linux-ext4 - Re: Any qualms about reverting 3d0518f4, ext4: New rec

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Date:	Tue, 03 Aug 2010 17:49:22 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	ext4 development <linux-ext4@...r.kernel.org>,
	Wei Yongjun <yjwei@...fujitsu.com>
Subject: Re: Any qualms about reverting 3d0518f4, ext4: New rec_len encoding
 for very large blocksizes ?

On 08/03/2010 05:44 PM, Andreas Dilger wrote:
> On 2010-08-03, at 16:30, Eric Sandeen wrote:
>> commit 3d0518f4758eca4339e75e5b9dbb7e06a5ce08b4 Author: Wei Yongjun
>> <yjwei@...fujitsu.com> Date:   Sat Feb 14 23:01:36 2009 -0500
>> 
>> ext4: New rec_len encoding for very large blocksizes
>> 
>> The rec_len field in the directory entry is 16 bits, so to encode 
>> blocksizes larger than 64k becomes problematic.  This patch allows
>> us to supprot block sizes up to 256k, by using the low 2 bits to
>> extend the range of rec_len to 2**18-1 (since valid rec_len sizes
>> must be a multiple of 4).  We use the convention that a rec_len of
>> 0 or 65535 means the filesystem block size, for compatibility with
>> older kernels.
>> 
>> It's a novel solution to the problem, but I'm not sure it's a
>> problem that needs to be solved today since we cannot even make
>> filesystems with
>>> 
>>> 64k blocks:
> 
> I don't object to reverting the > 64kB support, but we should strive
> to keep 64kB directory blocks working, if that is possibly going to
> break with reverting this patch.  I don't think any of the other extN
> code is expecting to handle block sizes larger than 64kB, so little
> value to do it just in the directory handling code.

As far as I know, reverting it won't break 64kb dir blocks...?

A simpler solution to the perf problem might be to just re-inline the
function, but I'm not sure what the point of all this is, as you say.

Thanks,
-Eric

>> Just for reference, the testing I did on RHEL6 was:
>> 
>> # bonnie++ -u root -s 0 -f -x 200 -d /mnt/test -n 32
>> 
>> (this does 200 iterations) and got this for the file creations:
>> 
>> ext4 stock:   Average = 21206.8 files/s
>> ext4 patched: Average = 22822.1 files/s
>> 
>> This is a 7.6% improvement...
> 
> Nothing to sneeze at.
> 
> Cheers, Andreas
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