Libguestfs - Sep 2015 - Question: running appliance commands over guest fs (resize2fs -P).

Hello everyone!

I am working on resizing qcow2 images using virt-resize+liguestfs. E.g. 
I when shrinking a partition, I have to resize filesystem using 
resize2fs-size. The problem is that I cannot find out minimal partition 
size (aka resize2fs -P). The only way is calling "resize2fs-size 1K", 
wait for resize2fs to claim "resize2fs: New size smaller than minimum 
(510050)" and parse this output.

It is known that resize2fs carries out complex calculations (no always 
correct) to get minimal size, so statvfs/tune2fs-l cannot help.

The questions are:
1) How to get minimal partition size (equal to resize2fs -P)?
2) More general, how to execute commands from appliance but make them 
run over image (which may not have anything but filesystem) - I saw 
something like that in source.

Thanks in advance!

-- 
Your sincerely,
Maxim Perevedentsev

On Tue, Sep 15, 2015 at 04:31:46PM +0300, Maxim Perevedentsev
wrote:
> Hello everyone!
> 
> I am working on resizing qcow2 images using virt-resize+liguestfs.
If you're shrinking, I believe a better way to do this is to sparsify
the image.
> E.g. I when shrinking a partition, I have to resize filesystem using
> resize2fs-size. The problem is that I cannot find out minimal
> partition size (aka resize2fs -P). The only way is calling
> "resize2fs-size 1K", wait for resize2fs to claim "resize2fs:
New
> size smaller than minimum (510050)" and parse this output.
> 
> It is known that resize2fs carries out complex calculations (no
> always correct) to get minimal size, so statvfs/tune2fs-l cannot
> help.
> 
> The questions are:
> 1) How to get minimal partition size (equal to resize2fs -P)?
There's no binding for that, but it would be a worthwhile addition to
the API.
> 2) More general, how to execute commands from appliance but make
> them run over image (which may not have anything but filesystem) - I
> saw something like that in source.
Not sure I understand the question?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

On 09/15/2015 04:57 PM, Richard W.M. Jones wrote:
> On Tue, Sep 15, 2015 at 04:31:46PM +0300, Maxim Perevedentsev wrote:
>> Hello everyone!
>>
>> I am working on resizing qcow2 images using virt-resize+liguestfs.
> If you're shrinking, I believe a better way to do this is to sparsify
> the image.
And how would it help? If I shrink, a want a VM to occupy less maximum 
space. virt-sparsify (I guess you are talking about it) will reduce 
space taken by guest disk image by now, but won't tighten the upper 
limit for guest.

-- 
Your sincerely,
Maxim Perevedentsev

On Tue, Sep 15, 2015 at 05:17:16PM +0300, Maxim Perevedentsev
wrote:
> On 09/15/2015 04:57 PM, Richard W.M. Jones wrote:
> >>2) More general, how to execute commands from appliance but make
> >>them run over image (which may not have anything but filesystem) -
I
> >>saw something like that in source.
> >Not sure I understand the question?
>
> As I understand, guestfs runs its own kernel-like daemon that can
> run its own commands. It is used to run e.g. resize2fs or e2fsck
> over loaded disk images. But those two utilities require the
> partition not to be mounted, so they are run by daemon and do not
> require anything to exists on partition.
> 
> In API I found only the 'command' command which requires the
> partition to be mounted and have kernel image on it. This cannot
> operate on empty or non-mounted partition.
You probably want to use the guestfs_debug API, ie:

  char *cmd[] = { "resize2fs", "-P", ..., NULL };
  char *ret = guestfs_debug (g, "sh", cmd);

It's better to add a new API however, and not difficult either:

  http://libguestfs.org/guestfs.3.html#adding-a-new-api-action

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

On 09/15/2015 05:37 PM, Richard W.M. Jones wrote:
> On Tue, Sep 15, 2015 at 05:17:16PM +0300, Maxim Perevedentsev wrote:
>> On 09/15/2015 04:57 PM, Richard W.M. Jones wrote:
>>>> 2) More general, how to execute commands from appliance but
make
>>>> them run over image (which may not have anything but
filesystem) - I
>>>> saw something like that in source.
>>> Not sure I understand the question?
>> As I understand, guestfs runs its own kernel-like daemon that can
>> run its own commands. It is used to run e.g. resize2fs or e2fsck
>> over loaded disk images. But those two utilities require the
>> partition not to be mounted, so they are run by daemon and do not
>> require anything to exists on partition.
>>
>> In API I found only the 'command' command which requires the
>> partition to be mounted and have kernel image on it. This cannot
>> operate on empty or non-mounted partition.
> You probably want to use the guestfs_debug API, ie:
>
>    char *cmd[] = { "resize2fs", "-P", ..., NULL };
>    char *ret = guestfs_debug (g, "sh", cmd);
Yeah! This is just what I was looking for. Thank you :-)
> It's better to add a new API however, and not difficult either:
>
>    http://libguestfs.org/guestfs.3.html#adding-a-new-api-action
>
> Rich.
I'll try to submit a patch when I'll have time.

-- 
Your sincerely,
Maxim Perevedentsev