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Re: [Qemu-block] "qemu-img: File contains external, encrypted or compres

From: Eric Blake
Subject: Re: [Qemu-block] "qemu-img: File contains external, encrypted or compressed clusters."
Date: Thu, 28 Mar 2019 17:18:23 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.0

On 3/28/19 7:30 AM, Eric Blake wrote:
> On 3/28/19 5:59 AM, Richard W.M. Jones wrote:
>> This error message is confusing and looks wrong to me:
> Confusing, but intentional design decision at the time (that said, I
> would also welcome an improvement).
>> $ ./nbdkit -U - data data="1" size=1M --run 'qemu-img map $nbd'
>> Offset          Length          Mapped to       File
>> qemu-img: File contains external, encrypted or compressed clusters.
> It's due to the fact that the human output WANTS to fill in the "mapped
> to" column, but can't, because there is no "File" that you can open and
> read from the "Mapped to" offset of that file to see the same thing as
> the guest would see at "Offset".
> Perhaps if we tweaked the output to just say:
> $ qemu-img map file
> Offset          Length          Mapped to       File
> 0               0x1000          N/A
> (try again with --output=json for full details)
> $

That's still a worthwhile idea for 4.1 for compressed/encrypted images.
But for NBD, I can do one better; I'll post a patch for 4.0 that makes
nbd behave more like file-posix - in short, the protocol level should be
returning BDRV_BLOCK_OFFSET_VALID in addition to everything else it
learns from the server, improving the output to:

Offset          Length          Mapped to       File
0               0x1ff           0               nbd://localhost:10809

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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