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

[Marc Lehmann <schmorp@schmorp.de>]

On Fri, Sep 25, 2015 at 10:45:46AM -0700, Jaegeuk Kim <jaegeuk@kernel.org> wrote:
> > He :) It's a nothing-special number between 64 and 128, that's all.
> 
> Oh, then, I don't think that is a good magic number.

Care to share why? :)

> It seems that you decided to use -s64, so it'd better to keep it to address
> any perf results.

Is there anysthing specially good for numbers of two? Or do you just want top
reduce the number of changed variables?

I'f yes, should I do the 3.18.21 test with -s90 (as the 3.18.21 and 4.2.1
tests before), or with -s64?

> > And just filling these 8TB disks takes days, so the question is, can I
> > simulate near-full behaviour with smaller partitions.
> 
> Why not? :)
> I think the behavior should be same. And, it'd good to set small sections
> in order to see it more clearly.

The section size is a critical parameter for these drives. Also, the data
mix is the same for 8TB and smaller partitions (in these tests, which were
meantr to be the first round of tests only anyway).

So a smaller section size compared to the full partition test, I think,
would result in very different behaviour. Likewise, if a small partition
has comparatively more (or absolutely less) overprovision (and/or reserved
space), this again might cause different behaviour.

At least to me, it's not obvious what a good comparable overprovision ratio
is to test full device behaviour on a smaller partition.

Also, section sizes vary by a factor fo two over the device, so what might
work fine with -s64 in the middle of the disk, might work badly at the end.

Likewise, since the files don't get larger, the GC might do a much better
job at -s64 than at -s128 (almost certainly, actually).

As a thought experiment, what happens when I use -s8 or a similar small size?
If the GC writes linearly, there won't be too many RMW cycles. But is that
guaranteed even with an aging filesystem?

If yes, then the best -s number might be 1. Because all I rely on is
mostly linear batched large writes, not so much large batched reads.

That is, unfortunately, not something I can easily test.

> Let me test this patch for a while, and then push into our git.

Thanks, will do so, then.

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