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

Johnny Hughes johnny at centos.org
Sun Jul 8 19:06:41 UTC 2007


Bart Schaefer wrote:
> 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.
>

Well ... if you are using hardware RAID, then striping LVM will likely
not give you any benefit speed wise.  (Unless multiple controllers are
used and read/write can be done in parallel).

The speed benefit happens if LVM can stripe the sectors to SATA or SCSI
drives where simultaneous read/writes can happen.  With hardware RAID,
the controller normally handles all that, so LVM striping is not
normally done.

Even w/o the speed benefit of striping, you can still use LVM on top of
hardware RAID to be able to increase drive size ... (add more physical
drives to your RAID controller, then use pvresize to extend the PV, then
use vgextend to extend the VG and lvextend to extend the LVs .. then
ext2online (CentOS-4) or resize2fs (CentOS-5) can be used to grow the
ext3 file system.


>> 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.
> 
Correct.

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