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Re: [Qemu-block] [PATCH] qcow2: Optimize L2 table cache size based on im

From: Ed Swierk
Subject: Re: [Qemu-block] [PATCH] qcow2: Optimize L2 table cache size based on image and cluster sizes
Date: Tue, 4 Oct 2016 08:36:31 -0700

On Tue, Oct 4, 2016 at 7:31 AM, Alberto Garcia <address@hidden> wrote:
> That might be a lot of memory if the image is big. 1 TB qcow2 image ==
> 128 MB L2 cache.
> See https://bugzilla.redhat.com/show_bug.cgi?id=1377735#c2
> If we want to add this feature, I think I'd rather make it explicit.

I agree explicit is generally better than automagical, but unlike say
the VM RAM size, the qcow L2 table cache is an implementation detail
no user should be expected to know exists, let alone needs tweaking
according to a specific formula to avoid an arbitrary performance

At least giving users a way to skip the math would be an improvement.
Would you be okay with an explicitly-set option like
l2_cache_size=auto or =max that optimizes for performance at the
expense of memory?


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