mkfs.xfs: cannot (re)set log sunit=0

[Dave Chinner]

On Mon, Jul 26, 2010 at 12:12:39PM +0200, Mario 'BitKoenig' Holbe wrote:
> Hello,
> 
> I intend to set up a XFS filesystem on a RAID0 (Linux md) with 512k
> Chunk Size. Since I cannot align the log section sunit to 512k, and
> since the filesystem will have not much write activity anyways (in fact,
> it'll be mounted read-only most of the time), I tried not to align the
> log section sunit at all via -l sunit=0, but this does not appear to
> work:

Of course - when you specific a value yourself, it has to be a valid
value and for version 2 logs that minimum value for th lsunit is one
log block.  Given that:

....
> log      =internal log           bsize=4096   blocks=2560, version=2
....
                                   ^^^^^^^^^^
Your log block size is 4k, then:

> Is this a bug or a feature?
> 
> The minimum I was able to limit the log sunit to was -l sunit=8, i.e. 1
> block.

This is the correct minimum you are allowed to specify.

> Is there a difference between sunit=0 and sunit=1 in practice or
> are single log entries aligned to blocks anyways?

sunit=0 is an invalid configuration for a version 2 log. Filesystems
without a data sunit default to a lsunit = 1 log block, so in
practice version 2 logs are always aligned/padded in some way.

> Btw...
> Since the filesystem is mostly read-only, I was also thinking about
> reducing the size of the log section. The planned filesystem will be
> 5.5T or bigger, which results in 2G log section per default. I thought
> about limiting it to 128M or 64M. There will be no concurrent write
> activity on the fs. Together with the zero-alignment, what do you think
> about that?

That'll be fine - the maximum log size until a couple of years ago
was 128MB, and I still do most of my benchmarking with a log that
size....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com