Re: Small Cache Dev Tuning

[Matthias Ferdinand <mf@mferd.de>]

On Tue, Jun 16, 2020 at 10:57:43AM -0400, Marc Smith wrote:
> This certainly helps me allow more dirty data than what the defaults
> are set to.

I only have production experience with slightly older kernels (4.15) and
~40GB partition of an Intel DC SATA SSD (XFS fs). Average latency of the
bcache device improved a lot with _reduced_ writeback_percent. I guess
dirty block bookkeeping adds its own I/O.
Currently I run them even at writeback_percent=1.

Not exactly answering your question, though :-)

Matthias


 But a couple other followup questions:
> - Any additional recommended tuning/settings for small cache devices?
> - Is the soft threshold for dirty writeback data 70% so there is
> always room for metadata on the cache device? Dangerous to try and
> recompile with larger maximums?
> - I'm still studying the code, but so far I don't see this, and wanted
> to confirm that: The writeback thread doesn't look at congestion on
> the backing device when flushing out data (and say pausing the
> writeback thread as needed)? For spinning media, if lots of latency
> sensitive reads are going directly to the backing device, and we're
> flushing a lot of data from cache to backing, that hurts.
> 
> 
> Thanks,
> 
> Marc