On Tue, May 08, 2012 at 11:24:52AM +0800, Zhu Han wrote:
> On Tue, May 8, 2012 at 7:59 AM, Dave Chinner <
david@xxxxxxxxxxxxx> wrote:
>
> > On Mon, May 07, 2012 at 08:44:17PM +0800, Zhu Han wrote:
> > > Seems like xfs of CentOS 6.X occupies much more storage space than
> > desired
> > > if fallocate is used against the file. Here is the step to reproduce it:
> >
> > You test case is not doing what you think it is doing.
>
> Thanks for pointing it out.
>
> > > By the way, is it normal when the file is moved around after the
> > > preallocated region is filled with data?
> > >
> > > $ uname -r
> > > 2.6.32-220.7.1.el6.x86_64
> > >
> > > $fallocate -n --offset 0 -l 1G file ---->Write a little more data than
> > > the preallocated size
> > >
> > > $ xfs_bmap -p -vv file
> > > file:
> > > EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET
> > > TOTAL FLAGS
> > > 0: [0..2097151]: 2593408088..2595505239 21 (29420144..31517295)
> > > 2097152 10000
> > >
> > > $ dd if=/dev/zero of=/tmp/file bs=1M count=1026 conv=fsync
> >
> > That does a truncate first, removing all the preallocated space. Use
> > conv=notrunc to avoid this. Hence the space allocated by this
> > new write is different to the space allocated by the above
> > preallocation. The file has not been moved, the filesystem just did
> > what you asked it to do.
> >
> > >
> > > $ xfs_bmap -p -vv file
> > > file:
> > > EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET
> > > TOTAL FLAGS
> > > 0: [0..4194303]: 2709184016..2713378319 22 (23101408..27295711)
> > > 4194304 00000
> >
> > And so now you've triggered the speculative delayed allocation
> > beyond EOF, which is normal behaviour. Hence there are currently
> > unused blocks beyond EOF which will get removed either when the next
> > close(fd) occurs on the file or the inode is removed from the cache.
> >
>
> Close(fd) should be invoked before dd quits. But why the extra blocks
> beyond EOF are not freed?