On Fri, 2003-03-07 at 16:05, Rick Smith wrote:
> By preallocation are you referring to an ioctl after opening a file
> with XFS_IOC_RESVSP64? If so, does this only have an effect with
> unwritten=1?
> Does using the XFS_IOC_RESVSP64 ioctl guarantee contiguous extents when
> the file is written, or at least make more of an effort to ensure contiguous
> extent allocation? As you may remember from some of my previous emails, my
> goal is to have each file written as contiguous as possible and have each
> consecutive file as close to the previous file as possible. I have been
> experimenting with the realtime subvolume (due to the limited documentation
> speaking of reduced fragmentation), but I have not had much success so far.
> Thanks.
They do not guarantee continuous space, but they go to the allocator and
ask for it all in one go. If it is available, you will get continuous
space. I did not realize you had control over the app writing the
data. The calls are always available, but with unwritten extent
support off, they are restricted to root.
Steve
>
> Rick
>
> >From: Steve Lord <lord@xxxxxxx>
> >To: Rick Smith <rgsmith72@xxxxxxxxxxx>
> >CC: linux-xfs@xxxxxxxxxxx
> >Subject: Re: unwritten extents vs. fragmentation
> >Date: 07 Mar 2003 15:32:54 -0600
> >
> >On Fri, 2003-03-07 at 15:26, Rick Smith wrote:
> > > A couple questions. Does enabling unwritten extent flagging in XFS
> > > ultimately reduce filesystem fragmentation? What is the downside of
> >enabling
> > > unwritten extents other than slower write performance? Thanks.
> >
> >It should make no difference to physical fragmentation, and in the
> >normal I/O path unwritten extents should not be used. They only come
> >into play with space preallocation. In this case they will slow
> >down I/O somewhat, but provide more security in that you cannot
> >use unwritten extents to read old data. This is why the prealloc
> >calls on -d unwritten=0 filesystems are restricted to root.
> >
> >Steve
> >
> >--
> >
> >Steve Lord voice: +1-651-683-3511
> >Principal Engineer, Filesystem Software email: lord@xxxxxxx
>
>
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--
Steve Lord voice: +1-651-683-3511
Principal Engineer, Filesystem Software email: lord@xxxxxxx
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