Re: SMR drive test 2; 128GB partition; no obvious corruption, much more sane behaviour, weird overprovisioning

[Jaegeuk Kim <jaegeuk@kernel.org>]

On Thu, Sep 24, 2015 at 01:43:24AM +0200, Marc Lehmann wrote:
> On Thu, Sep 24, 2015 at 01:30:22AM +0200, Marc Lehmann <schmorp@schmorp.de> wrote:
> > > One thing I note is that gc_min_sleep_time is not be set in your script,
> > > so in some condition gc may still do the sleep with gc_min_sleep_time (30
> > > seconds by default) instead of gc_max_sleep_time which we expect.
> > 
> > Ah, sorry, I actually set gc_min_sleep_time to 100, but forgot to include
> > it.
> 
> Sorry, that sounded confusing - I set it to 100 in previous tests, and forgot
> to include it, so it was running with 30000. When experimenting, I actually
> do get the gc to do more frequent operations now.
> 
> Is there any obvious harm setting it to a very low value (such as 100 or 10)?
> 
> I assume all it does is have less time buffer between the last operation
> and the gc starting. When I write in batches, or when I know the fs will be
> idle, there shouldn't be any harm, performance wise, of letting it work all
> the time.

Yeah, I don't think it does matter with very small time periods, since the timer
is set after background GC is done.
But, we use msecs_to_jiffies(), so hope not to use something like 10 ms, since
each backgroudn GC conducts reading victim blocks into page cache and then just
sets them as dirty.
That indicates, after a while, we hope flusher will write them all to disk and
finally we got a free section.
So, IMO, we need to give some time slots to flusher as well.

For example, if write bandwidth is 30MB/s and section size is 128MB, it needs
about 4secs to write one section.
So, how about setting
 - gc_min_time to 1~2 secs,
 - gc_max_time to 3~4 secs,
 - gc_idle_time to 10 secs,
 - reclaim_segments to 64 (sync when 1 section becomes prefree)

Thanks,

> 
> -- 
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