Re: [PATCH 04/16] xfs: dynamic speculative EOF preallocation

Dave Chinner
2010-11-09T00:06:42Z

On Mon, Nov 08, 2010 at 06:43:25AM -0500, Christoph Hellwig wrote:
> > For default settings, ???e size and the initial extents is determined
>
> weird character.
>
> > The allocsize mount option still controls the minimum preallocation size, so
> > the smallest extent size can stil be bound in situations where this behaviour
> > is not sufficient.
>
> Do we also need a way to keep an upper boundary? Think lots of slowly
> growing log files on a filesystem not having tons of free space.

Perhaps - it's one of the things I've been debating backwards and
forwards and done nothing about yet.

It's hard to trim back preallocation before we hit ENOSPC via a
static threshold (e.g. 1% free space could be terabytes of space), but
once ENOSPC is hit we drop new preallocation completely. Perhaps a
gradual decrease in the maximum prealloc size based on freespace
remaining? e.g.

freespace max prealloc size
>5% full extent (8GB)
4-5% 2GB (8GB >> 2)
3-4% 1GB (8GB >> 3)
2-3% 512MB (8GB >> 4)
1-2% 256MB (8GB >> 5)
<1% 128MB (8GB >> 6)

I'm open to other ideas on what to do here.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com