[Sheepdog] Drive snapshots and metadata

[Chris Webb]

Hi Kazutaka.

MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp> writes:

> On second thought, "setattr/getattr" looks better to me because it is
> like an extended attribute of a file.  And if so, it may be better
> that "setattr" reads a value from stdin like the attr command by
> default;
> 
>   $ collie vdi setattr [key] < value_file
> or 
>   $ collie vdi setattr [key] -V [value]

I prefer this design too. For our purposes, I just hope to store short text
values in a handful of these keys, but this interface has the advantage that
it's eight-bit clean for people who want that.

Could probably get away with

  $ collie vdi setattr KEY < value_file
  $ collie vdi setattr KEY VALUE

as you can cleanly distinguish between the two cases by the presence or
absence of a VALUE argument.

> In that case, to unset key, a delete option would be required.
> 
>   $ collie vdi setattr [key] -d    
> 
> > 
> > In an ideal world, it would be good to have some sort of
> > no-clobber/exclusive mechanism too. For example:
> > 
> >   $ collie vdi setdata [key] [value] exclusive
> > 
> > which if [key] is unset, sets it to value [value], or if it is set, fails
> > with non-zero exit code. (This could be used by a management layer to
> > implement exclusive access to drives, for example.)
> 
> Yes, it would be useful to have the feature.

Perhaps a -x argument to specify exclusive/no-clobber behaviour, so the
final syntax might be

  $ collie vdi getattr KEY
  $ collie vdi setattr KEY [-x] < VALUEFILE
  $ collie vdi setattr KEY [-x] VALUE
  $ collie vdi setattr KEY -d

with getattr returning the value on STDOUT and exit status 0, or exit status
!= 0 if the key doesn't exist, and setattr returning exit status != 0 if the
key does exist and -x is given.

Best wishes,

Chris.