Dear all, I have been working on tuning the dm-thin performance on my storage server. I surprisingly find out that the write performance of newly created thin volume formatted as EXT4 has degraded significantly, when building thin-pool on top of RAID6 device. Thin pools in all experiment use 1MB block size, and I format all target volumes as EXT4 and mount them on /mnt. The following is the performance statistics I gather under different circumstances. Thin volume on top of a thin pool created by a Plextor M5P 128GB SSD as metadata device and a RAID6 block device composed by 7 disks (chunk size = 512KB) as a data device dennis@desktop:~$ sudo dd if=/dev/zero of=/mnt/zero.img bs=1M count=25000 25000+0 records in 25000+0 records out 26214400000 bytes (26 GB) copied, 245.808 s, 107 MB/s Directly make filesystem on RAID6 block device composed by 7 disks dennis@desktop:~$ sudo dd if=/dev/zero of=/mnt/zero.img bs=1M count=25000 25000+0 records in 25000+0 records out 26214400000 bytes (26 GB) copied, 129.543 s, 202 MB/s Thin volume on top of a thin pool created by a Plextor M5P 128GB SSD as metadata device and a RAID0 block device composed by 7 disks as a data device dennis@desktop:~$ sudo dd if=/dev/zero of=/mnt/zero.img bs=1M count=25000 25000+0 records in 25000+0 records out 26214400000 bytes (26 GB) copied, 46.1227 s, 568 MB/s Directly make filesystem on RAID6 block device composed by 7 disks dennis@desktop:~$ sudo dd if=/dev/zero of=/mnt/zero.img bs=1M count=25000 25000+0 records in 25000+0 records out 26214400000 bytes (26 GB) copied, 48.1104 s, 545 MB/s It is clear that the write performance has degraded significantly when we use RAID6 as thin-pool, but there is no similar behavior when using RAID0 as thin-pool instead. I dig a little deeper on this issue and find that if we can perfectly fit a pool block into a RAID6 stripe, there would be only 20~30% performance loss comparing to RAW RAID6. However, this poses a restriction on the disks number we can use to build a RAID6 for pool, and limit the maximum pool size as well. Is there any other way I can do to improve the write performance when using RAID6 as pool? Any help would be grateful. Thanks Dennis -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
