Re: Dumb question: Which is "better" SCSI or IDE disks?

[Leonard Zhang System Administrator ISD RVIB]

I think currently spinning SCSI hard disks on the world, either in servers
or workstation, either in Unix/Linux or NT, is 50-pin old guys, no DPT disk
cache or RAID.  My comparison is base on this current real world.  Not on
the most up-to-date technology.

At least Adaptec are still manufacturing and selling 16 bit SCSI card for 
about $A100.

No one denies SCSI's performance, expecially when you want hook up over 4
devices.  Those persons who asked the question do not have enough bucks in
their pockets. My comparison is based on this ground.  Everybody wants a 
Ferrari, but not everybody can afford. They buy Daiwoo not because they 
don't like Ferrari.

DPT cache + RAID, not including SIMM, or error correction RAM is about $A2000

If the disk assembly is the same, the date output from the physical disk is the 
same either in SCSI or IDE. It depends on how many sectors/bits can pass under 
the aligned head. It depends on how fast the disk is spinning, and how many
sectors the manufacture can squeeze in a cylindar.

If the disk assembly is the same, the MTBF is the same either in SCSI or IDE.
The reliability is the same.

If the disk assembly is different, it is another story.

Maximun data tranfer rate for 8 bit ISA bus is 1.38MB/s, 16 bit is 5.5 MB.
The IDE interface of current mother board with built-in IDE is PCI.

Somebody attributes the smart functions of controller card to SCSI hard disk
itself, others just think that the CPU can not do anything except waiting, 
IDE disk to get data ready after issuing disk read command.

High data transfer rate of SCSI can only be envisaged in the situation when 
several disk activity happens at the same time, especially there are quite a
lot of devices on a same bus.

The bottle neck for disk access is in getting out data from the cylinda, not 
on the data tranfer rate.   Suppose, disk spins at 6000 rpm, the head right 
over the cylindar, and standard 63 sectors per cylindar, in this idea 
situation, data rate is < 3.2 MB (not including track seeking time.)  
The data transfer rate becomes critical only in the situation, when several 
disks get data ready, waiting for transfering.

If you can manage to hook up all your devices in IDE, switching to SCSI may 
not get real benefit.

For those get flamed by my previous mail, my good advice to them is to 
excercise their NETIQUETTE.

Leonard


On Sat, 5 Dec 1998, Robert Minichino wrote:

> > 1. Some hard disk manufactures are using same disk drive (physically) to
> > make SCSI and ATA.  They only change the PCB board.
> 
> Absolutely true; see Seagate Medalist drives.
> 
> > 2. SCSI-1 and SCSI-2 have 8 bit data bus, while ATA have 16 bit data bus.
> 
> Not necessarily.  SCSI-2 provides for a 16 bit (Wide) and a 32 bit (rarely 
> used) data bus, and both busses operate at up to 10MHz synchronously (Fast 
> SCSI).  Narrow Fast SCSI easily pushes 10MB/s over the bus, and Wide Fast 
> SCSI pushes 20MB/s.  Double those values for Ultra SCSI.  Compare that to ATA 
> which maxes out at 16.6MB/s (not necessarily sustainable), both PIO and DMA.  
> UDMA maxes out at 33MB/s burst.  Slower than widely available Ultra Wide 
> SCSI.
>  
> > 3. ATA got disk cache built in as well.
> 
> Often the difference between the ATA and SCSI version of a drive (same HDA) 
> is the amount of buffer on the drive.  The buffering can surely be used more 
> effectively by SCSI as ATA does not provide for disconnects, and hence has 
> much reduced (eliminated?) concurrency.
>  
> > 4. Some low end of SCSI host card is only 8 Bit card.
> 
> And I can pick up those HBAs for about US$10.00, new.  Which, mind you, is 
> similar to the price of a similar ATA adapter.  You'd also be hard-pressed to 
> find a high-end ATA controller that supports caching and RAID with up to 15 
> drives for any price.  I can pick one up for approximately US$400.00.
> 
> Overall, the value of a SCSI storage solution in a PC is very high.  A Ultra 
> Wide-PCI HBA that outperforms any UDMA interface can be had for approximately 
> US$90.00.  Low-end SCSI drives now carry very little premium over their ATA 
> counterparts, sometimes only US$50-75.  There is absoultely no comparison 
> between a high-end SCSI drive meant for servers and workstations to the large 
> but not-necessarily-speedy UDMA drives.  Compare a 10.1GB IBM UltraDMA/33 
> drive, which can be had for approximately US$300.00, with a 9.1GB Seagate 
> Cheetah U2W drive which can be had for approximately US$700.00; the Cheetah 
> has twice the buffer, the IBM rotates at 7,200 RPM versus the Cheetah's 
> 10,000 RPM, and the seek time on the Cheetah is a mere 7.5ms, versus 9.5 for 
> the IBM drive.  And additionally, compare UDMA's 33MB/s burst transfer rate 
> with 80MB/s sustainable transfer rate with Wide Ultra2 SCSI.  What you don't 
> see in the specifications is how the drives perform with many outstanding 
> requests, and long-term reliability.  I administer and sell systems operating 
> with both drives, but I do believe you can guess which drive performs better 
> in the real world, and which is more reliable.  And that's what you pay for.
> 
> In summary:
> 
> If you're using Win95/98 only, get UDMA.
> 
> If you're using WinNT, stop ;)
> 
> If you're using a multiuser/multitasking operating system, but you rarely 
> perform more than one task at a time, keep your system load low, or have few 
> users, get UDMA if and only if you cannot afford the SCSI premium.
> 
> If you load your system down, run multiple tasks that are even moderately 
> disk-intensive, or have multiple users (NFS, locally, etc.), get SCSI.  Don't 
> even consider UDMA unless you're planning on using RAID-0, with each drive on 
> a discrete bus.  I've seen the best UDMA drives in multiuser systems, and it 
> is not a pretty sight.
> 
> If you have or need many storage devices, get SCSI.
> 
> ATA/UDMA's main advantage is cost.  The drives are cheap, and you likely 
> already have a controller in your system already.  However you are limited to 
> 2 drives per interface, and only one drive can be used at once per IF.  The 
> interface itself is slower, but this normally does not matter as few disks 
> can even approach the transfer rate of the bus; it only matters with 
> transfers previously buffered.
> 
> SCSI's main advantages are scalability, flexibility and reliability; you can 
> get better SCSI drives that are not available in ATA versions, the bus itself 
> is (or can be) faster, and you can attach more drives to the bus.  Many 
> (most) SCSI devices provide for disconnecting themselves from the bus, 
> allowing other SCSI transfers to take place while they're retrieving the 
> data.  You can attach up to 7 devices to a narrow SCSI bus, and 15 to a Wide 
> SCSI bus.  You can attach even more with SCSI-SCSI controllers (for example, 
> 30 device RAID towers that appear as one device on the bus).
> 
> At the same price and quality levels, they're quite competitive.  SCSI just 
> continues scaling where ATA levels off.  And rightly so.  ATA pales in 
> comparison to SCSI, just so long as you're disregarding price.  In the 
> low-end, ATA is the better value.
> 
> --
> Robert Minichino
> Chief Engineer
> Denarius Enterprises, Inc.
> http://www.denarius.com/
> 
> 


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