Hello list,
I want to implement libvirt Quorum support.
(https://github.com/qemu/qemu/commit/c88a1de51ab2f26a9a37ffc317249736de8c015c)
Quorum is a QEMU RAID like block storage driver.
Data are written on n replicas and when a read is done a comparison between the
replica read is done. If more than threshold reads are identical the read succeed
else it's and error.
For example a Quorum with n = 3 and threshold = 2 would be made of three QCOW2
backing chains used as identicals replicas. threshold = 2 means that at least
2 replica must be identical when doing a read.
I want to make use of the new backingStore xml element to implement quorum.
Proposed Quorum libvirt format:
-------------------------------
<disk type='quorum' device='disk'>
<driver name='qemu' type='quorum'/>
<threshold value=2/>
<backingStore type='file'>
<format type='qcow2'/>
<source file='/var/lib/libvirt/images/file1.qcow2'/>
</backingStore>
<backingStore type='file'>
<format type='qcow2'/>
<source file='/var/lib/libvirt/images/file2.qcow2'/>
</backingStore>
<backingStore type='file'>
<format type='qcow2'/>
<source file='/var/lib/libvirt/images/file3.qcow2'/>
</backingStore>
<target dev='vda' bus='virtio'/>
</disk>
Implementation plan:
--------------------
* Add VIR_STORAGE_TYPE_QUORUM
* In src/util/virstoragefile.h change _virStorageSource to contain a
virStorageSourcePtrPtr backingStores.
I think doing it at this level allow to keep a 1-1 mapping with the qemu
BlockDriverState hiearchy
* Add a int quorum_threshold field to the same structure
* Add support for parsing treshold in virDomainDiskDefParseXML
* Change virDomainDiskBackingStoreParse to virDomainDiskBackingStoresParse to parse
all the backingStore at once an use realloc to grow the backingStores field.
* Modify virDomainDiskDefFormat to call virDomainDiskBackingStoreFormat in a loop
for saving
* hook into qemuBuildDriveStr around line 3442 to create the quorum parameters
Do you feel that I am missing something ?
Best regards
Benoît