bcache not working on large files?

[Dirk Geschke <dirk-WMiem5eIfR0n5izryJqWLw@public.gmane.org>]

Hi all,

I was just playing a little bit with bcache and it works fine. But
if I try random IOPS writes (writeback) on a file larger than the
cache, it seems not to work? At least I get a performance as without
bcache.

I used a RAID-6 of 8 SSDs (each 238 GiB) as a cache and a RAID-6 of
10 HDDs (2794 Gib). For my test I did random writes of 4k blocks.
With 8 threads writing on one 1000 GB file, I get IOPS in the range
of 20000, but with a file of 10000 GB I end up with about 400 IOPS.
That is the same as without cache. 

Did I miss something? Is caching disabled in such cases?

I would expect, that even in this case (writeback and random writes)
it should work as well. Even after a few minutes I get these results,
so it is not a problem of a full cache.

Has anyone a hint for me, what is going wrong?

Best regards

Dirk

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