[lustre-discuss] non-empty files found when backing up MDT with LVM snapshots and tar

[Dauchy, Nathan (ARC-TNC)[Computer Sciences Corporation]]

On  April 22, 2016 at 10:07 Dilger, Andreas wrote:
> The fact that the MDT inodes are storing a size is an artifact of 1.8,
> and is not considered a problem. There is definitely no data actually
> stored on the MDT inodes. The fact that the files have a data blocks
> located is likely because of wide striping stored in an external xattr block. 

For everyone's future reference, I have confirmed that it is the "wide" striping that causes use of an external block.  And wide isn't really that wide... a stripe count of 8 is enough to cause a block to be allocated on the MDT.  It does not seem to matter whether any data is actually written to the file on the client.  From a stripe count of 8 up through a stripe count of 65 (as far as I tested) the number of blocks allocated does not change...

# stat file.7
  File: `file.7'
  Size: 0         	Blocks: 0          IO Block: 4096   regular empty file
Device: fd06h/64774d	Inode: 571005505   Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2016-04-25 13:21:05.870369459 -0700
Modify: 2016-04-25 13:21:05.870369459 -0700
Change: 2016-04-25 13:21:34.000000000 -0700

# stat file.8
  File: `file.8'
  Size: 0         	Blocks: 8          IO Block: 4096   regular empty file
Device: fd06h/64774d	Inode: 571005506   Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2016-04-25 13:21:05.872369447 -0700
Modify: 2016-04-25 13:21:05.872369447 -0700
Change: 2016-04-25 13:21:34.000000000 -0700

# stat file.65
  File: `file.65'
  Size: 0         	Blocks: 8          IO Block: 4096   regular empty file
Device: fd06h/64774d	Inode: 571005681   Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2016-04-25 13:21:06.025368603 -0700
Modify: 2016-04-25 13:21:06.025368603 -0700
Change: 2016-04-25 13:21:34.000000000 -0700

Thanks,
Nathan