On Tue, 16 Mar 2010 20:05:45 +0100 Nicolae Mihalache <mache@xxxxxxxxxxxx> wrote: > Hello, > > I have created a partitioned raid6 array over 6x1TB SATA disks using the > command (from memory): mdadm --create --auto=mdp --level=6 > --raid-devices /dev/md_d1 /dev/sd[b-g]. > > When I run a sequential read test using > dd if=/dev/md_d1p1 of=/dev/null bs=1M > I get low read speeds of around 80MB/s but only when the partition is > mounted. > > If I unmount, the speed is around 350MB/s. The filesystems I tried are > ext3 and xfs. Thanks for reporting this. I just did some testing and I get the reverse!! When a filesystem is mounted I get 135MB/s. When it isn't mounted I get 64MB/s. I cannot think what could cause this. I will have to explore. Can you please double check you results and confirm that it definitely is faster then unmounted. > > The partitions have been created with gparted, the partition table being > of type GPT. > > If I create normal /dev/sdx1 partitions on each disk and then make a > /dev/md1 raid6 array over them, the read speed is ok. > > I played with different read ahead settings and while they changed the > read speed, it's only marginally around the values reported above. > > Can somebody explain what is the difference when accessing a raw disk > when it is mounted or not? Also when playing with those read ahead > settings it was not clear how/if the read ahead of the individual disks > are taken into account. Only the read-ahead value of the array is considered. The read-ahead settings of the individual devices in the array are ignored. NeilBrown > > When setting big values of read ahead, I could see with iostat that tps > for the individual disks is double when accessing the mounted disk as > opposed to when accessing it unmounted (despite the speed being three > times lower). > It's like when accessing the mounted partition, it reads some other > parts of the disks. I could not find a way to print the blocks read from > the individual disks. The sysctl vm.block_dump=1 makes the kernel print > the block numbers on the md array but not on the components of the array. > > The system is debian 5 with kernel 2.6.26-2-686. > > Thanks for any hint on how to further debug the problem. > > nicolae > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
