I have a bcache device that has been acting up lately. I believe it
worked correctly until this week but now I cannot get any sequential
reads to use the cache anymore. Writes appear to be cached correctly,
but when reading back the same sectors the data comes from spinning rust
instead. I tried recreating the cache device but that did not help.
# dd if=/dev/bcache0 of=/dev/null bs=1M count=32768
34359738368 bytes (34 GB, 32 GiB) copied, 67.976 s, 505 MB/s
# dd if=/dev/bcache0 of=/dev/null bs=1M count=32768
34359738368 bytes (34 GB, 32 GiB) copied, 74.5009 s, 461 MB/s
# dd if=/dev/md/CacheRaid of=/dev/null bs=1M count=32768
34359738368 bytes (34 GB, 32 GiB) copied, 7.4617 s, 4.6 GB/s
Every repeated read on the same data shows similar speeds to the first
run. Accessing through a mounted btrfs shows similar results.
Expected: On repeated rounds speeds a few gigabytes per second, as per
the cache device speed.
# bcache-super-show /dev/mapper/BigRaw
sb.version 4 [backing device]
dev.sectors_per_block 8
dev.sectors_per_bucket 2048
dev.data.first_sector 4096
dev.data.cache_mode 0 [writethrough]
dev.data.cache_state 1 [clean]
# bcache-super-show /dev/md/CacheRaid
sb.version 3 [cache device]
dev.sectors_per_block 8
dev.sectors_per_bucket 512
dev.cache.first_sector 512
dev.cache.cache_sectors 3369647616
dev.cache.total_sectors 3369648128
dev.cache.ordered yes
dev.cache.discard no
dev.cache.pos 0
dev.cache.replacement 0 [lru]
# cat /sys/block/bcache0/bcache/cache/cache0/priority_stats
Unused: 91%
Clean: 8%
Dirty: 0%
Metadata: 0%
Average: 127
Sectors per Q: 9959504
Quantiles: [0 0 5 8 12 15 18 21 24 50 85 93 99 107 114 122 131 140 149
159 169 180 191 203 215 228 241 254 269 281 309]
Expected: Clean at least 50 % and unused below 50 % if all the data read
was cached (I read a much larger chunk than 32 GiB to test this).
/sys/block/bcache0/bcache/stats_total/bypassed 1.9G
/sys/block/bcache0/bcache/stats_total/cache_bypass_hits 57
/sys/block/bcache0/bcache/stats_total/cache_bypass_misses 890
/sys/block/bcache0/bcache/stats_total/cache_hit_ratio 3
/sys/block/bcache0/bcache/stats_total/cache_hits 15264
/sys/block/bcache0/bcache/stats_total/cache_miss_collisions 932
/sys/block/bcache0/bcache/stats_total/cache_misses 441927
/sys/block/bcache0/bcache/sequential_cutoff 0.0k
/sys/block/bcache0/bcache/cache/congested_read_threshold_us 0
/sys/block/bcache0/bcache/cache/congested_write_threshold_us 0
# uname -a
Linux xxx 5.14.8-zen1-1-zen #1 ZEN SMP PREEMPT Sun, 26 Sep 2021 19:36:16
+0000 x86_64 GNU/Linux
