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

[Andrea Arcangeli]

On Tue, Dec 11, 2001 at 03:43:31PM -0800, H . J . Lu wrote:
> 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.

do you have a 4k filesystm mounted on /dev/hdd? if so then you will get
the same performance with latest 2.4 (precisely after 2.4.1).

Andrea