setting up a RAID-1 array with a non-empty hdd?

[yawnmoth]

I have a hard drive whose contents I'd like to back up. I was
thinking about using RAID-1 mirroring to do this. The thing I'm
considered about is... will my drive be wiped? My second drive I
don't really care about - that drives purpose is to just serve as a
backup. But my main drive... I do care about that and reformating
would rather defeat the point.

 

[Arno]

I have a hard drive whose contents I'd like to back up. I was
thinking about using RAID-1 mirroring to do this. The thing I'm
considered about is... will my drive be wiped?
Yes.

My second drive I
don't really care about - that drives purpose is to just serve as a
backup. But my main drive... I do care about that and reformating
would rather defeat the point.

The way to do this with a decent RAID conttroller or software
RAID is as follows: Set up a degraded RAID1 with the second
driv. Partition it and create the filesystem. Then copy everything
over from the first fdirve. And then add the frist drive to
the degraded RAID1 array to get redundancy.

Side note: If the data on the frist drive is important, then surely
you have a backlup of it?

Arno

 

[Ed Light]

Arno,

Is a drive in a RAID1 array instantly usable as a normal drive with data
and booting OS outside it, all by itself, or does it have some special
format?

--
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Thanks, robots.

 

[yawnmoth]

The way to do this with a decent RAID conttroller or software
RAID is as follows: Set up a degraded RAID1 with the second
driv. Partition it and create the filesystem. Then copy everything
over from the first fdirve. And then add the frist drive to
the degraded RAID1 array to get redundancy.

Any idea as to what RAID controllers might provide that
functionality? <http://www.rosewill.com/products/942/
productDetail.htm> looked like a good RAID controller but when I asked
them here's the response I got:

Dear Customer we can not advise you on this, your data is important
there by we cant asusre you that your cofiguration will work in this
case .we will advise create your raid separated from the drive data
then
add
drive with data as a secondary .

And I'm not even sure what that means.

Side note: If the data on the frist drive is important, then surely
you have a backlup of it?

A backup is what I'm trying to create.

 

[David Brown]

A backup is what I'm trying to create.

RAID is not a backup solution. At best, you could perhaps manage to
make a single low-level copy of your disk in this way, which is not
normally the best way to do backups, or at least not if this is your
/only/ backup (for most people, it's their data that's most important).

If you want to make low-level copies of a disk, use some sort of
partition imaging software. There are plenty of such tools available to
suit all tastes - from "dd" to expensive automated hand-holding pretty
gui tools.

 

[Arno]

Arno,

Is a drive in a RAID1 array instantly usable as a normal drive with data
and booting OS outside it, all by itself, or does it have some special
format?

The controller typically places a RAID superblock on it and, if the
superblock is at the start, shifts the sector numbers. The position
and structure of the RAID superblock are unfortunately not
standardized. Anyways, you lose the old format, but apart from
that it is a nordinary drive, i.e. yes.

One exception: Some controllers (cheap Fake-RAID controllers)
may refuse to build a degraded array. I have been doing
thie procedure I described with Linux software RAID several
times, typically on resizing partition bases RAID.

Arno

 

[Arno]

Any idea as to what RAID controllers might provide that
functionality?

I don't know. Linux Software RAID does it and any good controller
should do it, but then the question is what constitutes a good
controller.

<http://www.rosewill.com/products/942/
productDetail.htm> looked like a good RAID controller

That is Fake Raid, i.e. not a RAID controller at all. I have
one with the same chip, this is a plain 2 port SATA controller.
What happens here is that this thing has software RAID in the
BIOS, which turns out to be the wortst possible way to do RAID.

Example of a real hardware controller:
http://www.3ware.com/products/serial_ata2-9650.asp
A bit more expensive though.

<http://www.rosewill.com/products/942/
productDetail.htm> looked like a good RAID controller

but when I asked
them here's the response I got:

Dear Customer we can not advise you on this, your data is important
there by we cant asusre you that your cofiguration will work in this
case .we will advise create your raid separated from the drive data
then
add
drive with data as a secondary .

And I'm not even sure what that means.

I have no idea either. If you add the drive with the data as
secondary, any proper RAID controller will overwrite the data,
and that is definitiely not what you want.

A backup is what I'm trying to create.

Repeat after me: RAID is not backup. Anyways, if you are reasonably
confident in your skills with handling data and the data is not
reallyt important, then RAID can be used as it does at least
some thing a real backup does.

I would advise you to get an external USB drive, make a backup
on that and then copy the data over. If you go with a Fake RAID
controller (which is ok, just don't expect too much), and are
doing this for the first time, the chances that you make a mistake
(or the documentation is wrong or unclear, see also the bizarre
answer you got) and lose all your data are just too high.

As an added benefit, that gives you a means to do a proper
backup.

Arno

 

[Arno]

RAID is not a backup solution. At best, you could perhaps manage to
make a single low-level copy of your disk in this way, which is not
normally the best way to do backups, or at least not if this is your
/only/ backup (for most people, it's their data that's most important).

Let me expand that a little
RAID1 Backup
Protects against disk failure y y
Protects against PC killing disks n y
(lightening, bad PSU,...)
Protects against user error n y
Protects against application/OS error n y
Works automatically y depends

The first item is why people mistake RAID1 for backup. But
backup serves a few additional critical functions that
RAID1 does not help with.

Arno

 

[yawnmoth]

I don't know. Linux Software RAID does it and any good controller
should do it, but then the question is what constitutes a good
controller.


That is Fake Raid, i.e. not a RAID controller at all. I have
one with the same chip, this is a plain 2 port SATA controller.
What happens here is that this thing has software RAID in the
BIOS, which turns out to be the wortst possible way to do RAID.

I assume BIOS software RAID is inferior to Linux software RAID just
because it's likely not going to be as well written as Linux's RAID
is?

Anyway, thanks for the feedback!

 

[yawnmoth]

I don't know. Linux Software RAID does it and any good controller
should do it, but then the question is what constitutes a good
controller.


That is Fake Raid, i.e. not a RAID controller at all. I have
one with the same chip, this is a plain 2 port SATA controller.
What happens here is that this thing has software RAID in the
BIOS, which turns out to be the wortst possible way to do RAID.

Example of a real hardware controller:http://www.3ware.com/products/serial_ata2-9650.asp
A bit more expensive though.

Also, is there a way to easily tell if an arbitrary card uses BIOS
software RAID or has "real" RAID controllers? And what advantage does
real RAID have over software RAID, be it Linux or BIOS? I assume less
CPU usage overhead?

 

[Arno]

I assume BIOS software RAID is inferior to Linux software RAID just
because it's likely not going to be as well written as Linux's RAID
is?

Basically: No suport for RAIDed partitions, very basic admin
tools, etc.. May still be ok for lower requirements. And
althoigh most of these do not state Linux support, the Linux
dm-raid driver is often able to access them under Linux as well.

Anyway, thanks for the feedback!

No problem.

 

[Arno]

Also, is there a way to easily tell if an arbitrary card uses BIOS
software RAID or has "real" RAID controllers? And what advantage does
real RAID have over software RAID, be it Linux or BIOS? I assume less
CPU usage overhead?

It used to be a speed question. It is not anymore.

Basically, hardware RAID has the following advantages:
- automated hotplug and rebuild, enclosure support
- low/no OS dependency

for Linux Software RAID you get
- can work on partitions
- very easy to monitor and administrate

Speed-wise they are pretty much the same if the software RAID
goes over a PCI-E attached controller.

As to recognition, if it is > 300EUR/USD, it is hardware RAID ;-)

Arno