Re: [Qemu-devel] [PATCH v6 0/2] add read-pattern for block qourum

[Liu Yuan]

On Thu, Aug 28, 2014 at 11:33:08AM +0100, Stefan Hajnoczi wrote:
> 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 <address@hidden>
> > Cc: Eric Blake <address@hidden>
> > Cc: Kevin Wolf <address@hidden>
> > Cc: Stefan Hajnoczi <address@hidden>
> > 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().

Thanks for your fix.
 
> Please extend the quorum qemu-iotests to cover the new fifo read
> pattern.  You can send the tests as a separate patch series.
> 

Okay, will do later.

Yuan