Re: [PATCH] ubi: Select fastmap anchor PEBs considering wear level rules

[Richard Weinberger <richard.weinberger@xxxxxxxxx>]

On Thu, Apr 30, 2020 at 4:29 PM Richard Weinberger
<richard.weinberger@xxxxxxxxx> wrote:
>
> On Thu, Apr 30, 2020 at 10:34 AM Arne Edholm <arne.edholm@xxxxxxxx> wrote:
> > Are you satisfied with my answers or are there additional
> > information/changes needed?
>
> Yes. In the meanwhile I did more testing and with your changes the
> anchor PEB selection
> is *much* better. Testing took some time and then I scheduled away to
> other stuff...
> Critical workloads are these where a fastmap is not written due to
> heavy write load,
> but other events like volume change/creation.
>
> A good reproducer seems something stupid like that:
> for i in `seq 1000` ; do ubimkvol -N test -m /dev/ubi0 >/dev/null &&
> ubirmvol /dev/ubi0 -n 0 ; done
> Wearleveling threshold is 50, btw.
>
> Without your patch, the erase counter of the first 64 PEBs:
> 4    4    4    4    4    4    4    4
> 4    4    4    4    4    4    4    4
> 4    4    4    4    4    4    4    4
> 4    4    4    4    4    4    4    4
> 4    4    4    4    4    4    4    4
> 4    4    4    4    4    4    22   4
> 4    19   4    4    4    4    4    4
> 4    4    4    4    8    908  906  1
>
> And with your patch:
> 95   95   95   95   95   95   95   95
> 95   95   95   95   95   95   95   95
> 95   95   95   95   95   95   95   95
> 95   95   95   95   95   95   95   95
> 95   95   95   95   95   95   95   95
> 95   95   95   95   95   95   95   95
> 95   95   95   94   94   94   94   94
> 94   94   94   94   94   94   94   95

While observing my own mail on the mailing list I discovered something
I didn't notice
last time on my terminal.
If we summarize all numbers in the squares it should be more or less 2000.
because the test triggered 2000 fastmap writes.
But 95 times 64 is much more than 2000.

Your patch produces almost a perfect distribution, the overall erase count
is three times as much as it is expected.
Hmmm.

-- 
Thanks,
//richard

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