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Date: Tue, 30 Jun 2009 17:44:50 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH] Allow adjustment of http block device's
	readahead size.
Message-ID: <20090630164450.GA18666@shareable.org>
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Nolan <nolan@sigbus.net>, Alexander Graf <agraf@suse.de>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

Anthony Liguori wrote:
> Alexander Graf wrote:
> >
> >On 29.06.2009, at 15:45, Anthony Liguori wrote:
> >
> >>Nolan wrote:
> >>>Allow adjustment of http block device's readahead size, via the
> >>>environment variable "HTTP_READAHEAD_SIZE".
> >>>
> >>>Signed-off-by: Nolan Leake <nolan <at> sigbus.net>
> >>>
> >>
> >>I'd rather a command line option than an environmental variable.
> >
> >Wouldn't it make sense to pass arbitrary options via -drive to backends?
> >
> >That way we could have -drive 
> >file=http://sample.com/test.iso,http-readahead=1024.
> 
> Where it gets complex is that you probably want those options to be 
> included as qcow2 backing_file too.  I've thought about this in the past 
> and haven't been able to come to a great solution.

And sometimes you want to change or override the qcow2 backing_file options...

It's already awkward when you want to rename a bunch of image files,
that the backing relative file path is fixed in the qcow2 file and
cannot be overridden by a management program (without rewriting the
qcow2 file).

Options like http-readahead=1024 on the backing file are the sort of
thing it's reasonable to want to change more often.  Imho they are in
the same category as the cache= option, which should be controllable
for backing files when invoking QEMU independently from the main file.

-- Jamie