Also check  congested_write_threshold_us, if the SSD is "congested" it bypasses it to speedup writes.

Killian De Volder


On 30-12-16 09:38, Jure Erznožnik wrote:

I have a large array sitting in my server containing a bit of data
that is used ofter and a lot of data that is used rarely.
My goal is to make the platters spin up as rarely as possible while
bcache handles most of the requests to the array.

I have set the disks spin-down time to 15 minutes so that during
high-activity periods they don't cycle power states too much.

I have set bcache parameters such:
cache_mode - writeback
readahead - 0
sequential_cutoff - 0
writeback_delay - 3600 (one hour)
writeback_percent - 40 (won't go higher)

I was hoping that this would make bcache cache everything leaving the
array practically untouched for an hour. However, stats_hour/bypassed
still shows megabytes of bypassed data and a cache_hit ratio of ~50%
even when I'm working ONLY with the frequently accessed data portion
(some 20% of total cache capacity). Also tiny writes are being made to
the array despite writeback_delay not expiring.

What other parameter am I forgetting to achieve what I want?

Thanks,
Jure
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