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

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Date:   Sat, 15 Oct 2022 16:27:12 -0600
From:   Jonathan Derrick <jonathan.derrick@...ux.dev>
To:     John Stoffel <john@...ffel.org>
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



On 10/14/2022 3:10 PM, John Stoffel wrote:
>>>>>> "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? 
I'll adjust it to chunk-count-based in the cover letter and make sure it
specifies bandwidth. I figured the chunk-count-based was a good way to
cover the desired percentage-based feature [1]. 

[1] https://elixir.bootlin.com/linux/latest/source/drivers/md/md-bitmap.c#L16


> 
> 
>> 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.  

40% or more of given workload I/Os being bitmap flushes.
Will be more clear in v3

> 
>> 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
>