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

[James A. Peltier]

----- Original Message -----
| ----- 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.

Oh!  One last case in point.  Partition Alignment.  This is very important to the performance of a disk subsystem.  With full disk LVM it's not an issue at all.

-- 
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

“A successful person is one who can lay a solid foundation from the bricks others have thrown at them.” -David Brinkley via Luke Shaw