[Lustre-discuss] MDT raid parameters, multiple MGSes

Cliff White cliffw at whamcloud.com
Fri Jan 21 12:31:51 PST 2011

On Fri, Jan 21, 2011 at 3:43 AM, Thomas Roth <t.roth at gsi.de> wrote:

> Hi all,
> we have gotten new MDS hardware, and I've got two questions:
> What are the recommendations for the RAID configuration and formatting
> options?
> I was following the recent discussion about these aspects on an OST:
> chunk size, strip size, stride-size, stripe-width etc. in the light of
> the 1MB chunks of Lustre ... So what about the MDT? I will have a RAID
> 10 that consists of 11 RAID-1 pairs striped over. giving me roughly 3TB
> of space. What would be the correct value for <insert your favorite
> term>, the amount of data written to one disk before proceeding to the
> next disk?

The MDS does very small random IO - inodes and directories.  Afaik, the
largest chunk
of data read/written would be 4.5K -and you would see that only with large
OST stripe
counts.   RAID 10 is fine. You will not
be doing IO that spans more than one spindle, so I'm not sure if there's a
real need to tune here.
Also, the size of the data on the MDS is determined by the number of files
in the
filesystem (~4k per file is good)
unless you are buried in petabytes 3TB is likely way oversize for an MDT.

> Secondly, it is not yet decided whether we wouldn't use this hardware to
> set up a second Lustre cluster. The manual recommends to have only one
> MGS per site, but doesn't elaborate: what would be the drawback of
> having two MGSes, two different network addresses the clients have to
> connect to to mount the Lustres?
> I know that it didn't work in Lustre 1.6.3 ;-) and there are no apparent
> issues when connecting a Lustre client to a test cluster now (version
> 1.8.4), but what about production?
> Cheers,
> Thomas
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