Hi When I was examining a reported performance degradation, I found out that the reason was buggy firmware in Maxtor Atlas 15K2 SCSI disks. If the requests are larger than 256kB, read rate drops excessively, even to one half of the normal read rate. Performance on these disks improves if the request size is limited with echo 256 >/sys/block/sdc/queue/max_sectors_kb I remember seeing similar problem (increased transfer size => decreased performance) with IDE and SATA Maxtor disks some times ago, but I don't have any such disk to test currently. I'd like to ask other people with Maxtor disks to run this script (with "sda", "sdb", etc. as an argument ... without "/dev/" prefix). The script measures read rate as a function of request size. Report the results, so that it can be determined which disks are buggy and which not and the default transfer size could be reduced to 256kB for the buggy ones. My results are: MAXTOR ATLAS15K2_146WLS HPM5 64 kB: 96,1 MB/s 448 kB: 54,3 MB/s 832 kB: 69,3 MB/s 128 kB: 95,9 MB/s 512 kB: 61,6 MB/s 896 kB: 72,8 MB/s 192 kB: 96,2 MB/s 576 kB: 63,4 MB/s 960 kB: 73,6 MB/s 256 kB: 96,4 MB/s 640 kB: 43,0 MB/s 1024 kB: 73,2 MB/s 320 kB: 47,3 MB/s 704 kB: 59,0 MB/s 384 kB: 47,5 MB/s 768 kB: 55,3 MB/s MAXTOR ATLAS15K2_146WLS HPM7 64 kB: 95,8 MB/s 448 kB: 54,3 MB/s 832 kB: 67,7 MB/s 128 kB: 95,5 MB/s 512 kB: 60,2 MB/s 896 kB: 73,1 MB/s 192 kB: 95,8 MB/s 576 kB: 62,8 MB/s 960 kB: 72,8 MB/s 256 kB: 96,3 MB/s 640 kB: 44,3 MB/s 1024 kB: 73,2 MB/s 320 kB: 47,2 MB/s 704 kB: 58,3 MB/s 384 kB: 48,3 MB/s 768 kB: 54,2 MB/s Note: fiddling with caching mode page (chaning DPTL, MAPF and MAPFC to -1) improved performance for large requests somehow (to 70-80MB/s), but I couldn't find a combination of settings that would restore 95MB/s rate, as for <= 256kB request. Thus, the correct workaround for these disks is to never send > 256kB requests. Mikulas #!/bin/sh DISK="$1" echo 1024 >/sys/block/$DISK/queue/max_sectors_kb echo `cat /sys/block/$DISK/device/vendor /sys/block/$DISK/device/model /sys/block/$DISK/device/rev /sys/block/$DISK/device/firmware 2>/dev/null` for i in `seq 64 64 1024`; do blocks=`expr 200000 / $i` echo -n "$i kB: " dd if=/dev/$DISK of=/dev/null iflag=direct bs=$i"k" count=$blocks 2>&1 | sed -n 's/.*s, //p' done -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
