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

From: Leonid Bloch
Subject: Re: [Qemu-block] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size
Date: Tue, 14 Aug 2018 15:29:05 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0

On 8/14/18 2:44 PM, Kevin Wolf wrote:
Am 14.08.2018 um 13:34 hat Leonid Bloch geschrieben:
On 8/14/18 11:18 AM, Kevin Wolf wrote:
Am 13.08.2018 um 18:42 hat Leonid Bloch geschrieben:
I don't actually think it's so bad to keep the cache permanently
allocated, but I wouldn't object to a lower default for non-Linux hosts
either. 1 MB may still be a little too low, 4 MB (covers up to 32 GB)
might be more adequate. My typical desktop VMs are larger than 8 GB, but
smaller than 32 GB.

And for a Windows VM just the OS installation takes above 40 GB. While we
probably are not running Windows VMs for our own needs, it is very common
that a customer of, for example, some cloud service uses QEMU (unknowingly)
for a full-blown Windows. So 100 GB+ images which are quite heavily used is
not a rare scenario. 256 GB - yeah, that would be on the higher end.

The OS installation is mostly sequential access, though. You only need
that much cache when you have completely random I/O across the whole
image. Otherwise the LRU based approach of the cache is good enough to
keep those tables cached that are actually in use.

Sorry, by "OS installation" I meant the installed size of the OS, which
should be available for fast and frequent access, not the installation
process itself. Obviously for one-time tasks like the installation process
it's not worth it, unless one installs all the time, instead of using ready
images, for some reason. :)

But you never use everything that is present in an OS installation of
40 GB (is it really _that_ huge these days?), and you don't read OS
files non-stop. The most frequently used parts of the OS are actually in
the guest RAM.

Yes, Windows 8.1, with all the desktop bloat - just above 40 GB. :]
I did a proper benchmarking indeed only on heavy I/O load, where full cache did show above 50% improvement, although just regular usage felt faster as well, but maybe it's just psychosomatic. :)


I don't think you'll really notice the difference in qcow2 unless you
have a really I/O intensive workload - and that is not usually for OS
files, but for user data. For only occasional accesses, the additional
64k for the metadata table wouldn't play a big role.


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