Re: [PATCH] ext4: fix how i_version is modified and turn it on by default V2

[Josef Bacik <josef@redhat.com>]

On Tue, May 15, 2012 at 10:33:16AM -0400, Josef Bacik wrote:
> This makes MS_I_VERSION be turned on by default.  Ext4 had been
> unconditionally doing i_version++ in a few cases anway so the mount option
> was kind of silly.  This patch also removes the update in mark_inode_dirty
> and makes all of the cases where we update ctime also do inode_inc_iversion.
> file_update_time takes care of the write case and all the places where we
> update iversion are protected by the i_mutex so there should be no extra
> i_lock overhead in the normal non-exported fs case.  Thanks,
> 

Ok did some basic benchmarking with dd, I ran

dd if=/dev/zero of=/mnt/btrfs-test/file bs=1 count=10485760
dd if=/dev/zero of=/mnt/btrfs-test/file bs=1M count=1000
dd if=/dev/zero of=/mnt/btrfs-test/file bs=1M count=5000

3 times with the patch and without the patch.  With the worst case scenario
there is about a 40% longer run time, going from on average 12 seconds to 17
seconds.  With the other two runs they are the same runtime with the 1 megabyte
blocks.  So the question is, do we care about this worst case since any sane
application developer isn't going to do writes that small?  Thanks,

Josef