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

[Eric Blake <eblake@redhat.com>]

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On 07/14/2014 10:13 PM, Liu Yuan wrote:

>>> +            '*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'
> 
> For startup command line, did it imply a change too? Sorry I'm not familiar with
> block-core.json.

Adding an enum in the .json file will generate an enum that you can
reuse in your parsing code (look at commit 82a402e99,
parse_enum_option).  The command line will be the same, but instead of
open-coding things, you can now use the same enum as what the QMP code
will use.

> 
> With your suggestion, how we pass read-pattern via qmp?
> 
>  read-pattern: fifo
>  or 
>  read-pattern: quorum

The QMP wire format will be identical - the user still passes
"read-pattern":"fifo" as part of building up their JSON.  It's just that
the string "fifo" is now validated for correctness against the enum of
valid strings, instead of being open-ended anything goes.

> 
> 
>>
>> 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).
> 
> I guess so. 'round-robin' in your context would benefit use case seeking for
> better read load balance.

Of course, if you do add 'round-robin', do it in a separate patch; and
that's also a case where having the enum list valid modes will make it
easier to add in a new mode.

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


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