XFS Preallocate using ALLOCSP

[Smit Shah]

On Mon, Jun 15, 2009 at 8:34 PM, Eric Sandeen <sandeen at sandeen.net> wrote:

> Smit Shah wrote:
> > Hi,
> >
> > I wanted to preallocate the file using the ALLOCSP so that the
> preallocated
> > space is zeroed out. Hence i used the xfsctl but the problem is that i
> > cannot execute  it as a non-root user.  So i tried using fallocate but
> when
> > i saw the xfs implementation of kernel it uses the RESVP cmd and  since
> the
> > unwritten flag is set i am assuming that the performance of writing to
> the
> > preallocated space will suffer because of the metadata updates. So is
> there
> > a way to preallocate using ALLOCSP as a non-root user or else i would be
> > required to modify the kernel ??
> >
> > TIA,
> > Smit
>
> ALLOCSP simply writes 0s to a file.... you may as well do it using posix
> calls, as a normal user, I think.
>

Even the man page of fallocate says that it allocates and initializes to
zero the disk space allocated but when i saw the code i did found out that
it does not zero it out.  Hence was a kindof confused. So posix_allocate is
similar to ALLOCSP when falloc is not supported by the underlying filesystem
 that is to ftruncate the file  and zero it out. So all of them try to
allocate contiguous blocks but the only difference is when we use the
fallocate in ext4/xfs it does not zero out the preallocated space. Am i
right ?
But  when i fallocate in ext4 i can see the write performance improvement
but not in xfs and reason i found out in one of your previous comments is
 because of the unwritten flag set in xfs. So how do we see if the unwritten
flag is set or not ? I did use xfs_info but it didnt show any such
information.

Thanks,
Smit


>
> -Eric
>
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