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
Liu Yuan (2):
qapi: add read-pattern support for quorum
block/quorum: add simple read pattern support
block/quorum.c | 181 +++++++++++++++++++++++++++++++++++++--------------
qapi/block-core.json | 19 +++++-
2 files changed, 149 insertions(+), 51 deletions(-)
--
1.9.1
