[Lustre-discuss] MDT raid parameters, multiple MGSes

Brian J. Murrell brian at whamcloud.com
Sat Jan 22 15:06:55 PST 2011

On Sat, 2011-01-22 at 11:23 +0100, Thomas Roth wrote: 
> Btw, files on the MDT  - why does the apparent file size there sometimes reflect the size of the real file, and sometimes not?

I believe in the cases where you are seeing a size, it's SOM (size on
mds) in action.

> For example, on a ldiskfs-mounted copy of our MDT, I have a directory under ROOT/ with
> -rw-rw-r-- 1  935M 15. Jul 2009  09000075278027.140
> -rw-rw-r-- 1     0 15. Jul 2009  09000075278027.150

Indeed.  It's pretty hazy at the moment (so please do check the
archives) but I think there was a thread here not that long ago that
explained that SOM was only activated for newly created files in the
release it showed up in.  Now it's interesting that the two examples
above have the same mtime.  Perhaps there are conditions for recording
an SOM for a preexisting file, like reading it perhaps.  Maybe one of
those has been read since you installed a SOM release and the other has

> As they should, both entries are 0-sized, as seen by e.g. "du".

Or ls -ls.

> On Lustre, both files exist and both have size 935M. So for some reason,
> one has a metatdata entry that appears as a huge sparse file, the other does not.


> Is there a reason, or is this just an illness of our installation?

As above, and per the archives.


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