Re: howto preallocate to minimize fragmentation

[Eric Sandeen <sandeen@xxxxxxx>]

Ying-Hung Chen wrote:
> > See the xfsctl man page from xfsprogs, specifically XFS_IOC_RESVSP
> >
> > creating a sparse file will not help with fragmentation, unfortunately.
>
>
> Hello again,
>
> Thanks for the info, here are more specifics on what we are trying to do:
>
> we have 200GB of harddive and would like to create 90-95 of 'video'
> file, each with 2GBs (data will be writting to each file continuesly,
> just like a tape). and we are hoping that if can write to the same
> physical place all the time, there won't be any fragmentation problem....
>
> we are wondering if there are any xfs parameters we can do (e.g. volumes
> groups?) for the above to 'gaurantee' the best layout since we are not
> going to create/delete files once created all those Video files, all we
> are doing are overwriting files continuesly.

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.

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

-Eric