lossless growing of linear RAID (resizing)

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Date: Wed, 17 Aug 2005 12:26:07 -0300
To: debian-user@lists.debian.org

I had great difficulty finding info. on how to convert from RAID-0 to
linear and then grow the system.

I have since found out, and am documenting the details here in case
others have similar needs.

My setup (for the compelte debian mirror at debian.fapeal.br) was 2 80
Gb disks on separate controllers, running RAID-0. This gave me about
150 Gb on /dev/md0 but the Debian mirror now needs more space. I
bought one more disk of 120 Gb but I needed to switch from RAID-0 to
linear, since with 3 disks raid-0 makes no sense if you have only 2
controllers.

This is what ended up working:

1. copy the data from RAID-0 /dev/md0 to the new disk, on an ext2 fs.
   the disk cannot become 100% full, leave some empty space.

2. then, destroy the old RAID-0 system by running mke2fs

3. edit /etc/raidtab to remove the old md0 device

4. edit raidtab to include a new raid linear device. The new disk should
   be the 1st disk, and the old (former RAID-0) disks can be 2 and 3.

5. create a new raid linear device, with persistent superblock, using
   mkraid -f. You will have to use --really-force and you will get a nasty
   warning about your data being lost. Just sit tight and ignore the warning.

6. The new raid device will have the full size of the 3 (or more) disks
   concatenated. However, the existing fs will still be in place, so that
   after fsck and mounting, df will show a total size only of the 1st disk.
   the full md device size will not be accessible. Your old data will still
   be there.

7. so after runnign fsck, run resize2fs /dev/md0 (or other device)
   and the fs size will expand to include the full space. No data
   will be lost.

I tested this and it works (and my mirror will be back online soon I
hope).

Of course, make backups, etc. (IN my case this was not possible due to
the sheer size and my limited resources).

There are other methods, like mke2fs -S, but of all the methods I
tested, this one is the best method for growing a raid-linear device.

-gandhi

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