Does breaking RAID1 mirror generally destroy drive contents?

[Sorry, I originally intended to post this msg here, but posted it into
comp.sys.ibm.pc.hardware.storage instead by mistake (selected wrong NG
in my newsreader). I think this might be the more appropriate NG.]



Hi,

I was working with an HP Proliant DL360 system with a "Smart Array 5i"
controller and 2 72GB SCSI drives.

The system was originally configured with the two drives in a RAID1
configuration, as one "Logical Drive". The Logical Drive was
partitioned as one 10GB partition and one 60GB partition.

I wanted to break the mirror, i.e., end up with 2 72GB drives, instead
of 1 72GB "Logical Drive".

To do this, I went into the Smart Array utility by pressing F8 during
bootup, and selected "Delete Logical Drive" (there were only 3 choices:
Create Logical Drive, Delete Logical Drive, and View Logical Drive).

After doing that, the system couldn't see either drive, and it wouldn't
let me (as far as I could tell) configure the drives as 2 separate
drives, so I went back and did Create Logical Drive, to put back the
mirror.

After all of that, when I looked at the drive, all the partitions were
completely gone.

I wasn't completely surprised about this, as I had been warned that this
might happen, but I thought that with a hardware RAID controller, the
two drives that are mirrored in a RAID1 configuration are still
essentially two independent drives that just happen to have the same
contents.

So, I was wondering: In general, is what I saw tonight always going to
be the case?

If I put 2 drives into a RAID1 configuration, and then later break the
mirror (in the controller configuration), will the contents of both
drives always be wiped out?

If so, why?

Does the RAID controller write some kind of configuration information
onto the mirrored drives when it mirrors the drives?

Thanks,
Jim

The answer to your question is "No." Details follow.

ohaya wrote:

....

Hi,

I was working with an HP Proliant DL360 system with a "Smart Array 5i"
controller and 2 72GB SCSI drives.

The system was originally configured with the two drives in a RAID1
configuration, as one "Logical Drive". The Logical Drive was
partitioned as one 10GB partition and one 60GB partition.

I wanted to break the mirror, i.e., end up with 2 72GB drives, instead
of 1 72GB "Logical Drive".

To do this, I went into the Smart Array utility by pressing F8 during
bootup, and selected "Delete Logical Drive"

Bad move #1. Does "Delete Logical Drive" really sound much like "Break
Mirror"?

(there were only 3 choices:
Create Logical Drive, Delete Logical Drive, and View Logical Drive).

With what one might consider to be fairly clear meanings, save possibly
for the last (which might actually have given you some additional
options, such as 'break mirror' - or not, since I'm not familiar with
the hardware in question).


After doing that, the system couldn't see either drive,

Of course not: you had just deleted it at the controller level, so the
controller provided nothing for the system to see. But the data was
almost certainly all still in place on the disks (though depending upon
just how the controller handles things you might have had to tweak some
high-level metadata to make it visible).

and it wouldn't
let me (as far as I could tell) configure the drives as 2 separate
drives, so I went back and did Create Logical Drive, to put back the
mirror.

No, to create a *new* mirror. Bad move #2, and it dutifully did just
what you told it to, apparently by clearing out everything on both
drives (or at least clearing out enough metadata to make them appear
empty: with a disk editor, controller internals documents, and some
patience you might find it all still accessible after cleaning up some
controller metadata).


After all of that, when I looked at the drive, all the partitions were
completely gone.

I wasn't completely surprised about this, as I had been warned that this
might happen, but I thought that with a hardware RAID controller, the
two drives that are mirrored in a RAID1 configuration are still
essentially two independent drives that just happen to have the same
contents.

Pretty much so, though the controller may well reserve a few disk
sectors for its own bookkeeping information.


So, I was wondering: In general, is what I saw tonight always going to
be the case?

If you repeat the same sequence of actions, most likely yes.


If I put 2 drives into a RAID1 configuration, and then later break the
mirror (in the controller configuration), will the contents of both
drives always be wiped out?

Almost certainly not. Just make sure next time that what you actually
do *is* break the mirror rather than something more destructive.


If so, why?

Does the RAID controller write some kind of configuration information
onto the mirrored drives when it mirrors the drives?

