SCSI RAID advice please!

[MONKEYMAN]

Hands up if you think its a good idea to buy a load of scsi drives and whack them on an uber raid card, would it be faster than a sata raid?, would it cost a fortune? I know nothing about scsi but I need a project and this could be it.

Is there a way to get round the limitations of the pci bus?

[Gizmo]

You may notice from my siggy that I am running a pretty mondo SCSI RAID array.

For my next rig, I am giving serious thought to a SATA-2 RAID array.

Here's the thing:

SCSI really does well under heavy load conditions. The way I use my system, building it with a SCSI RAID controller and array was the right thing to do at the time, and by shopping around I was able to build it relatively cheaply (I've got a total of about $250 in my entire array).

However, SATA RAID controllers have come a long way since then, as have SATA drives. Unless you can get some really good deals on SCSI components, I think a decent SATA RAID controller and some SATA drives makes a lot more sense than a SCSI array for all but the most demanding (i.e. high-end server) applications.

[Azriel]

I don't know about sata, but the difference used to be that ide drives were optimized for writing speed, while SCSI drives were very fast at reading, and terribly slow writing. At least that's the way the hardware availible to me benched out at the time.

If that still holds true, it would depend on your use. I have on the shelf behind me 2 arrays I am not using. RAID just became too much hassle for me. One tip I can give you is find out what the max cable lenght spec is for your raid card, and DO NOT exceed it. I smoked a very nice DAC960-PDU3 Mylex card because I thought 30 meters was okay, when actually the card was rated for only 20 meters.

I had a lot of fun playing with SCSI. Would be a good learning experience if you can keep it cheap.

[MONKEYMAN]

Ill have a look on ebay, I'm not looking to spend hundreds on it but thought it would be something new to have a go with.

[Áedán]

To get decent performance from a SCSI RAID array generally requires you to spend a fair chunk of money. Otherwise, you might as well go with SATA/IDE.

SCSI performs exceptionally well at high I/O loads - typically caused by multiple users accessing files. SATA and IDE both suck at this kind of operation, as they're designed to be used by a single user. No, multitasking does not count as multiple users!

If you end up buying low end SCSI drives, then you might have well spent your money on SATA/IDE. Low end SCSI drives do not perform well, and for single user purposes are outperformed by SATA/IDE.

To overcome the limitations in PCI bandwidth, many cards use 66MHz 64bit PCI (Also known as 64bit PCI-X), which boosts the performance of PCI to 528Mbyte/sec. These cards are usually backwards compatible with 33MHz 32bit PCI, but will drop down to the 132MByte/sec bandwidth. The 64bit PCI cards are obvious, as they have a PCI connector that is about twice as long as normal.

If you don't have anything other than standard PCI, then you cannot overcome the 132Mbyte/sec limit.

Edit: Take a look at Storage Review's benchmark database to get an idea of how various drives perform.

[Gizmo]

Originally Posted by Azriel

I don't know about sata, but the difference used to be that ide drives were optimized for writing speed, while SCSI drives were very fast at reading, and terribly slow writing. At least that's the way the hardware availible to me benched out at the time.

What kind of drives were you using, what array configuration? That can have a LOT to do with your write performance. The array in my sig is capable of handling everything the PCI bus can throw at it (90 MiB/Sec, according to ATTO), both Read and Write. Unfortunately, I don't have a 64-bit PCI bus, so I can't really push this controller, but since it is only an Ultra-2 SCSI controller and I'm only using two channels on it, it really doesn't matter.

Originally Posted by Azriel

If that still holds true, it would depend on your use. I have on the shelf behind me 2 arrays I am not using. RAID just became too much hassle for me. One tip I can give you is find out what the max cable lenght spec is for your raid card, and DO NOT exceed it. I smoked a very nice DAC960-PDU3 Mylex card because I thought 30 meters was okay, when actually the card was rated for only 20 meters.

If you were going 30 meters, you were running differential signaling. If you were using the PDU3 Mylex, then you were running high-voltage differential signaling in Ultra-SCSI mode. If memory serves, single-ended mode is only rated for about 1.5 meters. The official SCSI spec only rates HVD for 25 meters, IIRC.

In any case, this all suggests that you were either working with very old equipment, or you did this a fair while back (like 8-10 years or so), or a combination of the two.

Originally Posted by Azriel

I had a lot of fun playing with SCSI. Would be a good learning experience if you can keep it cheap.

Yeah, the only problem is that cheap and SCSI are almost always contradictions in terms. Even cheap SCSI usually isn't all that cheap.

[Azriel]

I'm not sure on the particulars, but I know the card was capable of 20 meters.

I was running raid 5 on the array with the 7 4.3gig 7200rpm cudas, and raoid 0 on the array with the 4 full height 5.25 23gig drives. Really a shame that I used those long cables. I was just too lazy to move things closer so I could use the 1 meter cables.

For entry level playing with SCSI, I'd go for an adaptec AAA-131u2 card. Cheap on ebay. For drives, you can usually find nice lots of SCA drives cheap, and the adapeters for them are like 4 or 5 bucks each. Plan to spend about $150 to get a 50 or 60 gig array going. RAID zero sucks. Go raid 5. With raid5, you lose the space of one drive. You get striping and redundancy.

If you really just want to play with it and don't care that it will be dirt slow, you can go with 50-pin drives. But this would be just for fun.

[Gizmo]

Originally Posted by Azriel

For entry level playing with SCSI, I'd go for an adaptec AAA-131u2 card. Cheap on ebay. For drives, you can usually find nice lots of SCA drives cheap, and the adapeters for them are like 4 or 5 bucks each. Plan to spend about $150 to get a 50 or 60 gig array going. RAID zero sucks. Go raid 5. With raid5, you lose the space of one drive. You get striping and redundancy.

I played with a AAA-131 card back with they first came out. It was an absolutely HORRID product. I understand that the later generation cards actually aren't too bad.

As for the SCA drives, I haven't seen any cheap SCA drives on E-Bay for a while now (I define cheap SCA as $1/G or less for 10000 RPM 3.5" drives), although it IS possible to find some in the $1.25-$1.50 range if you shop for a bit.

Originally Posted by Azriel

If you really just want to play with it and don't care that it will be dirt slow, you can go with 50-pin drives. But this would be just for fun.

I wouldn't recommend 50-pin SCSI for drive storage to my worst enemy. That's just cruel and unusual. LOL