On Tue, 2011-01-04 at 09:32 -0500, Trond Myklebust wrote: > On Tue, 2011-01-04 at 00:44 -0800, Daniel Stodden wrote: > > Hi anyone. > > > > If somebody's got a sec to enlighten me, there's some phenomenon I > > recently came across and found somewhat counterintuitive first. > > > > Whenever I > > > > 1. Dirty a bunch of pages backed by an NFS mount on some server. > > > > 2. Block the traffic with iptables (TCP, assuming that mattered). > > Still plenty of writeback pending. > > > > 3. Sync > > > > I see #3 drive the dirty count in /proc/meminfo drop back to > > almost-zero, immediately. The sync itself blocks, though. > > > > So the pages are called clean the moment the write got queued, not > > acked? Leaving the rest just to retransmits by the socket then? Is this > > just done so because one can, or would that order rather matter for > > consistency? > > Take a look at the 'Writeback:' count, which should turn non-zero when > you hit #3. > > The VM allows pages to be either dirty or in writeback, but not both at > the same time. This is not NFS-specific. The same rule applies to local > filesystems. Ah. That explains everything. Actually a question then, thanks for the clarification :) Rob Landley's comment regarding tx queue size somewhat made a good point too. But, given the rates I see, this queues mostly cache pages on the transport, not copies, right? Thanks. Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
