On Mon, Aug 18, 2014 at 05:41:03PM +0800, Liu Yuan wrote: > v6: > - fix a unused warning introduced by last version > > v5: > - simplify a for loop in quorum_aio_finalize() > > v4: > - swap the patch order > - update comment for fifo pattern in qaip > - use qapi enumeration in quorum driver instead of manual parsing > > v3: > - separate patch into two, one for quorum and one for qapi for easier review > - add enumeration for quorum read pattern > - remove unrelated blank line fix from this patch set > > v2: > - rename single as 'fifo' > - rename read_all_children as read_quorum_children > - fix quorum_aio_finalize() for fifo pattern > > This patch adds single read pattern to quorum driver and quorum vote is > default > pattern. > > For now we do a quorum vote on all the reads, it is designed for unreliable > underlying storage such as non-redundant NFS to make sure data integrity at > the > cost of the read performance. > > For some use cases as following: > > VM > -------------- > | | > v v > A B > > Both A and B has hardware raid storage to justify the data integrity on its > own. > So it would help performance if we do a single read instead of on all the > nodes. > Further, if we run VM on either of the storage node, we can make a local read > request for better performance. > > This patch generalize the above 2 nodes case in the N nodes. That is, > > vm -> write to all the N nodes, read just one of them. If single read fails, > we > try to read next node in FIFO order specified by the startup command. > > The 2 nodes case is very similar to DRBD[1] though lack of auto-sync > functionality in the single device/node failure for now. But compared with > DRBD > we still have some advantages over it: > > - Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed > storage. And if A crashes, we need to restart all the VMs on node B. But for > practice case, we can't because B might not have enough resources to setup 20 > VMs > at once. So if we run our 20 VMs with quorum driver, and scatter the > replicated > images over the data center, we can very likely restart 20 VMs without any > resource problem. > > After all, I think we can build a more powerful replicated image functionality > on quorum and block jobs(block mirror) to meet various High Availibility > needs. > > E.g, Enable single read pattern on 2 children, > > -drive driver=quorum,children.0.file.filename=0.qcow2,\ > children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1 > > [1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device > > Cc: Benoit Canet <ben...@irqsave.net> > Cc: Eric Blake <ebl...@redhat.com> > Cc: Kevin Wolf <kw...@redhat.com> > Cc: Stefan Hajnoczi <stefa...@redhat.com> > Liu Yuan (2): > qapi: add read-pattern enum for quorum > block/quorum: add simple read pattern support > > block/quorum.c | 177 > +++++++++++++++++++++++++++++++++++++-------------- > qapi/block-core.json | 20 +++++- > 2 files changed, 148 insertions(+), 49 deletions(-)
I dropped the \n from the error_setg() error message while merging. Please do not use \n with error_setg(). Please extend the quorum qemu-iotests to cover the new fifo read pattern. You can send the tests as a separate patch series. Thanks, applied to my block tree: https://github.com/stefanha/qemu/commits/block Stefan
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