SCSI questions

[Bozo]

I've been contemplating going to a SCSI drive in my computer. I've got a couple of questions.
If the PCI (32 bit ) bus can only handle 133MB/sec of data, whats the point in installing a SCSI drive that can transfer 320MB/sec?
Would a SCSI drive rated at 160MB/sec saturate the PCI bus, or is that more of a 'rating' and not a real world performance level?
Which would be better to have: a 320MB/sec SCSI card with a 160MB/sec hard drive or a 160MB/sec SCSI card with a 320MB/sec hard drive? Or matched componates? ( the reason I ask this dumb questions is a matter of buying used parts or new and trying to stay within my budget)
Is it really worth spending the money on a 15K RPM hard drive?
I was planning on using Seagate hard drives and Adaptec controller cards. Is there anything better, cheaper, faster......?

Thanks,
Bozo :mrgrn:

 

[blakerwry]

The reason for u320 controllers is server use. A basic u160 controller is more than enough for use in a PC. LSI makes some cheap (<$50 @ newegg.com) u160 controllers that are going to be as fast and good as the more expensive adaptec ones.

just like ATA, SCSI drives cannot saturate the bus they operate on. However, all SCSI drives you run are going to have to share the channel and it's bandwidth. Which is why many current drives are u320 capable, it allows you to run many drives before saturating the SCSI channel.

u320 drives will work just fine with a u160 controller. Take a look at SR for the benchmarks on the fastest 10k drive. It will likely be the best SCSI choice for PC use.

As far as I know there would be no performance penalty operating the fastest 10 or 15k drive on an 80MBps SCSI channel and only minor performance difference on a 40MBps one.

 

[Santilli]

Greg's new view on SCSI-
"I was planning on using Seagate hard drives and Adaptec controller cards. Is there anything better, cheaper, faster......? "

YES! Buy the single channel LSi controller. Hook it up to one of these:

http://www.supermicro.com/products/accessories/mobilerack/CSE-M35S.cfm

Mine wa 170 dollars from Newegg. Why?
ALWAYS, when shopping for scsi drives, you will see some fantastic close out price. EVERYTIME, it will be an SCA/scsi drive, 80 pin. You can plug in 5 drives, and, remove at will. Unless you buy a raid scsi card(expensive) you can boot off a single, smaller X15.4, and raid 1 your storage drives.

PCI 32 bit really can only go 120 mb/sec. No drive that I know of can go over LVD limits, at 75 mb/sec, except solid state.

Yes, cheetahs are that good, and they last forever. My 1997 models are still going strong in my mac.
The ease of SCA installation, and the speed, are simply worth the box price.

s

 

[Howell]

http://storageforum.net/forum/viewtopic.php?p=57854#57854

 

[sechs]

What's our budget here?

You can pick up a 18GB 15k.3 for $100 or less. Those refurbed 36GB drives were going for around $130. x15lps go for even less.

Ultra 2 wide cards can be found for $10 or less (see the ones I have posted in the for sale forum); used Ultra160/m cards are $40 or less.

Picking up the appropriate equipment to start out on SCSI isn't that expensive.

 

[The JoJo]

LSI or Adaptec 160 for controller, in my opinion. This way you'd have some expansion room, bus wise, if you decide to put another drive in there. I'm assuming one drive in the beginning.

How many drives are you planning on attaching?

As for a drive, go for Seagate X15.3, so you'll feel some real speed. If you just want to play with SCSI then the drive don't matter that much.

The X15.4 is hopefully coming soon, I'm going to get one to use as a boot drive in my linux. Max STR should be 90+MB, so therere I wouldn't go for a U2W controller.

 

[Grim]

The reason for u320 controllers is server use. A basic u160 controller is more than enough for use in a PC.

Just to expound upon this detail, the main reason servers may need more SCSI channel bandwidth than PCI bandwidth is RAID, especially RAIDs 2-5. (RAID 1 typically takes twice as much SCSI bandwidth for writes as PCI bandwidth. But the fact that you're going to be using other PCI devices in the system compensates for this.)

For a really good discussion of this, see a RAID primer. But, basically, writing a block of data to a RAID 2-5 normally requires reading from most of the disks in the RAID set. In an 11 disk RAID 5, small writes or misaligned writes have a bit of a bandwidth bottleneck that the u320 controllers alleviate.

There are some other possible reasons for u320, but generally, they're rather esoteric (such as IP over SCSI, for when gigabit just doesn't do it for you.)

 

[Bozo]

Looking at an LSI single channel U320 RAID card (LSI20320RKIT) and a Segate 15K RPM U320 hard drive. (ST318453LW)
This way I'll be ready to add a second hard drive for a RAID 1 or 0 setup.
Can two SCSI hard drives be accessed at the same time on the same cable? Or is it like ATA; one drive at a time.

