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SCSI

From 32009318

Answered By Mike Orr, John Karns

what are the benefits of SCSI over IDE and what types of SCSI are there eg LVD68,ultra wide,wide etc.

what are the general specs of ech SCSI type

i want to buy a 73gb 15,000rpm LVD68 seagate cheetah and want to know is this the fastest drive i can get and the best for performance in a machine thats role is Games/server/lan/burning/movie theatre/jukebox/music editor, producer/3d animator or you can call it a jack of all trades computer

thanks a million form elliot

[John] I would place the least importance on the rotational speed - it would be the factor least likely to be noticeable in the overall system performance. On the negative side, it would contribute considerably to the amount of heat to dissipate inside the box. Many of these types of drives require extra cooling - added fans bringing air into the case; naturally this also contributes to the noise factor - from the fans and the drive(s). I myself prefer a bit slower rotational speed (5k or 7.5k) to avoid the noise and extra heat.

Is your CD burner SCSI as well? This could make a difference. My desktop system uses SCSI for the system drive and I use a Yamaha 4x4x16 CD re-writer. I can't speak for the currently available devices which are capable of burning at speeds of up to 20x, but my 4x works well, allowing me to do other things while the burning a CD.

[Mike] SCSI handles parallel operations better. Thus, two SCSI devices can perform I/O independently at the same time. To achieve this with IDE, each device has to be on a separate controller. If you try to access two devices on the same IDE controller simultaneously, one will wait until the other is finished.

[John] SCSI disk controllers offload the system board, as they have instruction queues which buffer several instructions at a time to send to the disk as it is ready to accept them. In other words, it's a more intelligent type of controller.

[Mike] SCSI drives have traditionally been faster (and bigger) than IDE drives, but advances in IDE technology has narrowed the gap. The current EIDE (Enhanced Integrated Drive Electronics) technology and the Linux IDE driver have actually borrowed ideas from SCSI.

[John] I recently bought a 40GB Quantum ATA-100 drive. I wasn't so much interested in the data band width (bw) as much as the large capacity. In perusing the Quantum Internet site for the specs on the drive, I discovered that although the drive electronics has the ATA-100 interface, the physical design of the platter and spindle mechanism doesn't provide 100 MB/s bw anyway. I suspect that there many drives out there with similar aspects. IOW, just because the drive has an ATA-100 interface doesn't guarantee that the drive fully supports the spec.

[Mike] SCSI is also more expandable since that you can connect several devices (6-15) on one controller, where as IDE is limited to two.

[John] Additionally, SCSI devices port nicely between different machines. For example, my CD burner is an external device. I also have a tape drive, and Iomega Jaz and Zip drives all of which are external SCSI devices. I can connect these to a desktop machine as well as a laptop, in various combinations. Although they are now offering USB versions of the Iomega devices, USB is slower, and requires a driver for each device, which in many cases are not available for Linux. With SCSI, the commands are more or less standard; one only needs the driver loaded for the controller. With Linux, the pkg which provides the set of commands for tape drives works for IDE as well as SCSI tape devices, AFAIK.

[Mike] Most people building high-demand servers still insist on SCSI. However,

[John] SCSI multi-tasks much better than IDE, and in a server environment, this can make a big difference, and in many cases is mandatory because the difference can be critical.

[Mike] for general use, IDE is plenty adequate. I use only IDE. Performance is acceptable if I put each hard drive on a separate controller (and put infrequently-used devices such as CD-ROMs as a slave on either controller). IDE drives and controllers are cheaper than their SCSI counterparts. SCSI is anal about its configuration: if you don't terminate the SCSI chain properly or the controller doesn't like your cables' electrical properties for some reason, it goes into fits.

[John] Agreed - for many single user situations IDE is adequate, especially with the faster interfaces such as ATA-66 and ATA-100.

[Mike] There are several SCSI technologies with buzzwords like "fast", "wide", etc. I'll let somebody else say which currently has the best performance (and how much you'd pay for the drive + controller + cables compared with an ordinary IDE drive).

[John] "Fast" doesn't really mean much, as it's been used in a couple of different SCSI implementations - there was Fast SCSI I, and FAST SCSI-II.

"Wide" means 16-bit bus, which translates to 68 pins or more. AFAIK, the 50 pin interfaces are 8 bit bus devices. It really isn't important except in hdd devices. For CD's and tapes, and slower devices such as Zip drives, the 8 bit is adequate, as the bw exceeds that of the device. Other things being equal, it doubles the i/o bw.

"Ultra" doubles the clock speed of the SCSI bus.

A general rule of thumb would be something like this:

Fast SCSI-II: 10 MB/s

Wide SCSI: 20 MB/s

Ultra SCSI: 20 MB/s

Ultra-wide SCSI: 40 MB/s

68 / 80 pin LVD SCSI: 160 MB/s. AFAIK, 320 MB/s devices are now becoming available.

It is important to note though, that the PCI bus (most mobo's run them at 33 MHz) maxes out at around 40 MB/s -

Allow me to correct myself; although the original PCI bus spec (and AFAIK most machines still adhere to this, and most PCI cards are designed for 33Mhz operation) called for a 33Mhz clock, PCI is a 32 bit bus. Thus it can handle up to 4 bytes per cycle, making theoretical maximum bw 33M/s x 4B = 132MB/s. But the reality remains that actual performance is far below this.

There is a way to test this on a Linux system:

hdparm -t /dev/diskdev

where diskdev is hda, sda, hdb, sdb depending on your systems disk cfg. For most people it would be hda, assuming a single IDE hdd.

On my Asus P5A 500Mhz mobo with a Tekram ultra-wide SCSI controller (40 MB/s), the above test shows 15MB/s which is 37% (about 1/3) of the rated 40MB/s. This type of situation seems to be very common - i.e., actual performance specs are only a fraction of the rated spec.

So the devices which deliver a bw exceeding the local system, are really only advantageous in RAID configurations, where the data is transferred between the drives (or other devices) which are directly connected to the controller at higher speeds. Any data bound for the mobo is still limited. This means that for anything but RAID, the LVD drives really don't offer an advantage, and you are wasting your $$. So one can buy a 40 MB/s controller (I've been using the TekRAM 390U with good results - can be bought for < $100 ) and drive Ultra-Wide drive which will make maximal use of the PCI bus, giving essentially the same performance as a much more expensive LVD system, in a non-RAID configuration.

[Mike] By "LAN" do you mean just being a client on the LAN? Or is it a high-demand server too? If you plan to do intensive network serving, CD burning and game playing simultaneously, I would consider separate computers, especially since network serving affects others on the network and not just you. (After all, if your CD burning slows down you game, too bad. But if it slows down other people too, they may resent it.)

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Published in issue 72 of Linux Gazette November 2001

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