Re: [PATCHv6 3/5] ext4: remove unnecessary superblock dirtying

On Wed 11-07-12 12:07:26, Jan Kara wrote:

On Wed 11-07-12 12:58:16, Artem Bityutskiy wrote:

...

quoted

And this is the behavior this patch modifies: we stop using 's_dirt' and just
mark the superblock buffer as dirty right away. Indeed:

1. It does not add any value to delay the I/O submission for cases 1-3 above.
   They are rare.
2. Case number 4 above depends on whether we have file-system checksumming
   enabled or disables.
   a) If it is disabled (most common scenario), then it is all-right to just
      mark the superblock buffer as dirty right away and it should affect
      performance.
   b) If it is enabled, then we'll end up doing a bit more work on deletion
      because we'll re-calculate superblock checksum every time.

So case 2.b is a bit controversial, but I think it is acceptable. After all, by
enabling checksumming we already sign up for paying the price of calculating
it. The way to improve checksumming performance globally would be to calculate
it just before sending buffers to the I/O queue. We'd need some kind of
call-back which could be registered by file-systems.

  Actually, the most common case of adding orphan inode used
ext4_handle_dirty_super_now() so for that case there is no difference. And
other cases are so rare it really does not matter... So there shouldn't be
any measurable difference.

									Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR