Re: RAIDING different size drives

Rod Speed wrote:
David Brown wrote
Rod Speed wrote
David Brown wrote

The extent to which your RAID offers protection depends on your
failure recovery strategy.

Nope.

The benefits of *any* redundancy system are totally dependent on
what happens when part of it fails.

Wrong. Mirroring is a useful protection against hard drive failure
regardless of what what happens when part of it fails.


Consider a setup like this:

Disk A, 750G:
partition 1, 250G
partition 2, 500G
Disk B, 500G:
partition 1, 500G

You put your OS, programs, etc., on "C drive" on partition A.1. You mirror partition A.2 and B.1, and put your data on this "D drive".

Disk A dies.

You have a safe copy of all your data on the mirror B.1. But your OS drive is dead.

How do you get your data off B.1 again?

If you want to tell me it's easy, that's fine - as I've said before, I know about software raid on Linux but not on Windows. If it's easy, then you have a good system because you can easily recover your data after a failure.

If you want to tell me it's a stupid setup because you can't continue working, and the "C" drive should also be on the mirror, then that's also fine. You'd then be saying that it's a bad idea because recovery is hard.

If it is hard to recover data here, then you can certainly argue that it is still better than nothing - hard but not impossible. But the benefits of the protection are much smaller if recovery is difficult, since you want to get your data back in a safe and timely manner.

If you are are not interesting in recovering when something has
gone wrong,

Obviously the OP is.

you might as well backup to write-only media.

Having fun thrashing that straw man ?


There are a *lot* of people who effectively backup to write-only media - I'm sure you've met plenty who think they have a good backup system until the day comes when they need to do a restore. But fair enough, it's a straw man here.

If the setup is a hardware RAID of the two drives, it's easy.
But if the setup is based on Windows software RAID, then I
don't know about recovery (having never used this setup), and
it may be difficult if the OS disk is trashed.

Wrong again.

What part of that paragraph was "wrong"?

The last half of the last sentence.


So it's not difficult to recover after failure using Windows software raid? Does that apply when the whole disk (OS and data) are installed on a software raid? And does it apply when you have just the data on the software raid?

Remember, I had originally made a suggestion of just using 500 GB of the 750 GB for raid for storing data (see the details earlier in this post). I know it's straightforward to access this data on a Linux system even if drive with the OS has failed - you can boot a live CD, and mount it directly. So for Linux, it's a perfectly reasonable setup with a different balance between efficient use of disk space and redundancy and uptime than you would get with a full drive raid.

But I'd like to be absolutely clear on this one - is it equally simple with Windows software raid, or was it such a stupid idea for windows that you didn't realize I'd suggested it for consideration? I don't have the experience with windows software raid to know here - instinct tells me it's a stupid idea (hence my original sarcastic post), but I could be wrong.


No, I don't know what backup system he has now or has planned to
use. But his original post looked very much like he thought RAID
would give him data security for his files. And as we all know,
RAID is not for backup.

It does however give useful extra protection against hard drive failure which can be handy when the drive fails between backups.


This point is certainly true, and one that I hadn't taken into account - I was thinking only of longer term data security.

Do you care to give a reference or example showing how a RAID setup
can satisfy the requirements of a good backup system?

Thats an entirely different matter to your original 'does not
noticeably improve data security'


Fair enough.

I think we have a slightly different idea of what the risks are regarding data - we have different backgrounds, different experiences, and different types of hardware, software, users, and data use. For my users, I don't expect to see any noticeable data loss if a desktop harddrive dies. It means lost productivity, and an inconvenience, but not data loss other than perhaps files that are open at the time.

For home users without a server (with backup), I firmly believe that using raid mirroring is of very little benefit compared to spending the same money on a backup system (for home users, that typically means DVD writers, or perhaps an Internet-based backup system), or simply a second drive to which you make regular copies.

Either way, I don't think desktop raid is a big gain for data security. But clearly it does provide *some* gain - and if you already have a good backup system, then it adds a little more (as well as speed and uptime benefits).

Of course it mirroring improves data security, most obviously with
the new data that has showed up between backups.

.

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