On Wed, Sep 13, 2017 at 01:20:03PM +0200, Peter Krempa wrote: > On Wed, Sep 13, 2017 at 17:21:23 +0800, Liu Qing wrote: > > Random write IOPS will drop dramatically if qcow2 l2 cache could not > > cover the whole disk. This patch gives libvirt user a chance to adjust > > the qcow2 cache configuration. > > > > Three new qcow2 driver parameters are added. They are l2-cache-size, > > refcount-cache-size and cache-clean-interval. > > > > The following are from qcow2-cache.txt. > > The amount of virtual disk that can be mapped by the L2 and refcount > > caches (in bytes) is: > > disk_size = l2_cache_size * cluster_size / 8 > > disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits > > > > The parameter "cache-clean-interval" defines an interval (in seconds). > > All cache entries that haven't been accessed during that interval are > > removed from memory. > > > > Signed-off-by: Liu Qing <liuq...@huayun.com> > > --- > > Change since v3: a) copy qcow2 cache configurion from source to backing$ > > to backing file source.$ > > This looks like one of the tuning parameters which really is hard for > users to set and thus it should be justified properly if we need it. [1] > > From the commit message above it looks like that there are guidelines > how to set them. Can't we just make them implicit and not expose > anything to tune the settings? Does it make sense to do so? Are there > any drawbacks? > > If any of them need to be configured by the user, please describe that > in detail why it's necessary. > > Peter > > [1] There's discussion I can link to for other tuning parameters. The > gist is that allowing users to set something withoug giving them > guidance is pointless since they might not use it. Also if the guidance > is strict (e.g. a formula, libvirt or qemu should set the defaults > properly and not force users to do the calculation) The guidance could be found in doc/qcow2-cache.txt in qemu source code. As John Ferlan suggested I have added the file locaton in formatdomain.html.in.
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