Re: Extra Drives
Re: Extra Drives
- Subject: Re: Extra Drives
- From: email@hidden
- Date: Wed, 8 May 2002 16:22:33 EDT
>
ARG...it's not the *drive* that chokes, it's the *BUS*. That's an important
>
difference. I have used, and seen used, ATA drives attached to a SCSI bus
>
RAID system under *INSANE* heavy usage, and they *never* choke
ATA drives on a SCSI bus? Only through an emulation (such as some very early
ATA drives in low-end macs of by-gone eras)... ATA is a different
communications method than any flavor of SCSI.
Below find something I wrote for a white paper a while back on this issue
that might help clarify...
SCSI IS ROBUST:
One thing most users don't know is that there are different standards for
calculating Mean Time Between Failures (MTBF) for hard drives. The industry
standards for E/IDE (ATAPI) is based on a 8.33 hour per day, 5 day-per-week
usage (think in terms of heat generated & cooling times). SCSI drives on the
other hand are considered the defacto standard & mission-critical by drive
manufacturers; the MTBF for SCSI drives are calculated based on a 24/7
heavy-duty usage as would occur in a server. As a result, SCSI drives (and
SCSI in general as a set of communications standards) are more robust, and
are designed to have speed advantages over E/IDE drives in real-world usage.
SCSI IS FAST:
Most power users understand that SCSI is faster than E/IDE (ATAPI), but
tend to use raw numbers to compare data transfer speeds. While EIDE transfers
at 33Mb/sec (and a new revision supports 66Mb/sec transfers), SCSI can
transfer (on this particular card) at speeds of 160Mb/sec. But that's not the
end of the speed difference. IDE does not support blind transfers, request
optimization, request queing or stacking; SCSI supports all of these to give
it a real-world speed benefit over IDE. So what are these issues, that they
provide a real-world speed benefit?
Blind Transfers - if you are copying something from a SCSI CD drive to a SCSI
hard drive, the SCSI controller tells the CD drive to target the hard drive
and start reading at position X. After that, the controller can step out of
the transaction as the CD streams the data across the SCSI chain to the hard
drive, straight from one to the other. The same process under IDE would
require the controller to perform substancially more steps. First, it would
have to request the attention of the entire chain, effectively locking out
any other requests; then it would get the data from the CD and move it into
memory, close that request, start a new request to the hard drive, and then
write from the memory to the hard drive. As you might imagine, this consumes
more than twice the time, even at the same transfer speed!
Request Queing or Stacking - SCSI drives are capable of taking multiple
requests to read and/or write data at once. Let's say you open 5 small files
-- under SCSI, the request for all five would be sent from the controller to
the drive(s) as single request, and the card would then disengage from the
SCSI chain and wait for the data from the drives to come flowing back. And
because multiple requests are possible, the previous example of transfering
data from a CD to a hard drive could continue during the time it takes the
drive to grab the five files and return them. IDE on the other hand, only
supports a single request at a time, and the controller must wait for the
data to be returned before releasing the connection. For five files, the IDE
controller would have to make 5 separate calls, each with it's own session
overhead; the CD to hard drive transfer mentioned previously would have to
complete before the IDE controller could even begin to grab the file files
(at least on the hardware level). Again, SCSI is the clear speed winner at
the same transfer speed.
Request Optimization - As noted in the previous paragraph, SCSI drives can
store multiple requests to read and/or write data. On many SCSI drives, there
is on-board request optimization. On-board request optimizaton means that the
drive can optimize the order in which it reads (or writes, or both) the data
requested -- in the fastest order it can fulfull the requests. In our
example, we asked for five files; the SCSI drive can figure out that file 4
is on the way to file 1, and optimize the getting of data to reduce the total
time required to grab all the files. IDE does not support multiple requests
and would grab file 1, complete the transaction, then grab file 2, complete
the transaction, etc (you get the picture).
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