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Re: [Qemu-block] [PATCH v3 2/5] qcow2: Make the default L2 cache suffici

From: Leonid Bloch
Subject: Re: [Qemu-block] [PATCH v3 2/5] qcow2: Make the default L2 cache sufficient to cover the entire image
Date: Wed, 8 Aug 2018 17:35:19 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 08/08/2018 04:58 PM, Alberto Garcia wrote:
On Wed 08 Aug 2018 03:07:10 PM CEST, Leonid Bloch wrote:
On 08/08/2018 03:39 PM, Alberto Garcia wrote:
On Wed 08 Aug 2018 09:10:48 AM CEST, Leonid Bloch wrote:
Sufficient L2 cache can noticeably improve the performance when using
large images with frequent I/O. The memory overhead is not significant
in most cases, as the cache size is only 1 MB for each 8 GB of virtual
image size (with the default cluster size of 64 KB). For cases with very
large images and/or small cluster sizes, there is an upper limit on the
default L2 cache: 32 MB.

I find this description a bit confusing.

First of all, because it's not true that the default will cover the
whole image: we're just increasing it, but any image > 256GB is going to
need more than 32MB (with 64KB clusters, that is).

How about the following:

qcow2: Make the default L2 cache try to cover the entire image

That's what I think it's misleading, the patch only increases the
default cache size value. The current 1MB cache also tries to cover the
entire image, if the entire image is <= 8GB. But there's no new feature
that extends the cache size to cover the entire image. There's no "we
used to cover 8GB images by default, but now we cover all images".

In other words, the message that I think the user should get is: set
l2-cache-size as high as the maximum amount of memory you're willing to
use for the cache, and QEMU will ensure that it will only use as much as
it needs and no extra memory will be wasted. If you're working with a
100GB image it doesn't matter if you set l2-cache-size to 32MB or
1GB. It will use the exact same amount of RAM.


qcow2: Make the L2 cache size relative to the size of the image

[Something like] Now the L2 cache will be only as large as needed to cover the entire image, until a preset maximum (which is 32 MB by default).

The way I see it: there are two simple changes from the user's point of
view (they can even be two separate patches).

1) The default l2-cache-size is now 32MB. DEFAULT_L2_CACHE_CLUSTERS is
     useless now and disappears.
I don't think that it can be a separate patch, because unless the other
logic is changed, the cache will occupy 32 MB *always*, regardless of
the image size, and that's quite a big and unneeded overhead.

Change the order of both patches then :-)

Do you really think it's necessary? The increase of the default max size is directly tied to the functionality change: it will be harmful to increase the maximum before the new functionality is implemented, and there is no need to change the functionality if the default max is not increased.


  1) If l2-cache-size > l2_metadata_size, then make l2-cache-size =
     l2_metadata_size. This is already useful on its own, even with the
     current default of 1MB.

  2) Increase the default to 32MB. This won't waste additional memory for
     small images because of the previous patch, and will cover images up
     to 256GB. If you have larger images you would need to increase
     l2-cache-size manually if you want to cache all the L2 metadata.

The way I see it is that the important change is (1): it's safe to
increase l2-cache-size, it won't waste memory[*]. (2) is nice too
because it will cover larger images by default, but if you're running
VMs with disk images larger than 256GB (which is not an extreme
scenario) you still need to increase the cache.

[*] In practice it's not wasting too much memory even now, because even
if you allocate a 32MB cache but only need 1MB, those extra 31MB are
going to be uninitialized and unused, and therefore not taking up any
physical memory. But you still need to have Qcow2CachedTable structures
for those entries, and any loop that needs to walk Qcow2Cache.entries is
going to be slower. So it's a good idea not to allocate unneeded memory
and to make this an explicit promise to the user.


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