Re: howto preallocate to minimize fragmentation

[Eric Sandeen <sandeen@xxxxxxx>]

Ying-Hung Chen wrote:
> > pre-allocation before writing would still be your best bet.  If you
> > pre-allocate on a fresh fs before writing, you should get very large
> > extents.
>
> does this mean, if I create a 2GB file via dd (not sparse file), when i
> 'overwrite' to the same file, it will stay there? (same physical place)
Depends on if you truncate it before you re-write to it, I think.

But don't use dd - use xfs's preallocation calls, it will be MUCH more 
efficient.

> > Other things you could try; if you put each file in its own dir, it will
> > tend to go into its own allocation group.
> >
> > You could make the filesystem with allocation groups sized at 2GB
>
> I just thought of wild idea... since i am creating 90 files, what if i
> just create 90 allocation group via -d agcount=90, does this make sense
> or it won't work at all?
well, give it a shot.

But I'd try the preallocation calls first - that's what they are for.

-Eric