9fans archive / 2008 / 09 / 226 /    prev next

From: Nathaniel W Filardo <nwf@cs.jhu.edu>
Subject: Re: [9fans] 9p over high-latency
Date: Fri, 26 Sep 2008 21:17:18 -0400

On Thu, Sep 18, 2008 at 05:57:21PM +0800, sqweek wrote:
> On Fri, Sep 12, 2008 at 7:47 PM, erik quanstrom <quanstro@qua...> wrote:
> > as an aside: i don't think 9p itself limits plan 9 performance
> > over high-latency links.  the limitations have more to do with
> > the number of outstanding messages, which is 1 in the mnt
> > driver.
> 
>  Hm, but what's the alternative here? Readahead seems somewhat
> attractive, if difficult (I worry about blocking reads and timing
> sensitive file systems). But there's one problem I can't resolve - how
> do you know what offset to Tread without consulting the previous
> Rread's count?
>  Actually, I understand there has been discussion about grouping tags
> to allow for things like Twalk/Topen batching without waiting for
> Rwalk (which sounds like a great idea), maybe that would work here
> also...
> -sqweek
> 

This seems an appropriate time -- well, appropriate thread, if a few days
lagged -- to bring up Dave's/my cache journaling scheme for review... I
presented this at IWP9 last year and recieved mostly positive feedback, as I
remember, but sadly have not had a chance to work on it since.  (That said,
it remains on my todo list should nobody beat me to it.)

The protocol design is documented on my server at
https://wiki.ietfng.org/pub/Plan9/JournalCallbacks.  Sadly, it remains the
case that I don't have an implementation to offer.

The basic idea is to shim a client-side cache and a server-side cache
controller in front of a filesystem, like fossil or exportfs, and have the
cache controller snoop on all connections to the server, informing caches
when the contents of their caches have become invalid.  There are a few
known deficiencies ([in]coherency and authentication being the two big
ones), but I hope the idea remains sound.

Thoughts and criticisms welcome. :)
--nwf;