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

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Date:	Tue, 3 Aug 2010 16:44:11 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Eric Sandeen <sandeen@...hat.com>
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 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.

> 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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