[Libguestfs] nbdkit: how to require use of filter

[Nikolaus Rath]

On May 03 2022, rjones at redhat.com (Richard W.M. Jones) wrote:
> On Mon, May 02, 2022 at 10:25:44AM +0100, Nikolaus Rath wrote:
>> Hi,
>> 
>> Is there a way for a nbdkit plugin to *require* use of a nbdkit filter
>> (specifically the nbdkit-blocksize-filter) programmatically?
>
> No it's not possible in general, you just have to document it.
>
> However as it's *blocksize* filter that you're talking about, I assume
> you mean that your filter has blocksize constraints, eg. that it
> cannot handle sub-sector requests.  In this case it is possible to
> tell clients that they must not issue requests smaller than a certain
> size (and also hint about optimal and maximum sizes).  You can do this
> by writing a .block_size callback.  For example:
>
> https://gitlab.com/nbdkit/nbdkit/-/blob/1f5dc0679e570ca2fa149ceaa52922d70462b62c/plugins/vddk/vddk.c#L851
>
> Not all clients obey these (although these clients are buggy!)

Do you happen to know if the kernel's NBD client obeys them?

(I assume setting a blocksize constraint implies an alignment constraint
as well?)

Best,
-Nikolaus

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