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

[Stefan Hajnoczi]

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().

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

pgpkiCs1MBvTv.pgp
Description: PGP signature