +100%FREE /dev/mapper/crypto-srv
resize2fs /dev/mapper/crypto-srv
mount /srv

In the above example, md2_crypt is a crypto volume on top of a RAID-1 array, but it could be any device. crypto is the volume group (VG) and crypto-srv is the logical volume that holds the filesystem mounted on /srv.

Reminder: you should really use at least a RAID-1 array because if a PV fails in a linear logical volume (LV), you can loose all the data.

This will not make the /srv partition striped across the PVs. This is because there are no userland tools to support linear to striped conversions. Logically, this would be in the lvconvert command, but that only supports moving from linear to mirror mode and back, for example. And with all the new development going into BTRFS, I would be surprised if this ever documented.

One thing you can do however, is have the new data on the drive be striped. That is rather tricky, but it can be done. Let's assume we want to stripe crypto-tmp, the LV for /tmp. We first need to free up space for the stripes in the first PV:

pvmove -n crypto/tmp /dev/mapper/md1_crypt

This will reallocate the LV across the PVs, in my case it moved it all to the second PV:

root@foo:~# pvs --segments -o+lv_name,seg_start_pe,segtype
  PV                    VG     Fmt  Attr PSize PFree  Start SSize  LV   Start Type
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 10.00g     0   2384 root     0 linear
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 10.00g  2384    256 swap     0 linear
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 10.00g  2640   2560          0 free
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 10.00g  5200 709901 srv      0 linear
  /dev/mapper/md2_crypt crypto lvm2 a--  1.82t  1.81t     0   2560 tmp      0 linear
  /dev/mapper/md2_crypt crypto lvm2 a--  1.82t  1.81t  2560 474077          0 free

Now you can grow the filesystem (say we double its size) and tell LVM to stripe the new data:

root@foo:~# lvresize -i 2 -l 5120 /dev/crypto/tmp
  Using default stripesize 64.00 KiB
  Extending logical volume tmp to 20.00 GiB
  Logical volume tmp successfully resized

Now the new data for /tmp is striped across the two PVs:

root@foo:~# pvs --segments -o+lv_name,seg_start_pe,segtype
  PV                    VG     Fmt  Attr PSize PFree Start SSize  LV   Start Type
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 5.00g     0   2384 root     0 linear
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 5.00g  2384    256 swap     0 linear
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 5.00g  2640   1280 tmp   2560 striped
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 5.00g  3920   1280          0 free
  /dev/mapper/md1_crypt crypto lvm2 a--  2.73t 5.00g  5200 709901 srv      0 linear
  /dev/mapper/md2_crypt crypto lvm2 a--  1.82t 1.80t     0   2560 tmp      0 linear
  /dev/mapper/md2_crypt crypto lvm2 a--  1.82t 1.80t  2560   1280 tmp   2560 striped
  /dev/mapper/md2_crypt crypto lvm2 a--  1.82t 1.80t  3840 472797          0 free

But the original data is still in linear mode. It may be possible to move the filesystem to the striped extents, but consider that an exercise to the reader at this point. :)

See also Growing a LVM volume with striping and Redhat LVM changing stripes?.

Also note that the reverse (converting striped to linear) is possible, however: Is it possible to convert striped logical volume to linear logical volume?.