Re: ext2/3 performance regression in 2.6 vs 2.4 for small interleaved writes

[Hans Reiser <reiser@namesys.com>]

Jon Burgess wrote:

> Rik van Riel wrote:
>
>> Just for fun, could you also try measuring how long it takes
>> to read back the files in question ?
>>
>> Both individually and in parallel...
>>
> The original code did the read back as well, I stripped it out to make 
> the code smaller to post.
> It was the read back performance that I was most interested in. I 
> found that ext2/3 interleave all the blocks on the disk. With 2 stream 
> the read performance is 50%, 4 streams give 25% etc.
>
> I have one really bad case where I record a TV stream at 500kByte/s + 
> a radio one at 25kByte/s. These blocks are interleaved on the disk and 
> the read performance of the radio stream is reduced by the data ratio, 
> i.e. 1:20, so I get a miserable read performance of ~ 1MB/s.
>
> I found that ext2, ext3 and Reiserfs behave similarly.

Try Reiser4, it will probably cure it for you nicely.  I will be 
surprised if it does not, please let me know your results.  I would 
expect that tweaking the block preallocation code to preallocate more 
should cure it for ReiserFS V3 also....

> XFS and JFS appear to coalesce the data blocks during the write phase 
> and can read the data back at near maximum performance.
>
>    Jon
>
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