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Re: [Xen-devel] [xs-devel] blktap3 / tapdisk3 development

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Fabio Fantoni
> Sent: 02 October 2014 10:24
> To: Thanos Makatos; xs-devel@xxxxxxxxxxxxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; s.munaut@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [xs-devel] blktap3 / tapdisk3 development
> Il 02/10/2014 10:41, Thanos Makatos ha scritto:
> >> It seems upcoming XenServer 6.5 "Creedence" is going to use blktap3 /
> >> tapdisk3 after all, so if i understand this correctly blktap3 is
> >> being developed "only for XenServer", right?
> > Correct, although it shouldn't be too much work for making it work on
> upstream Xen.
> >
> >> Many interested persons have been asking on mailing lists and on IRC
> >> about the status of blktap3 development.
> >> Is it possible for someone from Citrix/XenServer to comment about this?
> >>
> >> I know there's a blog post about blktap3/tapdisk3 performance
> >> (improvements) here:
> >> http://xenserver.org/blog/entry/tapdisk3
> >>
> >> One of the interested developers is CC'd on this email. He has
> >> written RBD/Ceph support patch for blktap2, and he's interested in
> >> porting that to blktap3/tapdisk3..
> >>
> >> Also is this the correct repo with latest blktap3/tapdisk3 bits in it?:
> >> https://github.com/xapi-project/blktap
> > blktap3 is actively developed and is the I/O default data path in XenServer
> 6.5. The repo you mention is the right one (https://github.com/xapi-
> project/blktap, git branch "xs64bit", but we'll soon bring the "master" branch
> up to date. I'd be more than happy to assist you and the developer in porting
> that patch to blktap3!
> Sorry for a my probably stupid question.
> Why not use qdisk (that have very good performance, support many formats
> and is supported also by other project) and improve it if needed instead
> do/maintain another project xen-only?
> FWIK the only thing in what blktab is better is vhd support (not vhdx).

Tapdisk3 has many differences if compared to qdisk. One of them is the VHD 
support you already mentioned.
Another important one is the use of grant copy instead of grant mapping or 
persistent grants. In XenServer, we also have a tapdisk3 process per VBD, 
whereas qdisk uses one process per VM. Both these differences have performance 
implications and our studies show that tapdisk3 performs/scales better (at 
least) in the XenServer environment. I have presented some numbers at the 
XPDS14 and you can find my slides here (I suppose a video recording of the 
presentation will also be available at some point):

We were already working on tapdisk3 for XenServer 6.5 prior to decisions 
regarding upstreaming. Right now it is too soon to comment whether qdisk will 
be considered for future releases.


> Thanks for any reply and sorry for my bad english.
> >
> > Cheers
> >
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