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Re: [Xen-API] sharing NFS SRs

> On 26.05.2012 12:57, Dave Scott wrote:
> > Hi,
> >
> > IMHO one of the weaknesses of the current NFS SR backend in XCP is
> that it a single SR cannot be shared between pools. This is because the
> backend relies on the xapi pool framework to prevent
> >
> > 1. multiple hosts from coalescing the same vhds.
> >
> > 2. the same vhd being attached to two VMs at the same time.
> >
> > 3. a vhd being read one one node even after it has been coalesced and
> deleted on another
> >
> > If multiple pools could safely share the same NFS SR then a cross-
> pool migrate (which is possible with the current code) wouldn't have to
> actually mirror the disks.
> >
> > With this in mind I've been looking into NFS locking again. I realize
> this is a... tricky thing to get right... and google turns up lots of
> horror stories. Anyway, here's what I was thinking:
> >
> > For handling (1) and (2), we would only need one lock file (really a
> "lease file") per vhd. In the event of a network interruption we
> already know that running VMs are likely to fail after 90s or so -- the
> maximum time (IIRC) a windows VM will allow a page file write to take.
> So we could
> >
> > * explicitly tell tapdisk to shutdown after this long (since the VM
> will probably have blue-screened anyway)
> >
> > * periodically refresh our leases, setting them to expire well after
> the tapdisks are guaranteed to have shutdown
> >
> > So if a host leaves the network, all disks become unlocked a few
> minutes later and the VMs (and coalesce jobs) can safely be restarted
> on another pool. This could then be used as the foundation for a new
> "HA" feature, where only VMs whose I/Os have failed are shutdown and
> restarted.

George wrote:
> I've somehow afraid idea of 'leasing' operation (and periodic
> open/close
> operation).
> Here some scenarios to think about:
> 1) temporal loss of the host SAN connectivity. NFS on the host is going
> to interruptible sleep and continue IO as soon as we get connectivity
> back. We already kill tapdisk, remove lease, restart vm on other host
> and suddenly networking is revived... And pending NFS write operation
> is
> going straight in the middle of 'mission critical' database with fresh
> 'week after expiration date' data. May be weeks later after 'issue'
> with
> VM restart.
> 2) SR live migration is still very important feature I very hope to see.
> 3) Those leases will create additional IO. F.e. if we do have ~20k VMs
> (not really large number for clouds of new age) and lease is 10 minutes,
> it wll create ~33 IOPS - equivalent about 60-70 VMs (according to
> statistic from our cloud).
> 4) how do you plan to guarantee to tapdisk shutdown (this is NFS, if
> server is down or some issues with connectivity, there is no way to
> shut
> down locked in IO process)?
> 5) I think 30s is not very good number. Linux kernel starts to throwing
> IO errors after 120 seconds of IO wait.
> 6) about this library: '''you also need to make sure that your clocks
> are properly synchronized. """ I think this must add requirement to
> coexisting of hosts: do not allow to plug nfs sr until clock is synced
> with master. (Same for cross-pool migration - reject migration if clock
> is out of sync, but allow to shoot own leg with --force).

I see what you mean, perhaps leasing isn't a good idea.

Perhaps we can take advantage of driver domains (which we'll have good support 
for soon).

We could:

* acquire a lock (not a lease) whenever we use a .vhd file
* in the driver domain, a single process will send heartbeat I/O to the NFS 
* each tapdisk inside the driver domain is unmodified (so no tapdisk lease 
refresh generating extra I/O)
* when the heartbeat I/O fails for more than 'n' seconds (TBD), the whole 
domain is rebooted (either cleanly or forcibly by the hypervisor watchdog?)
* when another host (the pool master?) sees the driver domain has shutdown, it 
knows it can forcibly release the locks since all the buffers will have been 
thrown away

I quite like the idea of sending some heartbeats to the NFS server, since we 
don't really handle (or even detect) storage failure very well at the moment.

If we also support blkback/blkfront reconnect then the guest VBDs will be able 
to reconnect after the driver domain has rebooted-- that would be good.

What do you think?


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