Bozo :mrgrn:

 

[blakerwry]

I would only buy that controller if you're sure you're going to do RAID. Otherwise get the $32 controller with a $20 cable.


Technically, only one drive at a time can be transfering data, but the bus operates so much faster than an individual drive that they may as well be talking at the same time with SCSI.

A similar situation exists with ATA, however ATA drives cannot disconnect from the bus while waiting on mechanics which causes some perforamnce degradation when using ATA drives on the same channel simultaneously.

 

[Grim]

... Can two SCSI hard drives be accessed at the same time on the same cable? Or is it like ATA; one drive at a time...

Disclaimer: I don't know exactly how ATA works, so without further enlightenment, I'll have to assume it's the same as IDE.

Multiple SCSI drives can be active at the same time. All SCSI harddrives I've seen support having multiple outstanding commands as well (basically, the idea is, the drive knows exactly what it's currently doing, it should be able to best optimize operation order for reduced seek time), although that feature is generally not supported on the SCSI tapes and CD drives I've used.

The only contention is the brief bit where the command or data is on the wire.

My understanding on basic IDE behavior is that it requires each command to complete before the next is issued. So, compared to that, SCSI's a big win, although not quite so much if you have an OS like Linux that does drive command order optimization - of course, the drive itself still has more opportunity, as it can defer the current operation, whereas the OS can only juggle priorities for queued operations.

 

[Santilli]

That would probably explain the deference I've felt using scsi, in raid 0, or in general.

With Raid 0, I believe the extra cache, and more effective data transfer design, along with superior chips, and algorithims, make the effect more dramatic then the statistics might indicate. In other words, the effect of scsi raid 0 is considerably better then the tests would indicate, user feeling wise.

For example, this raid gives great small block burst rates, up to 90 mb/sec on atto(clearly reading the cache on the drives). Sustained transfer is around 30 mb/sec, yet, with the processor on the card, and the interface, plus softraid's excellent drivers, the difference is much more then the number might indicate. It gets better if you have a dual channel, or separate scsi cards for each drive.

s

 

[Bozo]

Thanks for all the help

blakerwry: I was thinking of doing a RAID setup in the future, a single drive for now.

Right now I have an Adaptec 2940UW with a 15K RPM Cheetah attached for testing (at work). Amazing the difference from Raptors and even a 10K RPM Cheetah. This is what got me to thinking about SCSI for home.

Bozo :mrgrn:

 

[Santilli]

I told you, I told you!

:mrgrn:

 

[The JoJo]

Has anyone read about the possible release date of X15.4 to the retail channel?

 

[Bozo]

I salvaged two drive cages from a couple of Compaq servers. The removable drive carriers are marked 'Wide-Ultra'. If I install U320 hard drives in the cage and connect it to an U320 SCSI card, will the system be U320 or will it revert to Ultra wide. ( there is a printed circuit board on the rear of the cage. I am presuming that this allows for hot swapping and assigns the drive number)

Bozo :mrgrn:

 

[Bozo]

Also, I'm guessing that the cage provides termination also????


Bozo

 

[blakerwry]

u2w is 80MB/sec IIRC. You may be able to get u160 out of it.. but i'd do some testing before you decide to go past its specs.

You won't notice a difference between 80 and u160 or u320 with a single drive. But if you're going to do RAID you'll want something u160 or higher.

 

[Santilli]

The cages should provide scsi id numbers, and termination. Of course your boxes are from compaq, so I'd check. Also, you need a driver for the box.

You can set scsi ID's to be before 7 or after 7 by jumper. If you use two boxes in series, I wonder if both need termination jumpered on?

Splash, Help!

s

 

[Bozo]

I attached a cage to an Adaptec AHA 2940UW and everything works. The drives were assigned numbers and recognized by the SCSI card. Seems each slot in the cage has a permanent number assigned to it. Cool

Bozo :mrgrn:

 

[Santilli]

When you start shopping for drives, you may well save the cost of the box, since every really good deal on a scsi drive seems to be an SCA 80 pin, with a converter, or without.

s

 

[Pradeep]

The cages should provide scsi id numbers, and termination. Of course your boxes are from compaq, so I'd check. Also, you need a driver for the box.

You can set scsi ID's to be before 7 or after 7 by jumper. If you use two boxes in series, I wonder if both need termination jumpered on?

Splash, Help!

s

You only need a driver if the enclosure supports hot-swap (SAF-TE or whatever it is). Otherwise the circuit board just provides power and termination. If you were daisy chaining, you would enable termination on the last enclosure on the chain.

 

[Santilli]

Thanks. Didn't know that...

s