Some (I suspect most) do, some don't. Any reasonable hardware RAID
controller should contain some NVRAM where it can keep most of the
critical metadata it needs, but still might want to place some kind of
identifier on the disks themselves to ensure that the right ones are
present if for some reason it doesn't choose to hold the disk IDs in its
NVRAM (or just to make the configuration recoverable if the controller
itself dies).

- bill

Bill,

Thanks. A couple of comments below...

Jim


So, I was wondering: In general, is what I saw tonight always going to
be the case?

If you repeat the same sequence of actions, most likely yes.

By "In general", I was trying to ask if this (loss of data/partitions,
etc.) would always occur, e.g., with other/all hardware-based RAID
controllers.

If a RAID controller does not maintain any information on the mirrored
drives themselves, would you expect that doing what I did would have
allowed me to get access to the individual, non-mirrored disk?



If I put 2 drives into a RAID1 configuration, and then later break the
mirror (in the controller configuration), will the contents of both
drives always be wiped out?

Almost certainly not. Just make sure next time that what you actually
do *is* break the mirror rather than something more destructive.

I had looked, but the only options that they provided were the ones that
I mentioned (create, delete, and view), and I couldn't find anything
under those to, for example, remove an individual drive from a "Logical
Drive". Also, the way that the utility worked was such that when I
tried to create a Logical Drive, it wouldn't allow me to create one with
just a single 'real' drive... it automatically selected both drives.



If so, why?

Does the RAID controller write some kind of configuration information
onto the mirrored drives when it mirrors the drives?

Some (I suspect most) do, some don't. Any reasonable hardware RAID
controller should contain some NVRAM where it can keep most of the
critical metadata it needs, but still might want to place some kind of
identifier on the disks themselves to ensure that the right ones are
present if for some reason it doesn't choose to hold the disk IDs in its
NVRAM (or just to make the configuration recoverable if the controller
itself dies).

I've been doing a lot of searching, and I think that I may have found
something in some of the HP documents on these "Smart Array 5i"
controllers.

The documents that I've looked at are a bit vague, but it kind of looks
like when they say "RAID1", they actually mean "1+0".

I guess that if I'm only mirroring a single pair of drives, "1+0"
becomes effectively "1", functionally, but I'm beginning to think that
even if there is only a single pair of drives, this controller must be
putting some kind of striping information on each of the drives in the
first pair, because the controller also seems to support extending the
RAID configuration (e.g., by adding on additional pairs of drives).

Thanks for your comments.

Jim

ohaya wrote:
Bill,

Thanks. A couple of comments below...

Jim



So, I was wondering: In general, is what I saw tonight always going to
be the case?

If you repeat the same sequence of actions, most likely yes.


By "In general", I was trying to ask if this (loss of data/partitions,
etc.) would always occur, e.g., with other/all hardware-based RAID
controllers.

If a RAID controller does not maintain any information on the mirrored
drives themselves, would you expect that doing what I did would have
allowed me to get access to the individual, non-mirrored disk?

No. Depending on how the controller handles a 'create' request, it
could easily wipe everything even if the controller maintains no private
data on the disks. I suppose it could even do so on a 'delete' request,
though I wouldn't normally expect it to bother.





If I put 2 drives into a RAID1 configuration, and then later break the
mirror (in the controller configuration), will the contents of both
drives always be wiped out?

Almost certainly not. Just make sure next time that what you actually
do *is* break the mirror rather than something more destructive.


I had looked, but the only options that they provided were the ones that
I mentioned (create, delete, and view), and I couldn't find anything
under those to, for example, remove an individual drive from a "Logical
Drive". Also, the way that the utility worked was such that when I
tried to create a Logical Drive, it wouldn't allow me to create one with
just a single 'real' drive... it automatically selected both drives.

Seems unlikely that the hardware wouldn't support this, but perhaps the
utility software is brain-damaged (or you just didn't find whatever
support for this function it contains).

If I wanted to break a mirror and couldn't find explicit support for
doing so in the related hardware, I'd just quiesce the storage
gracefully (if no specific mechanism was obvious, just waiting a while
with no access activity and then powering it down), remove the disks,
and attach them directly (if the controller really didn't have any
mechanism for acting as a JBOD) and see if they looked usable (I'd kind
of expect them to be even if the controller maintained some private
information on them, perhaps sequestered in a private partition or in
space at the beginning or end of the disk which it kept hidden from
external access).

