Re: Slow Disk I/O with QPS M3 80GB HD

[H . J . Lu]

On Mon, Dec 10, 2001 at 11:57:08PM -0800, H . J . Lu wrote:
> On Mon, Dec 10, 2001 at 08:34:52PM -0800, H . J . Lu wrote:
> > I have a very strange problem. The disk I/O of my QPS M3 80GB HD is
> > very slow under 2.4.10 and above. I got like 1.77 MB/s from hdparm.
> > But under 2.4.9, I got 14 MB/s on the same hardware. A 30GB HD has
> > consistent I/O performance under 2.4.9 and above on the same bus. Has
> > anyone else seen this? Does anyone have a large (>= 80GB) 1394 HD?
> > 
> 
> I did a binary search. 2.4.10-pre10 is the last good kernel. I got
> 
> # hdparm -t /dev/sda
> 
> /dev/sda:
>  Timing buffered disk reads:  64 MB in  4.40 seconds = 14.55 MB/sec
> 
> Even since 2.4.10-pre11 up to 2.4.16, I got about 1.77 MB/sec on the
> same hardware. However, I don't have problems with 80GB IDE HD. Has
> anyone seen I/O problems on large (>= 80GB) SCSI HD or HD with SCSI
> emulation?

I tracked own the problem to 40_blkdev-pagecache-17 in the 2.4.10
pre10aa1 patch. When it is applied, the disk I/O on some drives become
very slow. It not only happens to my 80GB 1394 HD, but also the second
IDE drive. Before the patch

# hdparm -t /dev/hdd

/dev/hdd:
 Timing buffered disk reads:  64 MB in  8.02 seconds =  7.98 MB/sec

After the patch

# hdparm -t /dev/hdd

/dev/hdd:
 Timing buffered disk reads:  64 MB in 21.09 seconds =  3.03 MB/sec

The slow down is not as bad as 1394. But it is still very significant.
I couldn't figure out why it only affects certain drives.

H.J.