[CentOS] Seeking pointers to info about LVM migration and performance

[Johnny Hughes]

Bart Schaefer wrote:
> We have a large MySQL database currently running on a CentOS 3.x
> server with an external SCSI RAID device.  This is currently an ext2
> filesystem because it was migrated once previously from an even older
> RedHat system.  We need to add storage capacity to this database and
> prepare for additional future growth.  One of the options under
> consideration is to move to CentOS 5 and use LVM.
> 
> Of some concern is that we run some pretty heavy-duty queries against
> this database in close to real time, so we don't want the switch to
> scalable storage to have a significant impact on read latency.
> 
> Consequently I'm looking for sources of information on two topics:
> - What kind of performance can we expect from an LVM group as compared
> to mounting the RAID array directly?

OK, the answer to this question is ...

RAID and LVM can be used together, or individually.

If you have 2 or more drives that are the same in the machine, you can
"Stripe" your LVM.  When doing this it behaves similarly to RAID0
(striping with no CRC / Redundancy).

LVM done this way is quite fast ... but obviously you would need to
provide another way to protect your data.

See this article for how to use LVM striping:

http://www.redhat.com/magazine/009jul05/features/lvm2/

http://www.tldp.org/HOWTO/html_single/LVM-HOWTO/#recipethreescsistripe

Obviously, LVM with striping is faster than a normal LVM setup.

> - What's involved in migrating our filesystem from ext2 to LVM with a
> minimum of downtime?



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