Re: Default mount options (that suck less).

[David Chinner <dgc@sgi.com>]

On Mon, Oct 29, 2007 at 09:01:52AM -0500, Eric Sandeen wrote:
> David Chinner wrote:
> > On Mon, Oct 29, 2007 at 06:56:57PM +1100, Niv Sardi wrote:
> >> Hello,
> >>
> >> XFS's default mount options are in most cases sub-optimal, we should try
> > 
> > Mkfs options ;)
> > 
> >> to have more sensible defaults, so far I'm following some quick dave-powered
> >> recomendations:
> >>
> >> - version 2 logs
> >> - attr2
> >> - lazy superblock counters
> >> - less allocation groups for single disk configs
> >>
> >> - imaxpct default can be reduced
> >>
> >> it is currently 25, what would be reasonable ?
> > 
> > Given that 25% on a 4GB filesystem will allow about 5million inodes,
> > I think it's probably reasonable to bring it down to 5% by the time we
> > pass 1TB and 1% by 50TB.....
> 
> But what does this affect?  It's a cap, but it doesn't affect allocation
> policy or anything does it?  What's the downside to 25%?

It reserves allocation groups as "metadata only" for inode32 filesystems
so data doesn't get placed in them untill all other space is full. So at
1TB, we don't use 25% of the filesystem until the other 75% is full, yet
to hold 4 million inodes we only need about 1GB of space. so 5GB out of
1TB gives us space in the filesystem for ~20m million inodes by default.
AFAICT, this is the only place it is used.

Likewise, at 50TB, we're only going to be able to use 2% of the filesystem
for inodes with inode32. At 100TB, it's meaningless.....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group