Hi,
Here's a very strange (and repeatable) result. Affects 2.2.x + raw device
patches (i.e. RH7.0). Also had a similar effect on 2.4.0test9!
The problem is best described with a little sequence. After using raw i/o
facilities, streamed block device reads from the same underlying device
exhibit much poorer performance than before the raw i/o.
Example
[root@localhost /root]# hdparm -t /dev/hda
/dev/hda:
Timing buffered disk reads: 64 MB in 3.81 seconds = 16.80 MB/sec
[root@localhost /root]# time dd if=/dev/raw/raw1 of=/dev/null bs=1024k
count=64
64+0 records in
64+0 records out
real 0m2.990s
user 0m0.010s
sys 0m0.450s
[root@localhost /root]# hdparm -t /dev/hda
/dev/hda:
Timing buffered disk reads: 64 MB in 6.12 seconds = 10.46 MB/sec
The read figures before and after the raw i/o are repeatable with only
little jitter.
Raw device reads are consistent and not affected by this phenomena.
Anyone know what's going on? Looks like a bug or inefficiency somewhere in
the kernel.
Cheers
Chris
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