Hi I am trying to use the xvm/xen in OpenSolaris on the top of zvol. The combination of xen with zfs might be the tricky and does not have to deliver what I expect. I am worried that the xen (or any other VM) with zvol device from ZFS (or any other file system.) may remove the benefits of ZFS, because the guest OS can write something to disk but the xen itself can do some buffering or lazy writing and it may delay to write to ZFS backend. Does it make sense to use ZFS with xen to protect the guest completely against accidental power outage or against the broken disk/cables? I understand there is no implicit persistence implemented in the xen/xvm, so the actual state of the guest system is lost on the power outage. However I would like to have a consistent file system in the guest OS stored physicaly on the mirrored backend zfs. to protect explicitly written data. Can someone point me to a relevent documentations so I can read more about it? I probably need some guarantees claims from ZFS and some guarantees claims from xen and then I can judge. I expect you (as a scientist) have much better overview of the proper documentation or at least the ideas on the topic the virtual machines and what in theory can and what cannot be guaranteed. Will ever the persistence be implemented the way the EROS/Coyotos or L3/L4 has? Marian Klein
Hello, marian klein, le Tue 15 Jul 2008 11:22:05 +0000, a écrit :> I am worried that the xen (or any other VM) with zvol device from ZFS > (or any other file system.) may remove the benefits of ZFS, > because the guest OS can write something to disk but the xen itself > can do some buffering > or lazy writing and it may delay to write to ZFS backend.Xen doesn''t do any kind of buffering or lazy writing. It provides barriers to the guest, to ensure any needed coherency. Samuel
> marian klein, le Tue 15 Jul 2008 11:22:05 +0000, a écrit : > > I am worried that the xen (or any other VM) with zvol > device from ZFS > > (or any other file system.) may remove the benefits of ZFS, > > because the guest OS can write something to disk but the xen itself > > can do some buffering > > or lazy writing and it may delay to write to ZFS backend. > > Xen doesn''t do any kind of buffering or lazy writing. It provides > barriers to the guest, to ensure any needed coherency. > > SamuelI''m not sure if this is related or not... I was party to some internal irc where it was believed to be the case that when a file-backed vbd using file:''...'' is used, dom0''s page cache may contain buffer pages that may be inconsistent with the disk, whereas a file-backed vbd using tap:aio:''...'' does not suffer from the same problem. Can someone confirm or deny (and if denied, was this fixed recently)? Thanks, Dan
Dan Magenheimer, le Tue 15 Jul 2008 12:32:53 -0600, a écrit :> > marian klein, le Tue 15 Jul 2008 11:22:05 +0000, a écrit : > > > I am worried that the xen (or any other VM) with zvol > > device from ZFS > > > (or any other file system.) may remove the benefits of ZFS, > > > because the guest OS can write something to disk but the xen itself > > > can do some buffering > > > or lazy writing and it may delay to write to ZFS backend. > > > > Xen doesn''t do any kind of buffering or lazy writing. It provides > > barriers to the guest, to ensure any needed coherency. > > I''m not sure if this is related or not... > > I was party to some internal irc where it was believed > to be the case that when a file-backed vbd using file:''...'' > is used, dom0''s page cache may contain buffer pages that > may be inconsistent with the diskAh, indeed, in that case the page cache is used, and the barriers may not have proper effect.> whereas a file-backed vbd using tap:aio:''...'' does not suffer from the > same problem.blktap uses O_DIRECT to avoid buffering completely indeed. Samuel