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Re: [Qemu-block] NBD structured reads vs. block size

From: Richard W.M. Jones
Subject: Re: [Qemu-block] NBD structured reads vs. block size
Date: Wed, 1 Aug 2018 15:13:07 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Aug 01, 2018 at 04:49:21PM +0300, Nir Soffer wrote:
> All this is an issue only when the underlying file is a file using raw
> format, right?

NBD should only really be used to send raw format data over the wire.
While it is possible to serve a qcow2 file over NBD as qcow2, it's not
really how NBD is intended to be used.

> Since NBD is meant for block devices, and block devices cannot have
> partial blocks, would it be simpler to round down unaligned file size,  and
> treat possible unaligned range at the end of the file as area that can not
> be accessed via NBD?

For nbdkit I would like to preserve the ability to serve arbitrary-
sized files.  nbdkit allows people to write plugins, and we cannot be
sure that every plugin everyone writes will always use sensible block

In a forthcoming nbdkit release you can add the "truncate" filter on
top of any plugin to round up the size to a multiple of any power of 2
(or round down if you really want).



Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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