[Lustre-discuss] Maximum OST Size
Bernd Schubert
bernd.schubert at fastmail.fm
Wed Oct 20 10:00:16 PDT 2010
On Wednesday, October 20, 2010, Andreas Dilger wrote:
> On 2010-10-19, at 08:27, Roger Spellman wrote:
> > I don't understand this comment:
> >> For the MDT, yes, you could potentially use "-i 1500" as about the
> >> minimum space per inode, but then you risk running out of space in the
> >> filesystem before running out of inodes.
> >
> > If we set -I to 512, then on an MDT, what else is there that would cause
> > require 1500 bytes per inode?
>
> With "-I 512" that means the actual inode will consume 512 bytes, so with
> "-i 1536" there would be 1024 bytes per inode of block space still
> available. That extra space is needed for everything else in the
> filesystem, including the journal, directory blocks, Lustre metadata
> (last_rcvd, distributed transaction logs, etc), and any external xattr
> blocks for widely-striped files (beyond 12 stripes or so).
>
I have to admit, I entirely fail to understand why we should need 2/3 of the
filesystem reserved for real file data.
- journal - 400MB -> negligible with recent decent MDT sizes (1TiB+)
- directory blocks, maybe, but I have noticed any system where that takes more
than 5%
- Lustre metadata > (last_rcvd, distributed transaction logs, etc) ->
negligible with recent decent MDT sizes
- external xattr for Lustre lov and additional ACLs: Maybe, depends on the
customer
With the default -i 4096, it looks like that for most customers I know of:
df -h:
973G 57G 861G 7% /lustre/lustre/mdt
df -ih:
278M 248M 31M 89% /lustre/lustre/mdt
So doubling inode ratio to -i2048 or even quadrupling it to -i1024 seems to be
recommendable.
Cheers,
Bernd
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