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

[Zdenek Kabelac]

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 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.


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