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Re: [Qemu-block] [PATCH v7 6/9] qcow2: Increase the default upper limit

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size
Date: Mon, 13 Aug 2018 17:16:43 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 2018-08-13 08:09, Leonid Bloch wrote:
> On August 13, 2018 4:39:35 AM EEST, Max Reitz <address@hidden> wrote:


>> Ideally we'd probably want a soft and a hard cache limit, but I don't
>> know...
>> (Like, a soft cache limit of 1 MB with a CCI of 10 min, and a hard
>> cache
>> limit of 32 MB with a CCI of 1 min by default.  So whenever your cache
>> uses more than 1 MB of RAM, your CCI is 1 min, and whenever it's below,
>> your CCI is 10 min.)
> Max, thanks for your insight. Indeed some good points.
> Considering this, I'm thinking to set the limit to 16 MB, and the CCI to 5 
> min. What do you think?

I think it's good for a preliminary solution, and then later increase
the limit with the soft and hard limits.

OTOH, if we implement the soft/hard limits, it doesn't really matter
what default you choose now...

> Modern Windows installations should gain performance from being able to 
> random I/O to >8 GB chunks, and data processing tasks where each data set is 
> 8+ GB for sure do (did benchmarks). And the maximum is only ever used if (a) 
> the image is large enough and (b) it is indeed used.
> While taking 256 GB images as the "limit" can be considered an overshoot, 128 
> GB is quite reasonable, I think.
> Your idea with "soft" and "hard" limits is great! I'm tempted to implement 
> this. Say 4 MB with 10 min., and 16 MB with 5 min?

32 MB and 2 or 3 min? :-)

If you do that, I'm fine with a plain default of 32 MB for now.


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