Hi all!
I'm using bcache and I think I have a rather low hit ratio and cache
occupation.
My setup is:
- Cache device: 82 GiB partition on a SSD drive. Bucket size=4M. The
partition is aligned on a Gigabyte boundary.
- Backing device: 3.6 TiB partition on a HDD drive. There is 732 GiB of
data usage on this partition. This 732 GiB are used by 9 qcow2 files
assigned to 3 VMs running on the host.
- Neither the SDD nor HDD drives have another partitions in use.
- After 24 hours of use, according to priority_stats the cache is 75%
Unused (63 GiB Unused - 19 GiB used), but...
- ... according to "smartctl -a" in those 24 hours "Writes to Flash" has
increased in 160 GiB and "GB written from host" has increased in 90 GiB
- cache_hit_ratio is 10 %
- I'm using maximum bucket size (4M) trying to minimize write
amplification. With this bucket size, "Writes to Flash" (160) to "GB
written from host"(90) ratio is 1,78. Previously, some days ago, I was
using default bucket size. The write amplification ratio then was 2,01.
- Isn't the cache_hit_ratio (10%) a bit low?
- Is it normal that, after 24 hours running, the cache occupation is
that low (82-63 = 19GiB, 25%) when the host has written 90 GiB to the
cache device in the same period? I don´t understand why 90 GiB of data
has been written to fill 19 GiB of cache.
Any ideas?
Thank you and regards.
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