[oops, let's try sending this again with reply-all instead of reply...] On 07/05/11 19:04, Justin T. Gibbs wrote: > On 7/5/11 7:14 PM, Colin Percival wrote: >> On 06/10/11 13:30, Justin T. Gibbs wrote: >>> On 6/9/11 9:26 PM, Colin Percival wrote: >>>> Has anyone seen anything like this? Is it possible that there's a bug >>>> in how our blkfront negotiates the request ring? Does anyone have >>>> ring_pages > 1 in use? >>> >>> The only backend driver I know of that can support more than one ring page >>> is FreeBSD. So, the problem likely is that blkfront is getting the >>> negotiation wrong and only 1 page is in use. >> >> Turns out that the linux backend in question really does support multiple >> pages. > > Can you provide a source or spec reference for this driver?
Ah, here it is, dating from 2008: http://xen.1045712.n5.nabble.com/attachment/2527534/0/big_ring.patch Sorry, I had several windows open and thought I was looking at kernel.org tree commits rather than a separate patch. > There was a > proposal (in 2009?) to add multiple page support to the XenSource > provided blkfront/back, but it didn't get committed to the Xen repository. > It also doesn't use the same XenStore variables that you list. Is this > something Amazon developed in house? I don't know what hand if any Amazon had in writing this, but it looks like they've convinced at least Ubuntu and RedHat to use this version. >> There's also an inconsistency about how multiple rings are negotiated: >> We set: >> * ring-pages to the number of pages blkfront wants to use >> * ring-ref to the reference for the first page >> * ring-refXX to the references for other pages >> while linux sets: >> * num-ring-pages to the number of pages blkfront wants to use >> * ring-refXX to the page references. > > You forgot to mention that the FreeBSD driver allows request size to > be negotiated and explicitly supports "chained" requests to break the > 44k request size limit imposed by the original interface. I was trying to highlight the compatibility issues, not list all the ways that FreeBSD is superior to Linux. ;-) >> It seems to be impossible to be compatible with both, since Linux > interprets >> having a value set for ring-ref to indicate that the single-ring > protocol is >> being used and doesn't check anything else. > > It's not "Linux". There is no multi-page support in either the kernel.org > hosted drivers or those in the Linux tree hosted in the official Xen > repository. Agreed. But it's multiple Linux distributions (and EC2). >> Is there any official source which defines the blkback/front protocol? > > Only the source code in the Xen repository. Both the FreeBSD extensions > that I did and those for the Linux system you are using are likely backwards > compatible to the original interface, but unfortunately are not compatible > with each other. Yes, both versions are definitely backwards compatible. >> Maybe >> the right option is to have a loader tunable dev.xn.linuxback to > control which >> version of the protocol is used? > > What a mess. Yep. Mess or not, shall I go ahead with having a loader tunable control this, or can you think of a better solution? -- Colin Percival Security Officer, FreeBSD | freebsd.org | The power to serve Founder / author, Tarsnap | tarsnap.com | Online backups for the truly paranoid _______________________________________________ freebsd-xen@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-xen To unsubscribe, send any mail to "freebsd-xen-unsubscr...@freebsd.org"
