Re: [Qemu-block] [Qemu-devel] [PATCH v2 2/2] qapi: allow blockdev-add for NFS

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 26.10.2016 um 09:23 hat Markus Armbruster geschrieben:
>> A few drive-by comments...
>> 
>> Eric Blake <address@hidden> writes:
>> 
>> > On 10/24/2016 02:27 PM, Ashijeet Acharya wrote:
>> >> Introduce new object 'BlockdevOptionsNFS' in qapi/block-core.json to
>> >> support blockdev-add for NFS network protocol driver. Also make a new
>> >> struct NFSServer to support tcp connection.
>> >> 
>> >> Signed-off-by: Ashijeet Acharya <address@hidden>
>> >> ---
>> >>  qapi/block-core.json | 56 
>> >> ++++++++++++++++++++++++++++++++++++++++++++++++----
>> >>  1 file changed, 52 insertions(+), 4 deletions(-)
>> >> 
>> >> diff --git a/qapi/block-core.json b/qapi/block-core.json
>> >> index 9d797b8..3ab028d 100644
>> >> --- a/qapi/block-core.json
>> >> +++ b/qapi/block-core.json
>> >> @@ -1714,9 +1714,9 @@
>> >>  { 'enum': 'BlockdevDriver',
>> >>    'data': [ 'archipelago', 'blkdebug', 'blkverify', 'bochs', 'cloop',
>> >>              'dmg', 'file', 'ftp', 'ftps', 'gluster', 'host_cdrom',
>> >> -            'host_device', 'http', 'https', 'luks', 'null-aio', 
>> >> 'null-co',
>> >> -            'parallels', 'qcow', 'qcow2', 'qed', 'quorum', 'raw',
>> >> -     'replication', 'tftp', 'vdi', 'vhdx', 'vmdk', 'vpc', 'vvfat' ] }
>> >> +            'host_device', 'http', 'https', 'luks', 'nfs', 'null-aio',
>> >> +            'null-co', 'parallels', 'qcow', 'qcow2', 'qed', 'quorum', 
>> >> 'raw',
>> >> +            'replication', 'tftp', 'vdi', 'vhdx', 'vmdk', 'vpc', 'vvfat' 
>> >> ] }
>> >
>> > Missing a comment that 'nfs' is since 2.8.
>> >
>> >>  ##
>> >> +# @NFSServer
>> >> +#
>> >> +# Captures the address of the socket
>> >> +#
>> >> +# @type:        transport type used for NFS (only TCP supported)
>> >> +#
>> >> +# @host:        host part of the address
>> >> +#
>> >> +# Since 2.8
>> >> +##
>> >> +{ 'struct': 'NFSServer',
>> >> +  'data': { 'type': 'str',
>> >
>> > Please make this an enum, instead of an open-coded string. It's okay if
>> > the enum only has one value 'tcp' for now; but using an enum will make
>> > it introspectable if we later add a second transport, unlike what we get
>> > with an open-coded string.
>> 
>> Yes.  When a JSON string has a compile-time fixed set of values, 'str'
>> is generally wrong.
>> 
>> > Must 'type' be mandatory if it must always be 'tcp'?
>> >
>> >> +            'host': 'str' } }
>> >> +
>> >> +##
>> >> +# @BlockdevOptionsNfs
>> >> +#
>> >> +# Driver specific block device option for NFS
>> >> +#
>> >> +# @server:        host address
>> >> +#
>> >> +# @path:          path of the image on the host
>> >> +#
>> >> +# @uid:           #optional UID value to use when talking to the server
>> >> +#
>> >> +# @gid:           #optional GID value to use when talking to the server
>> >
>> > Do we want to allow string names in addition to numeric uid/gid values?
>> > I'm not sure if NFS has name-based id mapping, but it's food for thought
>> > on whether we need to use an alternate type here (alternate between
>> > integer id and string name), or leave this as is.
>> 
>> As far as I know, NFS4 supports user and group names.  On Linux, see
>> rpc.idmapd(8).
>> 
>> How the name support affects C code I can't say.  If it's transparent,
>> i.e. you simply use local UID/GID, and the mapping happens below the
>> hood, then we'd have to translate string values to local UID/GID by the
>> usual means.  Looks like a minor convenience feature on first glance.
>> However, QMP is a *network* protocol.  A remote client can't easily do
>> this translation.
>> 
>> Consider a GUI like virt-manager: I guess we'd rather support user and
>> group names there.  But if we do, and QMP doesn't, either virt-manager
>> or libvirt need to map to numeric IDs.  Easy enough when running on the
>> host, probably impractical when not.
>> 
>> If we permit string values, are @uid and @gid still appropriate names?
>> The user name is not the user ID, it just maps to it.
>>
>> >> +#
>> >> +# @tcp-syncnt:    #optional number of SYNs during the session 
>> >> establishment
>> >
>> > Would tcp-syn-count be any more legible?
>> 
>> We generally write out things in long hand in the QAPI schema.
>> 
>> >                                           What is the default when omitted?
>> 
>> Whenever you write #optional, you must explain the default.  When
>> the default is a fixed value, specify it.  When the system picks a
>> default, state that, and think hard about what you need to specify on
>> how the system picks.
>> 
>> >> +#
>> >> +# @readahead:     #optional set the readahead size in bytes
>> 
>> @read-ahead
>> 
>> > What's the default when omitted?
>> >
>> >> +#
>> >> +# @pagecache:     #optional set the pagecache size in bytes
>> 
>> @page-cache
>> 
>> > Default?
>> >
>> >> +#
>> >> +# @debug:         #optional set the NFS debug level (max 2)
>> >
>> > Presumably default 0?
>> 
>> @BlockdevOptionsGluster calls this @debug-level.
>
> Are all of these renames really worth having to support two option
> names for each option in the driver? I'm not sure if I'm convinced of
> this.

I don't see "two option names for each option" in this patch, so please
explain.