[CentOS] to lvm or not to lvm - why/when to use lvm

[James A. Peltier]

----- Original Message -----
| Hi,
| 
| I was wondering, why/when it is useful or when should I avoid to use
| LVM.
| 
| I think the big advantage of LVMing is if you modify (rezising, ...)
| disk and filesystem layouts "a lot".
| 
| Are there any real pros or cons for following situations regarding
| e.g.
| management and speed?

The speed at which you can manage your disk environment through the use of LVM makes most of the tradeoffs worth while.  Of course, YMMV so you're best to test.

| e.g.:
| 
| I do have a server system raid for which the disk layout will not
| change; e.g. /var /usr /home will not change much in size.

This isn't so much the issue.  What if *any* partition requirements *do* change in the future.  LVM can account for that my allowing you flexibility to make a change should it be required.  Standard partitioning is less flexible in this regard.
 
| OR
| 
| I do have some file storage shares (iscsi raids) up to some TB each
| on
| one big storage device.
| 
| Sometimes (e.g. after a server crash) it is useful to remount the
| storage to a different server.

Standard caveats apply.  If the Volume Groups or the Logical Volumes are named the same moving them to another system with similar VGs or LVs can be problematic.  Same goes for file system labels, albeit both are relatively easy to fix in such a scenario.

| Should I use LVM on the iscsi storage volumes?

I would find it difficult to find a case where LVM shouldn't be used because of it's flexibility.  I tend to use full disk LVM (no partitions at all) and file system labels for mounting and the like (labels match LVs).

lvcreate -L 20G -n csgrad DATA
mkfs.xfs -L csgrad /dev/DATA/csgrad

/etc/fstab
----------

LABEL=csgrad /exports/csgrad xfs defaults 0 0


LVM offers other additional flexibility too in that you can migrate PVs from one device to another online.  So if you have one iSCSI server that is coming off support and you are replacing it with another, you can use pvmove to move the data from one target to another.

-- 
James A. Peltier
Manager, IT Services - Research Computing Group
Simon Fraser University - Burnaby Campus
Phone   : 778-782-6573
Fax     : 778-782-3045
E-Mail  : jpeltier at sfu.ca
Website : http://www.sfu.ca/itservices

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