xfs mount/create options (was: XFS status update for August 2010)

[Michael Monnerie]

On Mittwoch, 8. September 2010 Dave Chinner wrote:
> >  On machines with 32MiB or more 32k is the default, but most
> >  machines these days have multi-gigabytes of RAM, so at least for
> >  RAM>1GiB that could be made default.
> 
> That is definitely not true. XFS is widely used in the embedded NAS
> space, where memory is very limited and might be configured with
> many filesystems.  32k is the default because those sorts of machines
> can't afford to burn 2MB RAM per filesystem just in log buffers.
>
> Also, you can go and search the archives or git history as to why we
> don't tune the logbsize based on physical memory size anymore, too.

OK, then the man page should be updated to reflect this "newer logic". 
I've got the information directly from there.
 
> You're getting the wrong information there. largeio affects the
> output of the optimal IO size reported by stat(2). 'stat -f" does
> a statfs(2) call. Try 'stat /disk/db/<file> --format %o'....

Ah, that's better, thank you :-)
 
> >  And while I am at it: Why does "mount" not provide the su=/sw=
> >   options that we can use to create a filesystem? Would make life
> >   easier, as it's much easier to read su=64k,sw=7 than
> >   sunit=128,swidth=896.
> 
> You should never, ever need to use the mount options.

..except when a disk is added to the RAID, or it's RAID level gets 
changed. Then sw=7 becomes sw=8 or so - or better said: would become, as 
then you must use the (I call it strange, error prone) semantics of 
sunit/swidth.
 
> >  When I defined su/sw on mkfs, is it enough, or would I always have
> > to specify sunit/swidth with every mount too?
> 
> Yes, no. mkfs.xfs stores sunit/swidth on disk in the superblock.

So when I add a disk, I must only once mount with the new sunit/swidth, 
and that is stored? That's nice.

-- 
mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

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