[Lustre-discuss] mkfs.lustre stripe-count-hint

Brian J. Murrell Brian.Murrell at Sun.COM
Mon Nov 16 13:26:21 PST 2009


On Mon, 2009-11-16 at 21:03 +0000, Geoff Lustre wrote: 
> 
> My understanding of this parameter is:
> 
> 
> It provides the mkfs.lustre command with a hint about the expected
> level of striping.

Yes, indeed.

> The larger the (expected) stripe, the larger the resulting inodes.

To a limit, yes.  See bug 7240/7241.

> This allows (more) EAs to be embedded in the inode, allowing one read
> to bring the inode including EAs into memory.

Yes, that's the theory.

> Small inodes have the EAs located at the other end of a pointer,
> casing two seeks to bring the EAs into memory.

Correct.

> So much for the theory.  With Lustre 1.8.1 the output from the
> mkfs.lustre  with no stripe-count-hint and the output with
> stripe-count-hint =160 is the same.

That's right.  Because 160 is at the other end of the scale.

> Is stripe-count-hint ignored, as appears to be the case?

No.  The algorithm is as such:

if the stripe-count hint > 72, then 512 byte inode
if the stripe-count hint > 32, then 2048 byte inode
if the stripe-count hint > 10, then 1024 byte inode
otherwise, 1024 byte inode

Where the last match wins.

> It is possible to use --mkfsoptions="-I 4096 -i 4608" to force the
> creation of 4k inodes.

Yes.

> (This is the largest permissible size).

Correct.

> Please would someone confirm (or deny) the larger inode size has the
> effect of allowing lustre to handle inodes for heavily striped files
> more efficiently?

It does, with a cost as outlined by bug 7240/7241.

b.


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