just how dangerous is this??

just how dangerous is this??

[James Fillman]

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I've got a large number of linux server to build and I'm using SuSE's ALICE 
install program to build them. ALICE is SuSE's auto install program and has 
limited functionality. Most importantly, it doesn't do RAID. So what I'm 
doing is having ALICE partition two drives with the partitions being of type 
"FD". It installs linux just fine on "sda" and when it's all done, I boot a 
rescue disk, and do a "mkraid -c ./raidtab --really-force /dev/md[0-4]" to 
create the mirrors. I'm just trying it out now and it seems to be working but 
I've got very little experience working with SoftRAID and I thought I'd ask 
if I'm playing with fire!!

the mkraid command warns me that it will delete all the data on the partitions 
when creating the mirrors, but it doesn't. 

What sort of implications am I looking at with this? Will it be unstable?


cheers,
- -- 
#########################
#  James Fillman
#  Linux Systems Architect
#  CUCBC
#
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Re: just how dangerous is this??

[Danilo Godec]

On Tue, 28 May 2002, James Fillman wrote:

> I've got a large number of linux server to build and I'm using SuSE's ALICE
> install program to build them. ALICE is SuSE's auto install program and has
> limited functionality. Most importantly, it doesn't do RAID. So what I'm
> doing is having ALICE partition two drives with the partitions being of type
> "FD". It installs linux just fine on "sda" and when it's all done, I boot a
> rescue disk, and do a "mkraid -c ./raidtab --really-force /dev/md[0-4]" to
> create the mirrors. I'm just trying it out now and it seems to be working but
> I've got very little experience working with SoftRAID and I thought I'd ask
> if I'm playing with fire!!

I think this shouldn't work at all - have you rebooted the machine yet?

It's not quite clear how the mirrors reconstruct if one of the disks
(partitions) already has a filesystem on it, but I'm almost sure it's not
ment to reconstruct it properly...

AFAIK making a filesystem on a Linux SW RAID array is somewhat different
from making it on a 'normal' disk, so while you may end up with a
filesystem, it will probably be full of errors.

How does your raidtab look like?

The 'right' way of doing this is:

- create a 'degraded' mirror with sda* partitions marked as 'failed'
- copy all the data from sda* partitions to the appropriate mirrors
  (/dev/md*)
- modify /etc/fstab on the mirrored filesystem to use the mirrors
- reboot, use filesystems on the mirror (root=/dev/md?)
- hot add sda* partitions to approprate mirrors (using raidhotadd or
  mdadm)
- wait for the reconstruction to finish
- make sure that both disks have the boot loader installed in MBR, so they
  can both be used for booting

This should be it. Unfortunately, this procedure is not that easy to
script.

> the mkraid command warns me that it will delete all the data on the partitions
> when creating the mirrors, but it doesn't.

I guess it doesn't delete the data, it destroys it... :)

> What sort of implications am I looking at with this? Will it be unstable?

I am no super-expert or guru, but I'd say YES. Try booting the system with
a rescue disk and run e2fsck on your mirrored filesystems. If this goes
through OK... Well, then I might be wrong and it's OK to do this with new
kernel/raidtools...

   D.

Re: just how dangerous is this??

[Jakob Østergaard]

On Tue, May 28, 2002 at 09:20:39PM +0200, Danilo Godec wrote:
> On Tue, 28 May 2002, James Fillman wrote:
...
> I think this shouldn't work at all - have you rebooted the machine yet?
> 
> It's not quite clear how the mirrors reconstruct if one of the disks
> (partitions) already has a filesystem on it, but I'm almost sure it's not
> ment to reconstruct it properly...

Either it reconstructs it "right" or it does it "wrong".  It seems in his case
it did it right.   Lucky him.

Hoever, the persistent superblocks will overwrite the last few KB of his
*filesystem* on each partition.   So things *may* seem to work, but the
system will fail horribly later.  After an fsck the RAID suprblocks will
be damaged.  After another mkraid the filesystem will be damaged again.

> 
> AFAIK making a filesystem on a Linux SW RAID array is somewhat different
> from making it on a 'normal' disk, so while you may end up with a
> filesystem, it will probably be full of errors.

Yep.

> 
> How does your raidtab look like?
> 
> The 'right' way of doing this is:
> 
> - create a 'degraded' mirror with sda* partitions marked as 'failed'
> - copy all the data from sda* partitions to the appropriate mirrors
>   (/dev/md*)
> - modify /etc/fstab on the mirrored filesystem to use the mirrors
> - reboot, use filesystems on the mirror (root=/dev/md?)
> - hot add sda* partitions to approprate mirrors (using raidhotadd or
>   mdadm)
> - wait for the reconstruction to finish
> - make sure that both disks have the boot loader installed in MBR, so they
>   can both be used for booting

Yep.

> 
> This should be it. Unfortunately, this procedure is not that easy to
> script.

I suppose that if the installation kernel (the one booted during the installation
process) could be made to support RAID (with autodetection), you could just create
the RAID arrays prior to installation, and then simply install on those arrays.

But that may require some hacking of the SuSE install disk...

> 
> > the mkraid command warns me that it will delete all the data on the partitions
> > when creating the mirrors, but it doesn't.
> 
> I guess it doesn't delete the data, it destroys it... :)

Just a small amount of it.  It "almost doesn't totally f*ck up the system"   ;)

> 
> > What sort of implications am I looking at with this? Will it be unstable?
> 
> I am no super-expert or guru, but I'd say YES. Try booting the system with
> a rescue disk and run e2fsck on your mirrored filesystems. If this goes
> through OK... Well, then I might be wrong and it's OK to do this with new
> kernel/raidtools...

I'd say you're right about the failures.

-- 
................................................................
:   jakob@unthought.net   : And I see the elder races,         :
:.........................: putrid forms of man                :
:   Jakob Østergaard      : See him rise and claim the earth,  :
:        OZ9ABN           : his downfall is at hand.           :
:.........................:............{Konkhra}...............:
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Re: just how dangerous is this??

[Derek Vadala]

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On Tue, 28 May 2002, [iso-8859-1] Jakob Østergaard wrote:

> Either it reconstructs it "right" or it does it "wrong".  It seems in
> his case it did it right.  Lucky him.
> 
> Hoever, the persistent superblocks will overwrite the last few KB of his
> *filesystem* on each partition.   So things *may* seem to work, but the
> system will fail horribly later.  After an fsck the RAID suprblocks will
> be damaged.  After another mkraid the filesystem will be damaged again.

You should be able to pre-plan by creating initial ext2 file systems that
are smaller than the partition size. You can do this by specifying the
number of blocks in the file system when you run mke2fs. 

mke2fs [options] device blocks

You should be able to calculate the eventual location of the md superblock
and select an appropriate block size for each partition to insure that the
superlbock and the filesystem do not overlap. 

Even if you've had sucess thus far, it's likely that problems will arise
as the filesystem fills up-- when the last blocks are allocated, and they
overlap the md superblock.

This probably isn't worth the effort unless you are doing multiple
installs with the same partition layout and hardware, but it seems that it
might be the case here.

I think the formula should be something like:

blocks - (blocks % 64) - 64 = md offset

That's for 1k blocks... ( like from fdisk -l)

I'm a bit lazy to double check, but I think that formula works.

Then when creating a filesystem divide by block size and create a file
system that's the right size... 

---
Derek Vadala, derek@cynicism.com, http://www.cynicism.com/~derek

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