linux-kernel - Re: Disabling in-memory write cache for x86-64 in Linux II

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Date:	Fri, 25 Oct 2013 21:03:44 +0000 (UTC)
From:	"Artem S. Tashkinov" <t.artem@...os.com>
To:	neilb@...e.de
Cc:	david@...g.hm, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	axboe@...nel.dk, linux-mm@...ck.org
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

Oct 26, 2013 02:44:07 AM, neil wrote:
On Fri, 25 Oct 2013 18:26:23 +0000 (UTC) "Artem S. Tashkinov"
>> 
>> Exactly. And not being able to use applications which show you IO performance
>> like Midnight Commander. You might prefer to use "cp -a" but I cannot imagine
>> my life without being able to see the progress of a copying operation. With the current
>> dirty cache there's no way to understand how you storage media actually behaves.
>
>So fix Midnight Commander.  If you want the copy to be actually finished when
>it says  it is finished, then it needs to call 'fsync()' at the end.

This sounds like a very bad joke. How applications are supposed to show and
calculate an _average_ write speed if there are no kernel calls/ioctls to actually
make the kernel flush dirty buffers _during_ copying? Actually it's a good way to
solve this problem in user space - alas, even if such calls are implemented, user
space will start using them only in 2018 if not further from that.

>> 
>> Per device dirty cache seems like a nice idea, I, for one, would like to disable it
>> altogether or make it an absolute minimum for things like USB flash drives - because
>> I don't care about multithreaded performance or delayed allocation on such devices -
>> I'm interested in my data reaching my USB stick ASAP - because it's how most people
>> use them.
>>
>
>As has already been said, you can substantially disable  the cache by tuning
>down various values in /proc/sys/vm/.
>Have you tried?

I don't understand who you are replying to. I asked about per device settings, you are
again referring me to system wide settings - they don't look that good if we're talking
about a 3MB/sec flash drive and 500MB/sec SSD drive. Besides it makes no sense
to allocate 20% of physical RAM for things which don't belong to it in the first place.

I don't know any other OS which has a similar behaviour.

And like people (including me) have already mentioned, such a huge dirty cache can
stall their PCs/servers for a considerable amount of time.

Of course, if you don't use Linux on the desktop you don't really care - well, I do. Also
not everyone in this world has an UPS - which means such a huge buffer can lead to a
serious data loss in case of a power blackout.

Regards,

Artem
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