From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: Date: Fri, 9 Sep 2005 18:24:34 -0400 From: Russ Cox To: Francisco Ballesteros , Fans of the OS Plan 9 from Bell Labs <9fans@cse.psu.edu> Subject: Re: [9fans] reliability and failing over. In-Reply-To: <600308d6050909151067389b35@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline References: <600308d605090913554224787d@mail.gmail.com> <20050909210534.GI4207@server4.lensbuddy.com> <600308d6050909151067389b35@mail.gmail.com> Cc: Topicbox-Message-UUID: 86491dda-ead0-11e9-9d60-3106f5b1d025 > Funny. The 9p reliability project looks to me a lot like the redirfs > that we played with before introducing the Plan B volumes into the kernel= . > It provided failover (on replicated FSs, by some other means) and could > recover the fids just by keeping track of their paths. >=20 > The user level process I'm with now is quite similar to that (appart from > including the language to select particular volumes) it maintains a fid > table knowning which server, and which path within the server are the one= s > for each fid. It's what the Plan B kernel ns does, but within a server. >=20 > Probably, the increase in latency you are seeing is the one I'm going to > see in volfs. The 2x penalty in performace is what one could expect, beca= use > you have twice the latency. However, the in-kernel implementation has no > penalty at all, because the kernel can rewrite the mount tables. >=20 > Maybe we should talk about this. > Eric? Russ? What do you say? Is it worth to pay the extra > latency just to avoid a change (serious, I admit) in the kernel? I keep seeing that 2x number but I still don't believe it's actually reasonable to measure the hit on an empty loopback file server. Do something over a 100Mbps file server connection talking to fossil and see what performance hit you get then. Stuff in the kernel is much harder to change and debug. Unless there's a compelling reason, I'd like to see it stay in user space. And I'm not yet convinced that performance is a compelling reason. Russ