Re: Expanding existing Hammer FS

[Michael Neumann <mneumann@xxxxxxxx>]

Am 16.12.2008 20:28, schrieb Matthew Dillon:
> :Hi,
> :
> :I'd be nice to be able to add just another spare disk to an existing
> :Hammer FS to expand it's capacity. After taking a look at newfs_hammer,
> :it seems to be pretty easy to do that:
> :
> :   * run format_volume on the new disk
> :   * ondisk->vol_num += 1 (number of volumes)
> :     for each volume of the filesystem.
> :
> :I think that would be enough and I can hack it. My question is more
> :API-oriented. How would the command line look like?
>
>      Almost.  You also have to adjust the freemap to indicate
>      that the space is available.  That is a bit more difficult but
>      not too difficult.  Normally what you would do is allocate
>      space from the new volume itself for the freemap layer1 and layer2
>      pages involved, and then just map them into the master freemap's top
>      level array in the volume header.
>    
Yes, I think that is done by format_volume (initialize_freemap).

> :   How about this:
> :
> :       hammer newfs label [... specify some defaults...]
> :
> :   will create the following file:
> :
> :       /etc/hammer/label
> :
> :   which will list all devices that make filesytem "label".
>
>      In the hammer_newfs utility only, not in the main hammer utility.
>      The newfs function is just too dangerous to be in the main
>      hammer utility.
>    

Okay. But adding a volume also means formatting it. So I guess
I have to add an option to newfs_hammer to allow to initialize (format)
a single volume, without generating a filesystem.

> :   Now add a volume:
> :
> :     hammer add-volume label /dev/ad0
> :
> :   This will format /dev/ad0 and attach it to the filesystem
> :   "label" by iterating over all volumes in /etc/hammer/label and
> :    increasing the volume count.
> :
> :   Of course it would be more beautiful if the root volume would
> :   store those device names. That would also ease mounting of
> :   a multi-volume HAMMER fs:
> :
> :     # create multi-volume Hammer FS
> :     newfs_hammer /dev/ad0 /dev/ad1
> :
> :     # mount multi-volume Hammer FS by specifying the root volume
> :     mount_hammer /dev/ad0
> :
> :     # add /dev/ad2 to the filesystem specified by the root volume ad0
> :     hammer add-volume /dev/ad0 /dev/ad2
> :
> :Any suggestions?
> :
> :Regards,
> :
> :   Michael
>
>      On hammer add-volume, you should specify the filesystem and the device
>      to add, not the original device(s).  And it should only operate on a
>      live (already mounted) filesystem.
>
>      hammer add-volume /myhammermount /dev/ad2
>      hammer del-volume /myhammermount /dev/ad2
>    

Hm, but what happens after a reboot? It won't boot anymore, without
making a change to /etc/fstab. Ideally this information would be stored
in the root volume (or maybe somewhere else?).

So the sequence of expanding a HammerFS would then be:

  # newfs root volume
  newfs_hammer -L label /dev/ad0

  mount_hammer /dev/ad0 /hammer

  # prepare (format) a volume
  newfs_hammer -L label -F /dev/ad1

  # and add it to the filesystem
  # (checks if label of root-volume and that of /dev/ad1 matches)
  hammer add-volume /hammer /dev/ad1

  reboot

  mount_hammer /dev/ad0 /dev/ad1 /hammer

Regards,

  Michael