XFS Preallocate using ALLOCSP

[Smit Shah]

On Mon, Jun 15, 2009 at 11:44 PM, Smit Shah <getsmit at gmail.com> wrote:

>
>
> 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 ?
>
I guess i am not right here ftruncate simply does a lseek and wirtes to it
which might not be contiguous whereas fallocate tries to allocate contiguous
block so as to reduce fragmentation and hence i thought to reduce
fragmentation and for security reasons its better to use ALLOCSP rather than
something like ftruncate /posix_fallocate or RSEVSP which kindof performs
bad for writes with unwritten flag set and now there being a no direct way
while creating the fs to disable unwritten.


> 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
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://oss.sgi.com/pipermail/xfs/attachments/20090616/375839e7/attachment.htm>