Re: [Qemu-devel] [PATCH 4/4][RFC] Add logic to QEMU to read command line options from qcow2 images

[Anthony Liguori <anthony@codemonkey.ws>]

Brian Wheeler wrote:
> On Thu, 2007-08-09 at 15:32 -0500, Anthony Liguori wrote:
>   
>> Avi Kivity wrote:
>>     
>>> Anthony Liguori wrote:
>>>       
>>>> If you're looking for a low-end solution, another possibility would 
>>>> be having a "new" file format which consisted of:
>>>>
>>>> #!/path/to/qemu [<args> ...] <nl>
>>>> <standard disk image>
>>>>
>>>> And then make the appropriate changes to QEMU such that it can skip 
>>>> the first line in a disk image file.  This has a few nice side 
>>>> effects.  The disk image is directly executable and it makes it very 
>>>> clear to the user that they have to trust the disk image.  The other 
>>>> nice thing is that it would work with file formats other than qcow2.
>>>>         
>>> Well, it would be nice to align the disk image to a sector boundary 
>>> (or, better, a page boundary).  But yes, a very good idea.
>>>       
>> That has the very nice side effect of allowing you to edit the command 
>> line with a text editor provided you don't cross the sector boundary.  
>> Filling with spaces would be pretty nice.
>>
>>     
>
>
> Its a bad idea.  Either have a line terminated file, or a pure binary
> file, but don't mix them.  Besides firing up an editor to modify it
> being a royal pain, mystery breakages will occur when someone has their
> editor set to "insert mode" (which is the default) vs "overwrite mode"
> and doesn't notice those extra spaces moving beyond the 512-byte mark.
>
> Brian
>   

Assuming that you have a tool that edits the command line (I admit, 
using emacs is a bad idea), then aligning to a sector boundary means 
that you don't have to munge the data every time you update.  Since most 
command lines will be < 512 bytes probably, this means that you can 
usually get away with not having to copy any data on disk which is a 
very good thing when you have multi-gigabyte files.

Regards,

Anthony Liguori

>> Regards,
>>
>> Anthony Liguori
>>
>>
>>
>>     
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