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

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On 05/15/2012 08:53 PM, Josef Bacik wrote:

> 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_ב.
>> 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.  


do you mean that the "with the patch" is 40% slower then "without the patch"
	in the same "bs=1 count=10485760 test" ?

Then count me clueless. Do you understand this difference?

The way I read your patch the inode is copied and written to disk exactly the
same number of times, as before. Only that now i_version is also updated
together with ctime and/or mtime. What is the fundamental difference then?
Is it just that i_version++ in-memory operation?

> 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
Thanks
Boaz

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