convert legacy array in place?

convert legacy array in place?

[Stephen Walton]

Hi,

It seems like this should be a FAQ but careful google and mailing list 
archive searches didn't seem to turn up anything.

I have two machines with legacy RAID-0 arrays created with raidtools.  I 
have updated one to Fedora Core 4 and now use mdadm.  Am I correct in 
thinking I have to use "mdadm --build" every boot to access the array?  
My mdadm.conf contains

DEVICES /dev/hda1 /dev/hdb1 /dev/hdc1
ARRAY /dev/md0 level=raid0 num-devices=3 
devices=/dev/hda1,/dev/hdb1,/dev/hdc1

but the array doesn't seem to come up by itself on boot.

On the same lines, can this array be converted to a new style array just 
by writing new superblocks, or do I have to do the 
backup-reformat-restore thing?

TIA.

Re: convert legacy array in place?

[Neil Brown]

On Sunday August 14, stephen.walton@csun.edu wrote:
> Hi,
> 
> It seems like this should be a FAQ but careful google and mailing list 
> archive searches didn't seem to turn up anything.
> 
> I have two machines with legacy RAID-0 arrays created with raidtools.  I 
> have updated one to Fedora Core 4 and now use mdadm.  Am I correct in 
> thinking I have to use "mdadm --build" every boot to access the array?  
> My mdadm.conf contains
> 
> DEVICES /dev/hda1 /dev/hdb1 /dev/hdc1
> ARRAY /dev/md0 level=raid0 num-devices=3 
> devices=/dev/hda1,/dev/hdb1,/dev/hdc1
> 
> but the array doesn't seem to come up by itself on boot.

If it is a "legacy array" in that it was created with
"persisent-superblock = no"
then you are right. You need to use "mdadm --build" to assemble it.
--assemble ONLY works with arrays that have superblocks.
With --build, you need to make sure that you get the chunksize correct
of course (as it isn't stored in the superblock).

> 
> On the same lines, can this array be converted to a new style array just 
> by writing new superblocks, or do I have to do the 
> backup-reformat-restore thing?

An array with superblocks is slightly smaller than the legacy array
(due to the superblocks).  So you cannot do a straight conversion.
If you have afford to loose the last 64k-per-device of the array, then
you can simply re-create it as a with-superblock array.

If you are using ext2 or ext3, then you may be able to resize you
filesystem down with resize2fs and then remake the array.

NeilBrown