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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: Mon, 13 Aug 2018 18:24:21 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

Am 13.08.2018 um 18:08 hat Max Reitz geschrieben:
> >>> default should reflect that, especially considering that we only use
> >>> the memory on demand. If your image is only 32 GB, you'll never use more
> >>> than 4 MB of cache.
> >>
> >> Well, OK, yes.  This is an especially important point when it really is
> >> about hosts that have limited memory.  In those cases, users probably
> >> won't run huge images anyway.
> >>
> >>>                     And if your image is huge, but only access part of
> >>> it, we also won't use the full 32 MB.
> >>
> >> On Linux. O:-)
> > 
> > No, on any system where qemu_try_blockalign() results in a COW zero
> > page.
> OK, yes, but why would you only ever access part of it?  Then you might
> just as well have created a smaller disk from the beginning.

I always create my qcow2 images larger than I actually need them. It
costs basically nothing and avoids the need to resize my partitions
inside the guest later.

And anyway, a disk with 100% usage is not the common case, but the point
where the user will either delete stuff or resize the image. For
long-running VMs, deleting stuff doesn't get rid of the large cache on
non-Linux, but I think we agree that long-running guests aren't what we
expect on those hosts?

> > The Linux-only addition is returning memory even after an access.
> > 
> >> So it's good that you have calmed my nerves about how this might be
> >> problematic on Linux systems (it isn't in practice, although I disagree
> >> that people will find qcow2 to be the fault when their memory runs out),
> >> but you haven't said anything about non-Linux systems.  I understand
> >> that you don't care, but as I said here, this was my only substantial
> >> concern anyway.
> > 
> > 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.
> Will your typical desktop VMs gain anything from the cache covering
> more than 8 GB?

Good point. Probably not.

> Anyway, I certainly won't complain about 4 MB.
> (My point here is that on non-Linux systems, qemu probably does not have
> users who have use cases where they need to access 256 GB of disk
> simultaneously.  Probably not even more than 8 GB.  If you want to
> increase the cache size there to 4 MB, fine, I think that won't hurt.
> But 32 MB might hurt, and I don't think on non-Linux systems there are
> users who would benefit from it -- specifically because your "typical
> desktop VM" wouldn't benefit from it.)

Maybe 1 MB is fine for them, after all.


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