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Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [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.
Kevin
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, (continued)
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Kevin Wolf, 2018/08/13
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Max Reitz, 2018/08/13
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Kevin Wolf, 2018/08/13
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Max Reitz, 2018/08/13
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Kevin Wolf, 2018/08/13
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Leonid Bloch, 2018/08/13
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Kevin Wolf, 2018/08/14
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Leonid Bloch, 2018/08/14
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size,
Kevin Wolf <=
- Re: [Qemu-devel] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size, Leonid Bloch, 2018/08/14
[Qemu-devel] [PATCH v7 7/9] qcow2: Resize the cache upon image resizing, Leonid Bloch, 2018/08/10
[Qemu-devel] [PATCH v7 8/9] qcow2: Set the default cache-clean-interval to 10 minutes, Leonid Bloch, 2018/08/10
[Qemu-devel] [PATCH v7 9/9] qcow2: Explicit number replaced by a constant, Leonid Bloch, 2018/08/10