Preallocation with direct IO?

[Dave Chinner]

On Fri, Dec 30, 2011 at 08:37:00AM +0530, Amit Sahrawat wrote:
> On Fri, Dec 30, 2011 at 2:27 AM, Dave Chinner <david at fromorbit.com> wrote:
> > On Thu, Dec 29, 2011 at 01:10:49PM +0000, amit.sahrawat83 at gmail.com wrote:
> >> Hi, I am using a test setup which is doing write using multiple
> >> threads using direct IO. The buffer size which is used to write is
> >> 512KB.  After continously running this for long duration - i
> >> observe that number of extents in each file is getting
> >> huge(2K..4K..). I observed that each extent is of 512KB(aligned to
> >> write buffer size). I wish to have low number of extents(i.e,
> >> reduce fragmentation)... In case of buffered IO- preallocation
> >> works good alongwith the mount option 'allocsize'. Is there
> >> anything which can be done for Direct IO?  Please advice for
> >> reducing fragmentation with direct IO.
> >
> > Direct IO does not do any implicit preallocation. The filesystem
> > simply gets out of the way of direct IO as it is assumed you know
> > what you are doing.
> This is the supporting line I was looking for.
> >
> > i.e. you know how to use the fallocate() or ioctl(XFS_IOC_RESVSP64)
> > calls to preallocate space or to set up extent size hints to use
> > larger allocations than the IO being done during syscalls...
> I tried to make use of preallocating space using
> ioctl(XFS_IOC_RESVSP64) - but over time - this is also not working
> well with the Direct I/O.

Without knowing how you are using preallocation, I cannot comment on
this. Can you describe how your application does IO (size,
frequency, location in file, etc) and preallocation (same again), as
well as xfs_bmap -vp <file> output of fragmented files? That way I
have some idea of what your problem is and so might be able to
suggest fixes...

> Is there any call to set up extent size
> also? please update I can try to make use of that also.

`man xfsctl` and search for XFS_IOC_FSSETXATTR.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com