Re: [PATCHv4 3/5] ext4: remove unnecessary superblock dirtying

On Wed, 2012-07-04 at 15:11 +0200, Jan Kara wrote:

On Wed 04-07-12 15:21:52, Artem Bityutskiy wrote:

quoted

From: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

This patch changes the '__ext4_handle_dirty_super()' function which is used
by ext4 to update the superblock via the journal in the following cases:

1. When creating the first large file on a file system without
   EXT4_FEATURE_RO_COMPAT_LARGE_FILE feature.
2. When re-sizing the file-system.
3. When creating an xattr on a file-system without the
   EXT4_FEATURE_COMPAT_EXT_ATTR feature.
4. When adding or deleting an orphan (because we update the 's_last_orphan'
   superblock field).

This function, however, falls back to just marking the superblock as dirty
if the file-system has no journal. This means that we delay the actual
superblock I/O submission by 5 seconds (roughly speaking). Namely, the
'sync_supers()' kernel thread will call 'ext4_write_super()' later, where
we actually will submit the superblock down to the media.

However:
1. For cases 1-3 it does not add any value to delay the I/O submission. These
   events are rare and we may just commit submit the superblock for
   asynchronous I/O right away.
2. For case 4 - similarly, not terribly frequent event in most of workloads.
   It should be good enough to just submit asynchronous superblock write-out.

  Well, it happens for every inode being truncated / deleted to it can be
rather frequent. That's why I wanted to have now == 1 case everywhere -
i.e. just recompute the checksum and do mark_buffer_dirty(). I'd just
remove the 'now' test in this patch and then in patch 5 remove the now
argument from the function and callers as you did.

I am a bit confused.

It seems you consider that 'ext4_commit_super()' is a considerably
slower than just marking the buffer as dirty right away. But I do not
really understand why - all it does - it just updates a couple of
superblock fields and then marks the buffer as dirty (I assume sync ==
0). So from my POW they are almost the same. And when csum is enabled -
re-calculating csum will probably be the longest part.

More important is that we dirty the superblock on every deletion - this
mean that with my change we will re-calculate checsum on every deletion
and I am not sure it is nice. Ideally, we should be able to calculate
the checksum just before sending the buffer to the IO queue...

I'll prepare a new patch-set and send it to you. Thanks!

-- 
Best Regards,
Artem Bityutskiy