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

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size
Date: Tue, 14 Aug 2018 13:44:50 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

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.

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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