Topic: Addind two drives for raid

[Raymondh]

I have SME 5.5 installed on a one drive system.  Can I install two 20gig drives on the second IDE and use them as raid for /home?

Thank you for the help,

Ray

[Dan Brown]

Yes, it's possible, but (1) you'll have to do it all manually, and (2) it's a _really_ bad idea to try to RAID two drives on the same IDE channel.

[Raymondh]

So what is the best way to set one of these servers up?  If I have a system drive and a raid array will the initial install process set everything up correctly or if the install designed for single drive systems?

Ray

[Dan Brown]

SME is designed for (1) a single drive, (2) a pair of identical drives which the system will mirror by software, or (3) a single array of any variety of drives on a controller which appears to the OS as a single drive.

It can be made to work in the situation you describe (though, again, putting the drives as secondary master and slave is a very bad idea), but it would have to be done manually, AFAIK.

[ryan]

Dan,  

Seperate issue, but on this subject.  With just the motherboard IDE controllers, it is bad to use RAID one on the first channel if a IDE tape drive and CD rom exist on the other?

Ryan

[Dan Brown]

Because of the way IDE works, you don't ever want both RAID drives on the same IDE channel.  It will work, but it'll probably be very slow.

[ryan]

That was a quick reply!

So do you think it is OK to have both IDE drives as masters with the IDE tape drive and CD-ROM as slaves using RAID1?

Maybe hardware RAID is the most pratical if RAID plus tape bakup are needed?


Ryan

[Kelvin]

I would recommend hardware RAID. IDE based Mirroring controllers aren't very expensive. Make sure you have access to suitable drivers for the card you get. This way you get away from the problem about which drive to daisy chain with the slower IDE devices (ie. the Tape drive and CD-ROM Drive). Additionally, most hardware RAID cards have a much simpler means of reimaging a drive after a drive replacement (no linux commands necessary).

High end RAID cards (usually RAID 5 cards) can also support hot swap (with suitable hot swap trays). Software RAID cannot (tray or no tray).

Be aware that the SME installer is not designed to work with multiple drives in the system when they are not identical drives intended for software RAID-1. What exactly it does is supposedly "undefined" but one thing for sure, it wipes clean every drive it finds. I once had a system setup with 3 drives, 2 IDE (not for RAID) and 1 SCSI and the installer wiped all 3, created a small partition one drive 1, a bigger one on drive 2 for all the files (I could not find any files on drive 1 and partition magic reckons it is not a swap partition either, so I don't know what it is!) and nothing on the SCSI drive (which used to have data).

Kelvin

[Dan Brown]

Ryan, what you suggest would work fine.  A hardware RAID card would be better for the reasons Kelvin mentions, but of course it's cheaper to not buy one, and it isn't necessary.

[Kelvin]

Ryan,

As Dan says, what you are proposing works, but IMHO not ideal. There can be (and usually are) performance problems or issues when sharing the IDE channel with slower IDE devices.

Example :-
1) Why you should NEVER RAID drives on the same IDE channel (SCSI does not suffer this problem)
a) IDE controllers can only write to one device on a single channel at a time -> performance issue
b) in some cases, if a master drive fails, the slave drive becomes inaccessible as well until its master is replaced or it itself is reconfigured as master -> reliability issue

2) If you share a channel between a fast UDMA capable HDD and another device only capable of PIO modes, then most controllers will force both devices to run at the slower PIO mode -> performance issue.

3) If you are using older motherboards with slower disk controller chipsets, you will not be able to take advantage of the capabilities of the newer drives. If you use an add-in controller or for just a little bit more a hardware mirroring controller, you can run the drives at their faster speeds (after enabling hdparm of course !)

4) OK I'm the first to admit that I don't care to remember too many commands if I can avoid it at all (got lazy with Windows). So, between trying to remember how to recover a software RAID if the problem comes about, and using a menu driven BIOS interface from a hardware controller, no guesses which one I'm going to choose. Since RAID recovery is (hopefully) something you don't have to do often, there is even less chance that I'm going to remember the procedure or commands to do it (especially if you get a call from a client while you are on the road). No such problem with a hardware RAID controller. I have clients who are really far away -> they can easily get and install a replacement HDD, but less likely find someone with enough tech. knowledge to work on the linux software RAID.

Just my 2 cents.

Kelvin

[Kelvin]

Oh yes, one more thing, here in OZ, last time I looked, you can pick up a Highpoint based RAID-1 controller for round AUD$80.00 (possibly less if you hunted around). Not a lot of money for the convenience, IMHO.

Kelvin

[ryan]

Thanks for the info guys.

Ryan

[ryan]

Agreed

[Dan Brown]

Yeah, but the highpoint-based controllers generally aren't real hardware raid, and depend on their drivers to handle all the RAID stuff.  If you want a real hardware IDE raid controller, you're looking at stuff like 3Ware's 7500 series, which by all accounts is very nice (and very fast), but a bit pricier--they start at about $140 US for a 2-port model, or $250 for a 4-port.  However, they do support features like hot-swap (with appropriate cages) and hot spares, and come with up to 12 ports (want a terabyte in your machine?  No problem!).

Now, the caveat: I haven't personally worked with any of this hardware; this is all based on things I've read in various places.  If I were going to buy a hardware IDE RAID controller, it'd be a 3Ware, but I really don't have any need for one at the moment

[Kelvin]

Hi Dan,

> highpoint-based controllers generally aren't real hardware raid
Maybe, but whichever controller you choose, you still need a driver for it. And having used many of these cards before, I can't say I could tell that they are or they aren't true hardware controllers during normal operation (as it just works). All other points I made previously still justifies their use.

Anyway, if you want to get away from drivers altogether, you could try things like the Accusys ACS 7500 disk mirroring subsystem (approx. AUD $650). It's hot swap capable and OS independent (does not require additional drivers beyond that which is built into your OS to access the Motherboard's own onboard IDE channels). Some users on this phorum have indicated they even use one of these types of system as a backup method instead of a tape backup (I personally will still go for a tape drive). There's even a RAID-5 version as well. The downside to the RAID-5 version is performance as it connects to the M/B via 1 channel only (this does not matter for RAID-1). For another choice of RAID-5 card, try the Adaptec 2400A (the Adaptec 1200A is an example of a RAID-1 card based on the Highpoint 370 Chipset).

Kelvin