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. Kevin
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