From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57695) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1akw3O-0004sc-LP for qemu-devel@nongnu.org; Tue, 29 Mar 2016 11:59:39 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1akw3K-0004zH-Hc for qemu-devel@nongnu.org; Tue, 29 Mar 2016 11:59:38 -0400 References: <56EA06E0.7000409@cn.fujitsu.com> <56EA7C62.3090000@cn.fujitsu.com> <20160317094831.GA2504@work-vm> <56EA7F39.9060504@cn.fujitsu.com> <56FAA168.9090304@redhat.com> <56FAA2C4.3000002@redhat.com> <20160329155024.GH2240@work-vm> <56FAA4BB.3080300@redhat.com> <20160329155426.GI2240@work-vm> From: Max Reitz Message-ID: <56FAA65C.3080107@redhat.com> Date: Tue, 29 Mar 2016 17:59:24 +0200 MIME-Version: 1.0 In-Reply-To: <20160329155426.GI2240@work-vm> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="NvIjadCMuc99DtPsldrKU7wk9tS0bFRuQ" Subject: Re: [Qemu-devel] [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: Kevin Wolf , Changlong Xie , Alberto Garcia , qemu block , Jiang Yunhong , Dong Eddie , qemu devel , Markus Armbruster , Gonglei , Stefan Hajnoczi , zhanghailiang This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --NvIjadCMuc99DtPsldrKU7wk9tS0bFRuQ Content-Type: multipart/mixed; boundary="fsi5liXCCkSMcp5XXdubwI3u205QIReHn" From: Max Reitz To: "Dr. David Alan Gilbert" Cc: Eric Blake , Wen Congyang , Alberto Garcia , Changlong Xie , qemu devel , Kevin Wolf , Stefan Hajnoczi , Markus Armbruster , Dong Eddie , Jiang Yunhong , qemu block , zhanghailiang , Gonglei Message-ID: <56FAA65C.3080107@redhat.com> Subject: Re: [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child() References: <56EA06E0.7000409@cn.fujitsu.com> <56EA7C62.3090000@cn.fujitsu.com> <20160317094831.GA2504@work-vm> <56EA7F39.9060504@cn.fujitsu.com> <56FAA168.9090304@redhat.com> <56FAA2C4.3000002@redhat.com> <20160329155024.GH2240@work-vm> <56FAA4BB.3080300@redhat.com> <20160329155426.GI2240@work-vm> In-Reply-To: <20160329155426.GI2240@work-vm> --fsi5liXCCkSMcp5XXdubwI3u205QIReHn Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable On 29.03.2016 17:54, Dr. David Alan Gilbert wrote: > * Max Reitz (mreitz@redhat.com) wrote: >> On 29.03.2016 17:50, Dr. David Alan Gilbert wrote: >>> * Eric Blake (eblake@redhat.com) wrote: >>>> On 03/29/2016 09:38 AM, Max Reitz wrote: >>>>> On 17.03.2016 10:56, Wen Congyang wrote: >>>>>> On 03/17/2016 05:48 PM, Dr. David Alan Gilbert wrote: >>>>> >>>>> [...] >>>>> >>>>>>> The children.0 notation is really confusing in the way that Berto= >>>>>>> describes; I hit this a couple of months ago and it really doesn'= t >>>>>>> make sense. >>>>>> >>>>>> Do you mean: read from children.1 first, and then read from childr= en.0 in >>>>>> fifo mode? Yes, the behavior is very strange. >>>>> >>>>> So is this intended or is it not? In >>>>> http://lists.nongnu.org/archive/html/qemu-block/2016-03/msg00526.ht= ml >>>>> you said that it is. >>>>> >>>>> I myself would indeed say it is very strange. If I were a user, I w= ould >>>>> not expect this behavior. And as I developer, I think that how a BD= S's >>>>> child is used by its parent should solely depend on its role (e.g. >>>>> whether it is "children.0" or "children.1"). >>>> >>>> It sounds like the argument here, and in Max's thread on >>>> query-block-node-tree, is that we DO have cases where order matters,= and >>>> so we need a way for the hot-add operation to explicitly specify whe= re >>>> in the list a child is inserted (whether it is being inserted as the= new >>>> primary image, or explicitly as the last resort, or somewhere in the= >>>> middle). An optional parameter, that defaults to appending, may be = ok, >>>> but we definitely need to consider how the order of children is affe= cted >>>> by hot-add. >>> >>> Certainly in the COLO case the two children are not identical; and IM= HO we need >>> to get away from thinking about ordering and start thinking about fun= ctional >>> namingd - children.0/children.1 doesn't suggest the fact they behave >>> differently. >> >> To me it does. If quorum is operating in a mode call "FIFO" I would >> expect some order on the child nodes, and if the child nodes are >> actually numbered in an ascending order, that is an obvious order. >=20 > I don't understand why it's called 'FIFO'. Because in that mode quorum successively reads from all of its children and returns the first successful result. So the First successful Input is the one that becomes quorum's Output (there isn't much of a successive output, so it doesn't make much sense to call that the First Output, though...). I didn't name it, though. *waves hands defensively* :-) Max --fsi5liXCCkSMcp5XXdubwI3u205QIReHn-- --NvIjadCMuc99DtPsldrKU7wk9tS0bFRuQ Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQEcBAEBCAAGBQJW+qZcAAoJEDuxQgLoOKytfaEIAKQ/B/7ALbaOzpD1gIgdO73s j6C22rfA0GJCl3Xu22lvhQwaVlbxI1Ow4k3vxYX9O158Zod+WwHhSO3dGm7qrGRo jDhI8zqIoNrbUO3pWq+4QIQywTdD/3at5fYNHjA2nTLINs8UcJ2Zus8uKa+nehHS vPurcEVwqJBn4LyavOoUdceH5JEOB6wjzfBusU1/e/4rdlRIoT+V1o8Rn0kPQjYy qq2fOx+UxS7tygGcYMAwrUgn+FQ2tKZdRfFnEjN1Sg3hHG48iIzKrW0VOrPaYetv 13PiUZxjaE8CqjchltsAS9IX4gLnxddre6jB7qpCfR4WFq7lBVBNrqNVsgUjt7w= =CLV+ -----END PGP SIGNATURE----- --NvIjadCMuc99DtPsldrKU7wk9tS0bFRuQ--