Dne 28.5.2015 v 12:18 Dennis Yang napsal(a):
Hi, I have a workstation which runs Fedora 21 with linux-3.19 kernel and create a thin pool onto of a RAID0 (chunksize = 512KB) with five Crucial 256GB SSDs. [root@localhost ~]# dmsetup create pool --table "0 2478300160 thin-pool /dev/md0p1 /dev/md0p2 1024 0 1 skip_block_zeroing" Then, I create a small thin volume with the following commands. [root@localhost ~]# dmsetup message pool 0 "create_thin 0" [root@localhost ~]# dmsetup create thin --table "0 400000000 thin /dev/mapper/pool 0" After that, I use both dd and fio for throughput testing and get the following result. [root@localhost ~]# dd if=/dev/zero of=/dev/mapper/thin bs=2M count=25k 25600+0 records in 25600+0 records out 53687091200 bytes (54 GB) copied, 29.0871 s, 1.8 GB/s The 1.8 GB/s throughput looks pretty reasonable to me. However, after taking a single snapshot of this thin device, I get a pretty low throughput with the same command. [root@localhost ~]# dd if=/dev/zero of=/dev/mapper/thin bs=2M count=25k 25600+0 records in 25600+0 records out 53687091200 bytes (54 GB) copied, 191.495 s, 280 MB/s I am aware of that writing to a snapshotted device will trigger lots of copy-on-write requests, so I was expecting a 50~60% performance loss in this case. However, 85% performance loss can be observed in my test above. Am I doing anything wrong or is there anything I can tune to make this right? If someone can point direction for me, I am glad to test or even modify the source code to solve this case.
Hi Using 0.5MB chunks and expecting fast snapshots is not going to work. Have you measured speed with smaller chunks - i.e. 64k/128k ? Zdenek -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
