[Lustre-discuss] File sizes on MDT

Andreas Dilger adilger at sun.com
Mon Jul 27 13:47:03 PDT 2009


On Jul 27, 2009  14:24 +0200, Thomas Roth wrote:
> I'm copying around data between 2 MDTs in a test system. Having mounted
> the partitione as 'ldiskfs', I had a look in MDT/ROOT. I found all my
> test data there, but I'm puzzled by the indicated file sizes. For
> example I had put one of my holiday's movies, it's 40MB. On the
> ldiskfs-mounted MDT, I find a corresponding entry, which also has 40MB,
> as given by 'ls -lh'. Of course, the latter file doesn't have the
> contents of that movie, but why is it the same size? 'ls -li' also gives
> identical results, btw.
> On the other hand, there is another movie which is .6.4MB as such, but
> 0B on the MDT partition.

In Lustre 1.6.7 the "approximate" file size started to be stored on the
MDT inodes in order to facilitate[*] filesystem backup utilities to
allow them to have a fast estimate of the file size w/o having to access
the OST objects (that hold the authoritative size).  This size cannot
be used as the official file size in 1.x because there isn't sufficient
locking and recovery of the size in case of a crash, though a preview of
this feature (Size On MDS, SOM) will be available in the 2.0 release.

> Both movies play nicely, so there is no problem with this file system.
> On our production system, the MDT takes 13GB for 250TB of data,
> obviously there aren't entries on the MDT taking the size of the real
> data files ;-)
> 
> So my question is whether the file size reported by 'ls' on the MDT as
> any practical implication?

This size is not actively updated for pre-existing files, nor is it
always guaranteed to be written in case of a crash, which is why you
see some (likely older) files that do not have the size information.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.




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