Re: [Nbd] maximum size of a read/write request

[Folkert van Heusden <folkert@...421...>]

>> What is the maximum size of a read/write request? E.g. how much data can
>> be requested/transmitted using one read or write command?
>> And can this be limited using a nbd-client command-line setting and/or
>> nbd protocol handshake?
>
> In practice, I'd try to avoid reads that are more than 1 MB minus the reply header size, and writes that are more than 1 MB minus the request header size.  qemu-nbd used to choke on those, though the next release will only choke on more than 1 MB.
>
> A good way to see what kind of request the kernel might make is to use dd with the O_DIRECT flag.

That is too tricky: a new kernel version might show differnt behaviour.
Imho we should specify a limit or make it configurable or put it in
the protocol. We have 124 bytes of handshake left, we can use 8 of
them for a maximum blocksize.