On 01/05/2011 05:15 PM, Daniel Stodden wrote:
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.
Not nearly as good as I thought at the time.
> But, given the rates I see, this queues mostly cache pages on the
transport, not copies, right?
Easy enough to tell: there's a "Writeback" field in /proc/meminfo. Add
'em up and see what's missing.
Rob
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