Messages in this thread | | | Date | Mon, 29 Dec 2003 13:54:14 +0100 | From | Miquel van Smoorenburg <> | Subject | Speed drop /dev/sda -> /dev/sda1 -> /dev/vg0/test (3ware/LVM) |
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Hello,
I'm running Linux 2.6.0 with a 3ware 8506 controller in hardware RAID5 mode. The RAID5 array is built of 7+1 200 MB SATA disks.
Now it appears that more "mappings" on the array have a bad influence on speed. /dev/sda is the fastest, /dev/sda1 is quite a bit slower, LVM on /dev/sda is slower yet and LVM on /dev/sda1 is the slowest.
Results of a dd write directly on the RAID5 whole device:
time -p sh -c "dd if=/dev/zero of=/dev/sda bs=4k count=100000; sync" 100000+0 records in 100000+0 records out 409600000 bytes transferred in 5.120484 seconds (79992437 bytes/sec) real 5.23 user 0.03 sys 0.98
Results on the first partition:
time -p sh -c "dd if=/dev/zero of=/dev/sda1 bs=4k count=100000; sync" 100000+0 records in 100000+0 records out 409600000 bytes transferred in 6.141125 seconds (66697878 bytes/sec) real 6.24 user 0.04 sys 2.06
LVM is created on /dev/sda or /dev/sda1 using the following commands: pvcreate $DEVICE vgcreate vg0 $DEVICE lvcreate -L 100G -n test vg0
Results for a logical volume on /dev/sda:
time -p sh -c "dd if=/dev/zero of=/dev/vg0/test bs=4k count=100000; sync" 100000+0 records in 100000+0 records out 409600000 bytes transferred in 8.624306 seconds (47493676 bytes/sec) real 8.65 user 0.05 sys 1.03
time -p sh -c "dd if=/dev/zero of=/dev/vg0/test bs=4k count=100000; sync" Results for a logical volume on /dev/sda1: 100000+0 records in 100000+0 records out 409600000 bytes transferred in 9.223930 seconds (44406234 bytes/sec) real 9.27 user 0.07 sys 0.97
This is reproducable. I also tested this on a software RAID5 array, but with SW RAID5 it doesn't matter if I use the whole disk, a partition, or a logical volume; performance is always the same (slightly faster than HW RAID5; 100 MB/sec).
Any idea how to get the performance back ? How can I go about debugging this ?
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