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IDE, SCSI And Recording Everything
Raju writes: "For many years we were told that SCSI is superior to IDE. I always made my systems with SCSI and the others in the household got el-cheapo IDE disks. In the past SCSI beat IDE hands-down but now according to Simson Garfinkel, "today's IDE drives are significantly faster than SCSI drives". In the article at O'Reilly Network he talks about the tests they had run for storage of network data on disks. In the light of this article does anyone see any reason for going with SCSI in a desktop machine? For servers with heavy disk usage patterns it might be different due to command queuing." Disk types aren't what the article's really about, though -- it's a top-level look at network forensics (including advice on building a traffic-analysis system), and makes some interesting points about the unbalanced growth of storage and bandwidth.
The point of SCSI is that it allows the disk access and such to be offloaded from the CPU to the processor on the SCSI card. This way your programs don't freeze when heavy disk access occurs.
If you check out Storage reviews File Server Benchmark database, you'll see that the fastest ATA drive scores well below half what a 15,000 rpm Fujitsu drive does.
"Only in their dreams can men truly be free 'twas always thus, and always thus will be."
--Tom Schulman
FireWire Faq
Sure USB2.0 is about the same speed as FireWire, but FireWire hasn't been standing still - it's next version calls for speeds of 800Mbps and 1.2Gbps. There's even plans for fiber and wireless based versions.
However, even more import is that FireWire is PEER based. A computer is not required to transfer video from one device to another. There's already a bunch of video equipment that has FireWire support, camcorders as well as the Playstation 2(Sony calls it i.LINK instead of FireWire or IEEE 1394) come to mind.
While it might be possible to hack USB 2.0 for use without a computer, USB 2.0 wasn't designed for it. I suspect such a hack would be a successful as the "patched on security" we see in Windows.
Actually, SSA is rated at 180Mbps, whilst SCSI 3 is 160Mbps. Technically, the fastest drives are RAM drives. DATARAM used to make boxes (8 or 12 U, as I remember) of nothing but static ram. Blazing speed, sky high prices.
OK, I'm nit picking here.
Necessity is the plea for every infringement of human freedom. It is the argument of tyrants; it is the creed of slaves.
FreeBSD 4.3 flirted with turning off IDE write caching. This reduced write bandwidth to IDE disks but was considered necessary due to serious data consistency issues introduced by hard drive vendors. Basically the problem is that IDE drives lie about when a write completes. With IDE write caching turned on, IDE hard drives will not only write data to disk out of order, they will sometimes delay some of the blocks indefinitely when under heavy disk loads. A crash or power failure can result in serious filesystem corruption. So our default was changed to be safe. Unfortunately, the result was such a huge loss in performance that we caved in and changed the default back to on after the release.
[...]
There is a new experimental feature for IDE hard drives called hw.ata.tags (you also set this in the bootloader) which allows write caching to be safely turned on. This brings SCSI tagging features to IDE drives. As of this writing only IBM DPTA and DTLA drives support the feature. Warning! These drives apparently have quality control problems and I do not recommend purchasing them at this time.
So, SCSI is better both for performance and for data integrity.
Au contraire.
Apple didn't stop using SCSI as standard equipment because of its speed. They used it in their Macs for YEARS because of better speeds than any drives of the time. Apple chose IDE later (when Job returned) for reasons of cost, just as PC makers do. Removing SCSI as standard brought down Mac prices by a few hundred dollars.
For general daily use, and because of recent advances in IDE, there was no advantage to using SCSI as standard any longer for Apple.
However, SCSI, particularly the LVD (SCSI-3) will SMOKE any hard drive interface today, which is why Apple still equips various SCSI configs on build-to-order workstations and their Server models.
FireWire (1394) is theoretically as fast as SCSI-3, but few people can afford a true FireWire drive with genuine FW controllers (earlier FW drives were some IDE or SCSI to FW translator or used slow drives on a FW interface).
Apple is overdue to upgrade their logic boards (motherboards) to the faster buses found in the best PC boards now, so there should be improvements in their performance for that platform in the coming months.
Vos teneo officium eram periculosus ut vos recipero is.
Ummm, 600,000 hours is about 68 YEARS. I think I can manage crashes 68 years apart. Even if you worst worst worst case it to 100,000 hours you are at over 11 years.
All this means is that for any given drive in any given year, you have a 1.4705% chance of your drive failing, on average.
If you have 68 drives in your system, then, it is likely that one will fail per year.
That's stats for you.
So higher MTBF can actually work out to better reliability.
Of course, this is a simplistic analysis, which doesn't take into account the actual distribution of mortality for those drives (which, for any hardware, tends to have the stillborn/geriatric ends of the spectrum with the most failures)
Simon
Coming soon - pyrogyra
Look at "TCQ" -- Tagged Command Queueing -- that has been worked on by Andre Hedrick in the past, and is currently going into the Linux 2.5.x kernel series due to the work of Jens Axboe.
TCQ is where SCSI gets a lot of its speed, by allowing multiple device commands to be outstanding on the bus at any given time. TCQ really levels the playing field for IDE and SCSI... assuming your IDE driver supports it (most do not).
Firewire is 400Mbps, which is 50 MBps. That's faster than Ultra2 SCSI, but slower than Wide Ultra2, Ultra3 and Ultra160/320 SCSI. Check out this link for details. Firewire is still nice tech, and a fair bit smarter than USB2.0, but it's not the bandwidth king that SCSI is.
Ita erat quando hic adveni.