[Linux-cluster] Unformatting a GFS cluster disk

[DRand at amnesty.org]

Hi Bob,

Great, thanks for your info! The disk was previously a GFS disk and we 
reformatted it with exactly the same mkfs command both times. Here are 
more details. We are running the cluster on a Netapp SAN device.

1) mkfs.gfs -J 1024 -j 4 -p lock_gulm -t aicluster:cmsgfs /dev/sda [100Gb 
device]
2) Copy lots of files to the disk
3) gfs_grow /san   [Extra 50Gb extension added to device]
4) Copy lots of files to the disk
5) mkfs.gfs -J 1024 -j 4 -p lock_gulm -t aicluster:cmsgfs /dev/sda 

I have now read about resource groups and the GFS ondisk structure here..
  http://www.redhat.com/archives/cluster-devel/2006-August/msg00324.html

A couple more questions if you don't mind...

What exactly would the mkfs command have done? Would the mkfs command have 
overwritten the resource group headers from the previous disk structure? 
Or does it just wipe the superblock and journals?

If the resource group headers still exist shouldn't they have a 
characteristic structure we could identify enabling us to put 0xFF in only 
the correct places on disk?

Also is there anyway we can usefully depend on this information. Or would 
mkfs have wiped these special inodes too?

+ * A few special hidden inodes are contained in a GFS filesystem.  They 
do
+ * not appear in any directories; instead, the superblock points to them
+ * using block numbers for their location.  The special inodes are:
+ *
+ *   Root inode:  Root directory of the filesystem
+ *   Resource Group Index:  A file containing block numbers and sizes of 
all RGs
+ *   Journal Index:  A file containing block numbers and sizes of all 
journals
+ *   Quota:  A file containing all quota information for the filesystem
+ *   License:  A file containing license information

In particular there is one 11Gb complete backup tar.gz on the disk 
somewhere. I'm thinking if we could write some custom utility that 
recognizes the gfs on disk structure and extracts very large files from 
it?

Damon.
Working to protect human rights worldwide

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