On Tue, 15 Oct 2019, Coly Li wrote: > On 2019/10/12 10:23 下午, Sergey Kolesnikov wrote: > > Hello everyone. > > > > I'm trying to get my bcache setup running, but having almost all my > > traffic bypassing the cache. > > Here are some stats that I have: > > > > > > root@midnight:~# cat > > /sys/fs/bcache/9820f407-457a-46e3-abc0-f2214d39b64c/bdev0/stats_total/bypassed > > 2.8G > > root@midnight:~# cat > > /sys/fs/bcache/9820f407-457a-46e3-abc0-f2214d39b64c/bdev0/stats_total/cache_bypass_misses > > 247956 > > root@midnight:~# cat > > /sys/fs/bcache/9820f407-457a-46e3-abc0-f2214d39b64c/bdev0/stats_total/cache_bypass_hits > > 5597 > > root@midnight:~# cat > > /sys/fs/bcache/9820f407-457a-46e3-abc0-f2214d39b64c/bdev0/stats_total/cache_hits > > 233 > > root@midnight:~# cat > > /sys/fs/bcache/9820f407-457a-46e3-abc0-f2214d39b64c/bdev0/stats_total/cache_misses > > 243 > > > > And now for my machine setup. > > Running ubuntu 18.04 LTS with 5.0.0-31-lowlatency kernel. > > Cache device is a partition on NVMe PCI-e SSD with 4k logical and > > physical sector size. > > Backing device is LVM logical volume on a 3-drive MD RAID-0 with 64K > > stripe size, so it's optimal IO is 192K. > > I have aligned backing-dev data offset with > > make-bcache -B -o 15360 --writeback /dev/vm-vg/lvcachedvm-bdev > > > > I have tried all recommendations for routing traffic to SSD: > > > > echo 0 > /sys/fs/bcache/9820f407-457a-46e3-abc0-f2214d39b64c/congested_read_threshold_us > > echo 0 > /sys/fs/bcache/9820f407-457a-46e3-abc0-f2214d39b64c/congested_write_threshold_us > > echo 0 > /sys/fs/bcache/9820f407-457a-46e3-abc0-f2214d39b64c/bdev0/sequential_cutoff > > > > But I still get almost all traffic going to cache_bypass_misse. BTW, > > what does this stat mean? I don't get it from the in-kernel manual > > > > Any help?.. > > I have no much idea. The 4Kn SSD is totally new to me. Last time I saw > Eric Wheeler reported 4Kn hard diver didn't work well as backing device, > and I don't find an exact reason up to now. I am not able to say 4Kn is > not supported or not, before I have such device to test... We pulled the 4Kn SSD configuration, it wasn't stable back in v4.1. Not sure if the problem has been fixed, but I don't think so. Here is the original thread: https://www.spinics.net/lists/linux-bcache/msg05971.html -- Eric Wheeler > > -- > > Coly Li >
