[dm-devel] Performance degradation of DM devices after upgrading to linux-5.9 from linux-5.8

[Dennis Yang]

Hi, I have observed a severe performance degradation of DM devices with a
simple dd test in linux-5.9 comparing to linux-5.8.

This test contains the following steps:
1. Create a dm-thin pool
2. Create a volume in this pool
3. Run "dd if=/dev/zero of=/dev/mapper/vol bs=512K count=10k"

In my own setup, I use a SSD as thin pool's metadata device and a HDD as
data device. Here is what I get from both linux versions.

--- Thin Volume ---
linux-5.9.11         10.5 MB/s
linux-5.8.18         77.7 MB/s

--- Linear device over HDD ---
linux-5.9.11         77.0 MB/s
linux-5.8.18       136.0 MB/s

--- Linear device over SSD ---
linux-5.9.11        256.0 MB/s
linux-5.8.18        369.0 MB/s

>From iostat, I can tell that DM devices will get a smaller bio with
length equals to only 1 sector in linux-5.9.11 comparing to 8 sectors in
linux-5.8.18. I dig a little deeper to this issue, and it turns out this
patch made the size of bio of buffer I/Os equal to the logical block size
of target block device which is 512 bytes of all my HDD and SSD in my
cluster.
https://lore.kernel.org/patchwork/patch/1264139/

After reverting this patch in linux-5.9.11, I can have the same performance
as linux-5.8. However, I am not sure if this is the right thing to do or
something needs to be taken care of in the device-mapper layer.

Any comment would be highly appreciated.
Thanks for your time.

Dennis
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/dm-devel/attachments/20201208/d55f1bb6/attachment.htm>