[Lustre-discuss] MDS - RAID0 or RAID1

[Carlos Santana]

Hi,

What RAID level should be used for MDS configuration? The manual seems
to suggest RAID1, however, the FAQ suggests RAID0 level.
http://manual.lustre.org/manual/LustreManual16_HTML/RAID.html#50401396_pgfId-1290495
http://wiki.lustre.org/index.php/Lustre_FAQ#What_is_the_typical_MDS_node_configuration.3F

Also, is it possible to add more OSTs as the requirement grows? If
yes, can additional resources be made part of RAID or can we turn
existing non-RAID system into a RAID system? Any insights?

-
Thanks,
CS
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[Brian J. Murrell]

On Tue, 2009-06-23 at 15:14 -0500, Carlos Santana wrote:
> Hi,

Hi,

> What RAID level should be used for MDS configuration? The manual seems
> to suggest RAID1, however, the FAQ suggests RAID0 level.

These are not mutually exclusive. You can use RAID1 if you want
redundancy and RAID0 if you want to enhance performance with some
striping (although on an MDT, I'm not sure you'd get much bang for your
buck) or need to build a bigger volume than you get with a single disk.
If you use both RAID0 and RAID1, this is usually called or implemented
as RAID10.

Most people use RAID1 for their MDT. We recommend against RAID5 and
RAID6 for MDTs for performance reasons.

> Also, is it possible to add more OSTs as the requirement grows?

Yes.

> If
> yes, can additional resources be made part of RAID or can we turn
> existing non-RAID system into a RAID system?

I think this is orthogonal to the question of adding OSTs. How you
build a block device for an OST is up to you. Once you have a block
device (however you build it), it can be an OST and be added to an
existing filesystem.

b.

[Andreas Dilger]

On Jun 23, 2009 15:14 -0500, Carlos Santana wrote:
> What RAID level should be used for MDS configuration? The manual seems
> to suggest RAID1, however, the FAQ suggests RAID0 level.
> http://manual.lustre.org/manual/LustreManual16_HTML/RAID.html#50401396_pgfId-1290495
> http://wiki.lustre.org/index.php/Lustre_FAQ#What_is_the_typical_MDS_node_configuration.3F

The MDT should always use RAID-1. If you need more space than will fit
on one disk, or you need more IOPS than can be handled by a single disk,
then create sufficient RAID-1 pairs for your needs and then stripe them
with RAID-0. Having RAID-1 in pairs at the lower level minimizes the
chance that multiple disk failures will cause data loss.

I've clarified the FAQ in this regard.

> Also, is it possible to add more OSTs as the requirement grows?

Yes, adding new OSTs to an existing filesystem is a fairly common
procedure. Note if you are planning on expanding your filesystem
in this way it makes sense to configure the MDT to have enough
space/inodes for the OSTs that will be added in the future.

> If yes, can additional resources be made part of RAID or can we
> turn existing non-RAID system into a RAID system? Any insights?

I'm not sure I understand this question. OSTs should always use
RAID to avoid data loss, though RAID-5 or RAID-6 is OK for OSTs.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

[Carlos Santana]

On Tue, Jun 23, 2009 at 4:36 PM, Andreas Dilger<adi...@sun.com> wrote:
> On Jun 23, 2009  15:14 -0500, Carlos Santana wrote:
>> What RAID level should be used for MDS configuration? The manual seems
>> to suggest RAID1, however, the FAQ suggests RAID0 level.
>> http://manual.lustre.org/manual/LustreManual16_HTML/RAID.html#50401396_pgfId-1290495
>> http://wiki.lustre.org/index.php/Lustre_FAQ#What_is_the_typical_MDS_node_configuration.3F
>
> The MDT should always use RAID-1.  If you need more space than will fit
> on one disk, or you need more IOPS than can be handled by a single disk,
> then create sufficient RAID-1 pairs for your needs and then stripe them
> with RAID-0.  Having RAID-1 in pairs at the lower level minimizes the
> chance that multiple disk failures will cause data loss.
>
> I've clarified the FAQ in this regard.
>
>> Also, is it possible to add more OSTs as the requirement grows?
>
> Yes, adding new OSTs to an existing filesystem is a fairly common
> procedure.  Note if you are planning on expanding your filesystem
> in this way it makes sense to configure the MDT to have enough
> space/inodes for the OSTs that will be added in the future.
>
>> If yes, can additional resources be made part of RAID or can we
>> turn existing non-RAID system into a RAID system? Any insights?
>
> I'm not sure I understand this question.  OSTs should always use
> RAID to avoid data loss, though RAID-5 or RAID-6 is OK for OSTs.

My initial test setup does not have any RAID setup. I was planning to
further explore this into RAID setup. So I was curious abt it.
Thanks for the info..

-
CS.

[Brian J. Murrell]

On Wed, 2009-06-24 at 09:49 -0500, Carlos Santana wrote:
>
> My initial test setup does not have any RAID setup. I was planning to
> further explore this into RAID setup. So I was curious abt it.

OK. You probably know this already, but it does bear mentioning since
you are indeed testing without any RAID on your MDT... if you lose your
MDT (disk), your entire filesystem is gone.

b.