CentOS - Jul 2007 - Seeking pointers to info about LVM migration and performance

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

Thanks in advance for directions.

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.
umount /dev/sdb1
tunefs -j /dev/sdb1
vi /etc/fstab    (and, change ext2 to ext3 on the mount line for this 
filesystem)
mount /dev/sdb1


and voila, its EXT3 now, with journalling.


> 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?
> - What's involved in migrating our filesystem from ext2 to LVM with a
> minimum of downtime?
>
> Thanks in advance for directions.
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> http://lists.centos.org/mailman/listinfo/centos
>

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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On 7/7/07, Johnny Hughes <mailing-lists at hughesjr.com>
wrote:
> Bart Schaefer wrote:
> > - 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.
Thanks for confirming that, though in this case it's a hardware RAID
so I wouldn't expect LVM to need to treat it as anything other than
big disk.
> If you have 2 or more drives that are the same in the machine, you can
> "Stripe" your LVM.
Could you clarify "that are the same"?  The primary point of this
excercise is to make the filesystem resizable.  It's already
impossible to buy exactly the configuration of new hardware that's
already attached; we wouldn't want to be limited to adding identical
extents every time we expanded the storage.
> LVM done this way is quite fast ... but obviously you would need to
> provide another way to protect your data.
Presumably using hardware RAIDs as the physical volumes would take care of that.

Thanks for the information.