XFS & LVM: unexpected cp when issuing mv

[Dave Chinner]

On Sun, Nov 29, 2009 at 02:52:16PM +0100, Michael Monnerie wrote:
> I have an unexpected behaviour and I hope someone can explain me the 
> reasons:
> 
> This is an openSUSE 11.2 virtual machine within XENserver. XENserver can 
> only create 2TB disks, but I needed more. So I create 2x 2TB disks for 
> that VM. These disks have no partitions, but are straight LVM:
> # pvscan
>   PV /dev/xvdb   VG sharestore   lvm2 [1,95 TB / 0    free]
>   PV /dev/xvdc   VG sharestore   lvm2 [1,95 TB / 0    free]
>   Total: 2 [3,91 TB] / in use: 2 [3,91 TB] / in no VG: 0 [0   ]
> 
> I created one VG, and then one LV:
> # vgscan
>   Reading all physical volumes.  This may take a while...
>   Found volume group "sharestore" using metadata type lvm2
> # lvscan
>   ACTIVE            '/dev/sharestore/public' [3,91 TB] inherit
> 
> On that LV, I created an XFS filesystem, mounted from /etc/fstab:
> /dev/sharestore/public /disks/sharestore  xfs        
> noatime,nodiratime,logbufs=8,logbsize=256k,attr2,nobarrier,largeio,swalloc,inode64,prjquota

You are using project quotas.

> Now when I move from one dir to another, example
> mv /disks/sharestore/upload/* /disks/sharestore/download/
> 
> within some dirs it's a simple mv where only metadata is moved, but with 
> some dirs it's a physical cp+rm of the files. You can easily see that by 
> the speed of the mv, plus with iostat:
> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-
> sz avgqu-sz   await  svctm  %util
> xvdb              0,00     0,00    0,00  647,31     0,00 28424,75    
> 87,82    18,46   29,71   0,24  15,65
> xvdc              0,00     0,40  631,14    2,40 26928,54    76,65    
> 85,25     5,56    8,69   1,56  98,84
> 
> Until now I believed that a mv within one filesystem is always just a 
> metadata mv. But it seems I found a case now where even within the same 
> filesystem a physical cp+rm is done. Can someone explain me
> 
> 1) why this happens

It happens if you move from one project directory heirarchy to
another - rename is not allowed across project quota boundaries as
the moved data has to be correctly attributed to the new project.
Hence it causes a mv to do a copy/unlink by returning a EXDEV error
to the rename.

> 2) how I can prevent this?

You can't if you are moving from one project to another. If you
move within the project heirarchy, then it will be a rename as per
normal.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com