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Re: [Nbd] Thoughts on structured replies



Eric,

On 25 Apr 2016, at 14:55, Eric Blake <eblake@...696...> wrote:

>> Is the current structured-reply extension incredibly baroque for no gain?
> 
> No, I still think we want the full flexibility of multiple chunks.
> Remember, the BIG gain is the ability to read holes without sending
> zeros over the wire.

Ah yes, I forgot this.

>  You cannot get that without multiple chunks, or
> else by constraining your maximum block size to be relatively small
> (aren't there file systems with holes at the granularity of 32k?).

There certainly are if the offset isn't 32k aligned :-)

> Requiring the client to only read 32k at a time in order to efficiently
> report holes seems like it will cause more traffic overhead.
> 
> Other benefits from structured replies: error messages to accompany an
> error chunk, and the ability to implement the BLOCK_STATUS extension
> (proposals have been floated on list, but right now we still don't have
> an extension-* branch for it).

True.

>> Could we do something much simpler if we assume that the 'chunking' is done by the BLOCKSIZE stuff?
>> 
> 
> I don't think so, because the ability to read holes is the main reason I
> added chunking of the read reply.

Yes. Fair enough.


--
Alex Bligh




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