- bill

I had looked, but the only options that they provided were the ones that
I mentioned (create, delete, and view), and I couldn't find anything
under those to, for example, remove an individual drive from a "Logical
Drive". Also, the way that the utility worked was such that when I
tried to create a Logical Drive, it wouldn't allow me to create one with
just a single 'real' drive... it automatically selected both drives.

Seems unlikely that the hardware wouldn't support this, but perhaps the
utility software is brain-damaged (or you just didn't find whatever
support for this function it contains).

Hi,

I'm not sure if this makes any sense (maybe it's just too late at
night), but from some rather vague references in the HP controller
documents, it kind of looks like they might support a single drive off
of the controller by setting each of the individual 'real' drives up as
a RAID0 Logical Drive with 1 drive in it. In other words, since I have
two 'real' drives, I'd "create" two Logical Drives, where each Logical
Drive consists of a set of one 'real' drive...

Not sure if I want to try that, though, at this point, I don't have much
to lose, except for 'time' ...

Thanks again,
Jim

P.S. Did any of the stuff that I said about the "1+0" make any sense?

On Sun, 13 Feb 2005 23:56:27 -0500, Bill Todd
wrote:

[ Snip ]

Does the RAID controller write some kind of configuration information
onto the mirrored drives when it mirrors the drives?

Some (I suspect most) do, some don't. Any reasonable hardware RAID
controller should contain some NVRAM where it can keep most of the
critical metadata it needs, but still might want to place some kind of
identifier on the disks themselves to ensure that the right ones are
present if for some reason it doesn't choose to hold the disk IDs in its
NVRAM (or just to make the configuration recoverable if the controller
itself dies).

Yup.

Smart designers put the metadata at the *end* of each of the disks.
Dumb ones put it at the front.

There are a lot of dumb designers out there, apparently...

- bill

Malc.

ohaya wrote:

I had looked, but the only options that they provided were the ones that
I mentioned (create, delete, and view), and I couldn't find anything
under those to, for example, remove an individual drive from a "Logical
Drive". Also, the way that the utility worked was such that when I
tried to create a Logical Drive, it wouldn't allow me to create one with
just a single 'real' drive... it automatically selected both drives.

Seems unlikely that the hardware wouldn't support this, but perhaps the
utility software is brain-damaged (or you just didn't find whatever
support for this function it contains).

Hi,

I'm not sure if this makes any sense (maybe it's just too late at
night), but from some rather vague references in the HP controller
documents, it kind of looks like they might support a single drive off
of the controller by setting each of the individual 'real' drives up as
a RAID0 Logical Drive with 1 drive in it. In other words, since I have
two 'real' drives, I'd "create" two Logical Drives, where each Logical
Drive consists of a set of one 'real' drive...

Not sure if I want to try that, though, at this point, I don't have much
to lose, except for 'time' ...

Thanks again,
Jim

P.S. Did any of the stuff that I said about the "1+0" make any sense?


Hi,

I got some confirmation from HP yesterday re. configuring the individual
drives so that they appear individually. Apparently, with the Smart
Array 5i controller, you can do this by first creating a RAID0 Logical
Drive with just one drive, then creating a 2nd RAID0 Logical Drive with
the 2nd drive.

I think that the reason that deleting the Logical Drive earlier caused
the drive data to be unavailable was that, as some of you have
suggested, that at that point the controller didn't have any Logical
Drives configured (duh ).

I'm kind of curious if I had tried creating the two RAID0 Logical Drives
at that point (after deleting the RAID1 Logical Drive), if I would have
been able to see the original partitions/data from the 1st physical
drive (since the HP "RAID1" was apparently actually a "1+0"), but I
haven't tried that yet.

In other words, I'm thinking that in the original (RAID1+0)
configuration, the drives actually had:

Drive1: Striping + Partitions/Data
Drive2: Mirrored copy of Drive1 (i.e., Striping + Partitions/Data)

and, if I then deleted the Logical Drive and re-created two Logical
(RAID0) drives, I'd see two individual Logical Drives:

Drive1: Striping' + Partitions/Data
Drive2: Striping + Partitions/Data (which happen to be same as Drive1)

Seems like this might work if the RAID0 "Striping'" was exactly the same
as the RAID1(+0) "Striping"??

Jim