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
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