Re: [qemu-web PATCH] Add a blog post about FUSE block exports

[Hanna Reitz <hreitz@redhat.com>]

On 20.08.21 23:24, Eric Blake wrote:
> On Thu, Aug 19, 2021 at 12:25:01PM +0200, Hanna Reitz wrote:
>> This post explains when FUSE block exports are useful, how they work,
>> and that it is fun to export an image file on its own path so it looks
>> like your image file (in whatever format it was) is a raw image now.
>>
>> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
>> ---
>> You can also find this patch here:
>> https://gitlab.com/hreitz/qemu-web fuse-blkexport-v1
>>
>> My first patch to qemu-web, so I hope I am not doing anything overly
>> stupid here (adding SVGs with extremely long lines comes to mind)...
>> ---
> ...
>> +
>> +Besides attaching guest devices to block nodes, you can also export them for
>> +users outside of qemu, for example via NBD.  Say you have a QMP channel open for
>> +the QEMU instance above, then you could do this:
>> +```json
>> +{
>> +    "execute": "nbd-server-start",
>> +    "arguments": {
>> +        "addr": {
>> +            "type": "inet",
>> +            "data": {
>> +                "host": "localhost",
>> +                "port": "10809"
>> +            }
>> +        }
>> +    }
>> +}
> Rather than using a TCP port, is it worth mentioning that you can use
> a Unix socket?  If the point of this is local access to the disk
> contents, that feels a bit lighter weight.

Well, the point of this is local access through FUSE; the NBD part here 
just serves to introduce the concept of block exports, so it shouldn’t 
really matter whether we use TCP or Unix sockets here.  I like TCP 
sockets a bit more in this case, because I feel like for people who 
don’t know much about NBD, that may seem more natural.

>> +{
>> +    "execute": "block-export-add",
>> +    "arguments": {
>> +        "type": "nbd",
>> +        "id": "fmt-node-export",
>> +        "node-name": "fmt-node",
>> +        "name": "guest-disk"
>> +    }
> This defaults to a readonly image; you may want to include
> "writable":true in the JSON, especially if the purpose is to show how
> to modify guest-visible contents of an at-rest disk image.

Oh, yes, good idea.  I should do this in every export command line.

> Overall a nice post!  I hope my comments help in addition to all the
> other good reviews you got.

Thanks!  I think I’ll keep TCP for exporting, but I will add writable=on 
to every export example.

Hanna