Re: [Qemu-devel] [PATCH v2] block/quorum: add simple read pattern support

[Eric Blake]

On 07/14/2014 09:19 PM, Liu Yuan wrote:
> This patch adds single read pattern to quorum driver and quorum vote is 
> default
> pattern.
> 

> 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:
> 

> +++ b/block.c
> @@ -2212,7 +2212,7 @@ int bdrv_commit(BlockDriverState *bs)
>  
>      if (!drv)
>          return -ENOMEDIUM;
> -    
> +
>      if (!bs->backing_hd) {
>          return -ENOTSUP;
>      }

While this whitespace cleanup is nice, it doesn't belong in this patch,
when there is no other change to this unrelated file.


> +++ b/qapi/block-core.json
> @@ -1398,12 +1398,17 @@
>  # @rewrite-corrupted: #optional rewrite corrupted data when quorum is reached
>  #                     (Since 2.1)
>  #
> +# @read-pattern: #optional choose quorum or fifo pattern for read
> +#                set to quorum by default (Since 2.2)
> +#
>  # Since: 2.0
>  ##
>  { 'type': 'BlockdevOptionsQuorum',
>    'data': { '*blkverify': 'bool',
>              'children': [ 'BlockdevRef' ],
> -            'vote-threshold': 'int', '*rewrite-corrupted': 'bool' } }
> +            'vote-threshold': 'int',
> +            '*rewrite-corrupted': 'bool',
> +            '*read-pattern': 'str' } }

Raw strings that encode a finite set of values are bad for type-safety.
 Please add an enum:

{ 'enum': 'QuorumReadPattern', 'data': [ 'quorum', 'fifo' ] }

then use '*read-pattern': 'QuorumReadPattern'

Should we offer multiple modes in addition to 'quorum'?  For example, I
could see a difference between 'fifo' (favor read from the first quorum
member always, unless it fails, good when the first member is local and
other member is remote) and 'round-robin' (evenly distribute reads; each
read goes to the next available quorum member, good when all members are
equally distant).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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