[Cluster-devel] [PATCH 2/5] gfs2_edit savemeta: Don't read rgrp blocks twice

[Bob Peterson <rpeterso@redhat.com>]

----- Original Message -----
| When savemeta iterates over the rgrps and saves the rgrp header blocks
| it calls save_block which does a bread() for each block despite them
| already being read in earlier. Instead, call save_bh() on the existing
| rgrp buffers. This isn't likely to give a significant performance
| improvement as the duplicate reads would be cache hits but it should
| have a noticeable effect on very large file systems.
| 
| Signed-off-by: Andrew Price <anprice@redhat.com>
| ---

Hi,

IIRC, the reason I coded it the way I did was with the notion that
customers often save their metadata (and send it in) because it's corrupt.
If there's an rgrp block, such as a bitmap, which is corrupt, I wanted
savemeta to save the rgrp blocks regardless of the corruption so we could
see the trash with which it was overwritten. Function gfs2_rgrp_read
checks for corruption and if's not a bitmap, it frees ALL bi_bh buffers;
even the non-corrupt ones.

So we need to ensure that corrupt / blasted rgrps and bitmaps also get
saved, provided their rindex is healthy (which it also might not be).

Perhaps my fears are unfounded? You could verify this by doing:
(1) mkfs.gfs2
(2) blast / zero out a random rgrp or rindex
(3) gfs2_edit savemeta
(4) gfs2_edit printsavedmeta
(5) ensure the output contains the blasted block

Regards,

Bob Peterson
Red Hat File Systems