Re: [Qemu-block] [PATCH v7 6/9] qcow2: Increase the default upper limit on the L2 cache size

[Leonid Bloch]

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. :)

Leonid.

> 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