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

[Paolo B. <pbo...@re...>]

On 10/10/2011 03:00 PM, Folkert van Heusden wrote:
>>> 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.

O_DIRECT bypasses everything clever that the kernel might do.

Paolo