[dm-devel] Poor snapshot performance in linux-3.19

[Dennis Yang]

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 at 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 at localhost ~]# dmsetup message pool 0 "create_thin 0"
[root at 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 at 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 at 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.

Thanks,
Dennis