[ale] Questions on Linux software RAID

Robert L. Harris Robert.L.Harris at rdlg.net
Mon Feb 2 20:45:39 EST 2004

Assume you have:

/dev/md0 which is a mirror of /dev/hda1 and /dev/hdb1 which is / (root)
/dev/md1 which is a mirror of /dev/hda2 and /dev/hdb2 which is /usr (root)

Thus spake Greg (runman at speedfactory.net):

> I am somewhat confused about Linux software RAID.  Suppose I have 2 disks in
> a RAID 1 (mirrored) setup.  hda and hdb are the disks.  hda dies and I need
> to get to the data on disk hdb.  Is the following true ?
> 1. I can still get to the data on hdb, even though the raid array is broken
> (a status of _ for hda and U for hdb in /proc/mdstat).  My research leads me
> to believe this is true.

You can read /dev/md0 and /dev/md1 which exists on /dev/hdb? still even
though /dev/hda is dead.
> 2. If I break the array, I cannot get to the data on hdb, since the
> partition table is written to show hdb as part of an array.

If you break the array the data is still on the mirrors but you actually
can still mount /dev/hdb2 to access /usr if you reboot and your system
doesn't boot off the mirror for some reason (mine boot off /dev/hdb1
fine and then load the raids)

> 3. Can I re-do the partition table of hdb to make it a single disk (no RAID)
> that has the data on it still?

You can still find the data but it's not worth doing.

> Now for 3 questions:
> 1. Is there anything besides constant checking /proc/mdstat that would tell
> someone that the array is broken ???

You can use some options to "mdadm" which will give you a status.

> 2. Would rsync on an hourly basis be better ?

definitely not.

> 3. Would rsync on an hourly basis be better ?

See #2.
> I am trying to use Linux software raid for a friends pc.  They want to be
> able to take one of the disks away when they leave so if the house burns
> down, they won't lose all of their data.

Ugly solution but very workable.  If you have any problems let me know,
I use software raid ALOT at work and at home.

Robert L. Harris                     | GPG Key ID: E344DA3B
                                         @ x-hkp://pgp.mit.edu
      These are MY OPINIONS ALONE.  I speak for no-one else.

Life is not a destination, it's a journey.
  Microsoft produces 15 car pileups on the highway.
    Don't stop traffic to stand and gawk at the tragedy.
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