Storage

[Mike Schwartz]

On 5/10/06, Alan Dayley <alandd at consultpros.com> wrote:
>
> Trent Shipley said:
> > There has been a discussion about drive interface standards on the
> Postgresql
> > general list.  I don't understand it.
>
> Your questions are very broad.  Maybe some context of the discussion would
> help to narrow the focus a bit.  Let me provide some specific bullet
> points.
>
> > What is the difference between IDE, SSCI, ATA, and SATA?
>
> - IDE (Integrated Drive Electronics) is the early marketing term for ATA
> (Advanced Technology Attachment).  They are really the same. [1]
>
> - You didn't mention ATAPI (Advanced Technology Attachment Packet
> Interface) with started as an extension to ATA for support of removable
> media drives like CD-ROM and Iomega ZIP drives.  Probably because ATAPI
> hard disks don't exist.  In any case, ATAPI is part of the ATA protocol
> specification now.
>
> - SATA (Serial ATA) is an update to ATA to transfer the same command set
> over serial channels instead of a parallel bus. [2]
>
> - Because of SATA, the term PATA (Parallel ATA) is now being applied to
> the original ATA devices.  ie. IDE == ATA == PATA.
>
> - SCSI (Small Computer System Interface) has been around a long time
> compared to ATA with it's roots going back to 1979.  It handles devices of
> many types (scanners, tape drives, media changers, etc.) besides hard disk
> drives.
>
> - You did not mention a new tech called SAS (Serial Attached SCSI) which
> is similar in nature to the ATA-->SATA evolution.  That is, transmitting
> the same SCSI commands in serial packets. [4]
>
> > What are the
> > implications for:
> > drives on notebooks and PCs
>
> This maybe best answered by looking at the goals of each protocol.  I'll
> reduced them to ATA vs. SCSI since the rest are flavors of one or the
> other.
>
> The original goal of ATA was to provide an inexpensive "consumer grade"
> hard drive interface.  SCSI, from the beginning, was intended for reliable
> "systems or industrial grade" data operations, including higher
> performance.  As a result of these somewhat opposing goals ATA
> technologies are common in the consumer computer world and ATA devices
> tend to be less expensive per MB.  SCSI devices are very common in
> industrial applications, servers and main frames, for example and are more
> expensive per MB.  SCSI fans look down on ATA devices as cheap and prone
> to failure sooner rather than later.  ATA fans look down on SCSI devices
> as overly expensive and over engineered.
>
> Truth is that, whether by side effect of improving technologies or by
> design, ATA is evolving to be just as good and fast as SCSI.  Believe it
> or not but there are still far more SCSI storage systems in use today than
> ATA but ATA use is multiplying very fast and is taking over uses where
> SCSI was traditionally deployed.  It used to be that finding ATA drives
> with high duty cycles and MTBF was difficult but that is changing.
>
> The bottom line is that you'd be hard pressed to find a notebook with a
> SCSI hard drive in it.  Similarly you'd be hard pressed to find a
> mainframe or heavy industrial computer with an ATA drive.  But the two are
> converging both in terms of price and capabilities.
>
> > low TB drive arrays
> > SAN?
>
> Drive arrays and SAN and NAS, etc. are technologies either combining or
> sharing (or both) storage to a host or across a network.  This is a layer
> "above" the actual hard disk interface layer represented as SCSI or ATA or
> one of it's flavors.  In other words, you could have a SAN server that
> uses ATA drives and another that uses SCSI and both would interact with
> the network layer exactly the same.  Clients using the storage would not
> know what hard drive protocol is used.
>
> So, how the hard drive protocol effects the network storage technology
> manifests in only in how reliable, fast, etc. the interface operates.  One
> who believes SCSI is best in a SAN service probably believes it is best in
> every other situation for the same reasons.  Some dismiss ATA as a viable
> server interface because of it's history as consumer grade.  I can accept
> such a thought as a rule of thumb but not as universally true.
>
> A server is general powered and operating for 24/7 so the most important
> specification is the drive's duty cycle.  How many hours per 24 is the
> drive designed to be operating?  The duty cycle of most ATA drives is 10
> hours or less per 24.  (There are more and more that have a 24 hour cycle
> but they'll cost you.)  Using such a drive in a server is not "wrong" if
> you have a good data backup process that you actually follow and you are
> willing to save money on each drive but probably buy them more often.
>
> Corrections and discussions are invited.
>
> Alan
>
> [1] http://en.wikipedia.org/wiki/Integrated_Drive_Electronics
> [2] http://en.wikipedia.org/wiki/Serial_ATA
> [3] http://en.wikipedia.org/wiki/SCSI
> [4] http://en.wikipedia.org/wiki/Serial_Attached_SCSI
>
> PS Wikipedia is wonderful!
>
>
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If the
   Main PLUG discussion list <plug-discuss at lists.plug.phoenix.az.us>
has an FAQ archive, or any kind of
"BEST of" the all-stars (all-time helpful posts) log,
then I would nominate this [answer from Alan Dayley],
for such an honor.
-- 
Mike Schwartz
Glendale  AZ
schwartz at acm.org
Mike.L.Schwartz at gmail.com
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