Where are the High-Capacity SCSI Drives?

An anonymous reader asks: "Storage technology has really exploded in recent years, giving us ATA drives up to and exceeding 200-250 GB per drive. Why is it that SCSI drive technology has remained stagnant? I can't find a SCSI drive exceeding about a 146 GB capacity. Instead, businesses (and some individuals) wanting greater storage capacities are required to buy more drives which takes up more space, generates more heat, provides more points of failure, uses more electricity, etc. Why is this so?"

Comments

My Guess

[MBCook]

My guess is a simple one. Who buys SCSI stuff? It's expensive so it's mostly businesses and others who need high reliability (which one of the major reasons SCSI is more expensive). Now while normal people can "afford" to lose 250, 300, or more GB of data, for a business that could be worth billions of dollars.

The solution to this reliability problem is the RAID. There are two RAID levels that are ideal (there are more, but this is a simple explanation). There is 1, which is just a mirror; and 5, which is striping with parity.

With RAID 1, if you have 500 GB of data, you would need 2 500 GB drives. You lose 50% of the capacity you buy. The other option is RAID 5, where you lose (1/number of disks). So you could store 500 GB of data on 6 100 GB disks. This way you've only lost 100 GB of storage to redundancy as opposed to 500 GB.

So when businesses want to store large ammounts of data, it's more economical to use many smaller drivers than to large drives. Even if you don't need the redundancy (for example the disk is just being used for temporary storage while working on large digital picture or video files) they it's still better to use many small disks. While using a 500 GB drive will only go so fast (lets just say 60 MB/s sustained), by using a RAID, you can mulitply that. So by using 5 100 GB drives, you might be able to sustain 300 MB/s (assuming the bus can keep up, etc). Even if you only scale at 50% (that would be 150 MB/s) that's still 2 to 3 times faster than a single drive. That performance can save you money.

So, if you can afford it you can get much better performance or economics from using multiple smaller drives from one large one.

That's my theory/understanding. Begin tearing it apart!

Re:My Guess

[Robbat2]

RAID is a wonderful concept, but work needs to be on points of failure other than the drives.

Most decent external RAID units today have dual hot-swappable dual power supplies and fans. However there is still only a single backplane and RAID controller board (IBM PowerPC chips are very popular for this) involved. I've both a backplane and controller a fail on me in the span of 2 years, in both cases taking all the data with them. These units were 6x200GB IDE drives, 1TB usable, 1 parity drive, and we had several cold spares available to hot-swap in on a failure.

Sure I agree that statistcally your drives, fans and power supplies are much more likely to fail than the backplane or controller, but it can still happen.

Never forget the important of having backups, and make sure you can recover from them as part of implementing your backup solution. (1 month rotation of Ultrium tapes here).

There is a solution to the above, but it's very costly, and that's RAID over distributed storage (iSCSI and the like).

THE ANSWER

[icandodat]

This info is from an IBM Magnetic Storage Engineer. The reason is that the IDE market is a retail home market and very competitive. He said "If an IDE manufacturer can save 5 cents on a component he'll buy the cheaper one". The time from R and D to store shelf is less than a year. For SCSI drives on the other hand are primarily for servers and they have expensive components and are tested for a long time before they reach the market. The time from R and D to store shelf is about three years for SCSI. what was the bigest drive you could buy three years ago (ide)? Thats right about the same size as the biggest SCSI drive today. So ... what does this mean? IDE drives suck, they are cheap they are the zip lock bag of the storage industry. If you are going to grandmas with your data thats ok but if its going to the moon... buy tupperware, (SCSI).

They do exist!

[MarcQuadra]

Hitachi/IBM produce the 300GB UltraStar 10K300, which is a mighty drive if I've ever seen one.

The real reason is that when you move up to higher rotational sppeds to reduce latency, you have to reduce density relative to the motion of the disk under the head, so a 10K drive can generally pack only 60%-ish as much data per-inch as a 7200RPM drive.

The same can be seen in 15K disks, which are much lower density than their 10K counterparts. The 15K platters are smaller too, to keep them from flying apart.

Do you remember when the 5400RPM disks had higher capacity than the 7200 ones? I sure do, it was for the same reason.

Until the latency of the read-write head improves this will be the case.

Too slow to be useful?

[TheLink]

Drive speeds haven't really gone up tremendously. Still too slow.

Imagine you have a 1TB drive, but were stuck at a 100MB/sec max seq transfer rate. It takes you 2.7 hours to read/write the entire drive. And that's for _sequential_ access. Gets ugly for random seek.

A similar speed 10TB drive will take you more than a day (27+ hours) to read sequentially.

Before the point where it takes too long to read an entire single drive you might as well start using multiple drives to add capacity rather than having bigger drives.

Taking too long is subjective, but I'd say this: how long can you make your boss/customer wait whilst you are restoring an entire disk image from backup? 27 hours or 2.7 hours? or 25 minutes?

So 70GB would be about the limit if you have impatient users and bosses.

Larger capacities are OK if they are to hold data that aren't important enough to be backed up, and don't require masses of data to be available quickly. Or you are doing mirroring and read speeds are important but write speeds aren't as important (but remember that restoring from backup = writing ;) ).