On 3/29/23 02:38, Adriano Silva wrote:
Hey guys, I'm using bcache to support Ceph. Ten Cluster nodes have a bcache device each consisting of an HDD block device and an NVMe cache. But I am noticing what I consider to be a problem: My cache is 100% used even though I still have 80% of the space available on my HDD. It is true that there is more data written than would fit in the cache. However, I imagine that most of them should only be on the HDD and not in the cache, as they are cold data, almost never used. I noticed that there was a significant drop in performance on the disks (writes) and went to check. Benchmark tests confirmed this. Then I noticed that there was 100% cache full and 85% cache evictable. There was a bit of dirty cache. I found an internet message talking about the garbage collector, so I tried the following: echo 1 > /sys/block/bcache0/bcache/cache/internal/trigger_gc That doesn't seem to have helped. Then I collected the following data: --- bcache --- Device /dev/sdc (8:32) UUID 38e81dff-a7c9-449f-9ddd-182128a19b69 Block Size 4.00KiB Bucket Size 256.00KiB Congested? False Read Congestion 0.0ms Write Congestion 0.0ms Total Cache Size 553.31GiB Total Cache Used 547.78GiB (99%) Total Unused Cache 5.53GiB (1%) Dirty Data 0B (0%) Evictable Cache 503.52GiB (91%) Replacement Policy [lru] fifo random Cache Mode writethrough [writeback] writearound none Total Hits 33361829 (99%) Total Missions 185029 Total Bypass Hits 6203 (100%) Total Bypass Misses 0 Total Bypassed 59.20MiB --- Cache Device --- Device /dev/nvme0n1p1 (259:1) Size 553.31GiB Block Size 4.00KiB Bucket Size 256.00KiB Replacement Policy [lru] fifo random Discard? False I/O Errors 0 Metadata Written 395.00GiB Data Written 1.50 TiB Buckets 2266376 Cache Used 547.78GiB (99%) Cache Unused 5.53GiB (0%) --- Backing Device --- Device /dev/sdc (8:32) Size 5.46TiB Cache Mode writethrough [writeback] writearound none Readhead Sequential Cutoff 0B Sequential merge? False state clean Writeback? true Dirty Data 0B Total Hits 32903077 (99%) Total Missions 185029 Total Bypass Hits 6203 (100%) Total Bypass Misses 0 Total Bypassed 59.20MiB The dirty data has disappeared. But the cache remains 99% utilization, down just 1%. Already the evictable cache increased to 91%! The impression I have is that this harms the write cache. That is, if I need to write again, the data goes straight to the HDD disks, as there is no space available in the Cache. Shouldn't bcache remove the least used part of the cache?
I don't know for sure, but I'd think that since 91% of the cache is evictable, writing would just evict some data from the cache (without writing to the HDD, since it's not dirty data) and write to that area of the cache, *not* to the HDD. It wouldn't make sense in many cases to actually remove data from the cache, because then any reads of that data would have to read from the HDD; leaving it in the cache has very little cost and would speed up any reads of that data.
Regards, -Martin
Does anyone know why this isn't happening? I may be talking nonsense, but isn't there a way to tell bcache to keep a write-free space rate in the cache automatically? Or even if it was manually by some command that I would trigger at low disk access times? Thanks!
