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

[Jon Burgess]

Andrew Morton wrote:

> So you'll see that instead of a single full-bandwidth
> write, we do two half-bandwidth writes.  If it weren't for disk writeback
> caching, it would be as much as 4x slower.
Write caching does indeed make a big difference. Here is a test run on a 
drive with and without write caching (hdparm -W 0/1). The test was done 
on 2.6.2 with ext3 and shows the write speed in MB/s:

Write Cache   1 Stream   2 Streams
Enabled       21.54      3.66
Disabled      18.11      0.46  

The two stream case is almost 10x slower without write caching.
I don't think this explains the difference between 2.4 and 2.6 unless 
one of them changes the write cache mode of the drive.

   Jon

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