linux-kernel - Re: [PATCH v2 0/3] Bitmap percentage flushing

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Date:   Fri, 14 Oct 2022 17:10:30 -0400
From:   "John Stoffel" <john@...ffel.org>
To:     Jonathan Derrick <jonathan.derrick@...ux.dev>
Cc:     Song Liu <song@...nel.org>, <linux-raid@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, jonathan.derrick@...idigm.com,
        jonathanx.sk.derrick@...el.com,
        Mariusz Tkaczyk <mariusz.tkaczyk@...ux.intel.com>
Subject: Re: [PATCH v2 0/3] Bitmap percentage flushing

>>>>> "Jonathan" == Jonathan Derrick <jonathan.derrick@...ux.dev> writes:

> This introduces a percentage-flushing mechanism that works in-tandem to the
> mdadm delay timer. The percentage argument is based on the number of chunks
> dirty (rather than percentage), due to large drives requiring smaller and
> smaller percentages (eg, 32TB drives-> 1% is 320GB).

I've been reading and re-reading this and I still don't understand
what you're saying here.  You say you're adding a percentage based
mechanism, but then you say it's based on chunk counts, not
percentages.  I think you need to clean this up and re-word it.

Maybe you're trying to say that you only take a percentage of the
available write bandwidth per second or something like that? 


> This set hopes to provide a way to make the bitmap flushing more consistent. It
> was observed that a synchronous, random write qd1 workload, could make bitmap
> writes easily become almost half of the I/O. And in similar workloads with
> different timing, it was several minutes between bitmap updates. This is too
> inconsistent to be reliable.

> This first and second patches adds the flush_threshold parameter. The default
> value of 0 defines the default behavior: unplugging immediately just as before.
> With a flush-threshold value of 1, it becomes more consistent and paranoid,
> flushing on nearly every I/O, leading to a 40% or greater situation. From

What situation?  Please be more clear here.  

> there, the flush_threshold can be defined higher for those situations where
> power loss is rare and full resync can be tolerated.

> The third patch converts the daemon worker to an actual timer. This makes it
> more consistent and removes some ugly code.

> Jonathan Derrick (3):
>   md/bitmap: Add chunk-threshold unplugging
>   md/bitmap: Add sysfs interface for flush threshold
>   md/bitmap: Convert daemon_work to proper timer

>  Documentation/admin-guide/md.rst |  5 ++
>  drivers/md/md-bitmap.c           | 98 +++++++++++++++++++++++++-------
>  drivers/md/md-bitmap.h           |  4 +-
>  drivers/md/md.c                  |  9 ++-
>  drivers/md/md.h                  |  2 +
>  5 files changed, 93 insertions(+), 25 deletions(-)

> -- 
> 2.31.1