On Wed, 10 Mar 2010 09:40:42 +0100 Kevin Wolf <kw...@redhat.com> wrote:
> Am 10.03.2010 00:08, schrieb Anthony Liguori: > > On 03/09/2010 04:53 PM, Luiz Capitulino wrote: > >> Hi, > >> > >> This series is based on a previous series submitted by Uri Lublin: > >> > >> http://lists.gnu.org/archive/html/qemu-devel/2009-03/msg00864.html > >> > >> Details on the patches, except for this question: does it make sense to > >> have > >> a 'low' watermark for block devices? > >> > >> I think it doesn't, then the event (and the monitor accompanying command) > >> should be called BLOCK_HIGH_WATERMARK. But this makes the event very > >> unflexible, so I have called it BLOCK_WATERMARK and added parameters for > >> the > >> high/low watermark type. > >> > > > > The alternative way to implement this is for a management tool to just > > poll the allocated disk size periodically. > > Then we need to provide that information using the monitor. As far as I > know, we don't do that yet. No, we don't. We have a 'info blockstats' command though and it could provide image related stats info if needed. > Not doing that would mean that the > management tool would have to open an image which is already in use by > qemu (which is already something I feel uncomfortable about) and search > for the highest allocated cluster (which makes it completely inefficient > and therefore basically forbids the use case). > > Really, we have no choice but to implement the high watermark tracking > in the qemu block layer. The only question is if we have a monitor > command to ask for the current value or if we signal an event if it goes > above a given threshold. > > I don't think I'm really decided on that question. I'm not either and that's why I'd like to hear from the management tool people. > > It's no more/less safe than generating an event on a "watermark" because > > the event is still racy with respect to a guest that's writing very > > quickly to the disk. > > Being racy isn't a problem, a management tool doing this kind of things > needs to use werror=ENOSPC (at least) anyway. The watermark thing, as I > understand it, is only a mechanism to make it less likely that the VM > has to be stopped. > > Kevin
