The Four Hundred

When this newsletter described the new SCSI disk drive and PCI-X disk controller features that IBM announced in conjunction with the revamped iSeries line on January 20, pricing information was not available for these and other feature prices that can account for a large portion of a complete iSeries setup. I've gathered up this information, as well as some internal IBM information on the performance of the new disks and controllers, to help you make better buying decisions.

To recap briefly: There are two new RAID5 disk controllers for the iSeries, as well as a non-RAID disk/tape controller. All of these adapters plug into PCI-X slots in the new iSeries or their expansion towers. They all require OS/400 V5R2 at the February 2003 release level to function properly.

The feature 2782 adapter is a RAID5 Ultra3 SCSI disk controller that is available on the old Model 270 and 820 machines through expansion towers as well as on the new Model 800, 810, and 825 servers and their towers. This disk controller allows a RAID5 disk controller to have as few as three disk drives (compared with the minimum of four disks required with prior IBM RAID5 disk controllers for the iSeries). The feature 2782 controller can support up to 12 disk drives (an increase over the maximum of 10 disks on prior RAID controllers), has 40 MB of write cache memory (up from 10 MB on the feature 2763 RAID controller), and has two Ultra SCSI buses. Neither old feature 2763 nor new feature 2782 supports hardware disk compression, which was available on IBM's disk drives when used in conjunction with iSeries controllers. (Hardware disk compression is not supported on 70 GB or larger 15K RPM disks, but I think this may change in the future.) The new feature 2782 PCI-X controller costs $3,000, the same price as the old feature 2763 PCI-based controller.

The second new RAID5 disk controller is feature 2757, which is the new high-performance, large-cache disk controller for the iSeries line that replaces the feature 2778 and feature 4778 PCI RAID controller used in AS/400 and iSeries machines for the past two years. Feature 2757 has 235 MB of write cache memory; with data compression turned on, this cache memory turns effectively into a 757 MB cache. This Ultra3 SCSI controller also supports a RAID5 set with a minimum of three drives, but a RAID set can be expanded to 18 drives. This is a lot better than the 10-drive limit on feature 2778 and feature 4778 Ultra2 SCSI RAID5. The feature 2757 controller supports up to four SCSI buses, which run at 160 MB/sec compared to 80 MB/sec. The maximum PCI burst rate on the new controller is 532 MB/sec, four times that of the prior card. The compressed write cache, at 757 MB, is more than seven times as large as the 104 MB effective cache on the feature 2778 and feature 4778 cards. The new controller also supports SCSI bus tagged command queuing, which yields faster response times under heavy loads, and has new hardware-assisted array parity checking and cache memory scrubbing algorithms that are five times faster than with prior cards. The net effect, said IBM sources at the iSeries revamping in January, is that customers who move to these controllers, the new 15K RPM disk drives, and the PCI-X slots or expansion towers can see performance in their disk subsystems improve by a factor of three. The feature 2757 PCI-X RAID5 card can plug into new iSeries models in their internal PCI-X slots or in PCI-X slots in I/O towers. Older iSeries machines must attach these new cards to their servers through I/O towers, since older iSeries machines did not support PCI-X slots.

The old feature 2778 and feature 4778 cards cost $6,000 each, and the new 2757 costs $7,200. IBM clearly reckons that customers will, in effect, need to buy fewer RAID5 adapter cards given the new design, and with the increased performance it figures that it can command a premium, too. Two feature 2778 or 4778 cards managing 20 disks is still a lot more expensive than one feature 2757 card managing 18 drives. In this respect, the new drive is a bargain, but only if customers really will get better performance if they can really hang twice as many drives on the controller.

According to the performance information I have seen, the new controllers do not necessarily yield three times the performance (IBM might have been referring to real customer workloads), but there is clearly a performance benefit from IBM's CPW performance tests. IBM probably ran a lot of tests, but I have seen the results from two of them.

In the first benchmark test, IBM configured one old and one new iSeries server with V5R2 and 15 of its feature-6718, 18-GB, 10K-RPM disk drives. (IBM didn't say which iSeries models were tested.) The old iSeries machine was attached to the disks through the feature 2778 PCI RAID controller and was housed in a 5074 expansion tower, while the new machine linked up the same disks in the 5074 tower using the new feature 2757 PCI-X RAID card. Both machines were set up to run the CPW variant of the TPC-C online transaction processing benchmark test, and IBM plotted out system response times and system throughput as virtual users were added to the machines. On the old kit, system response times stay at around 0.05 seconds until a throughput of about 2,000 transactions per minute (TPM) is reached, and by the time the machine is pushing 3,000 TPM, response times have jumped up on a nice logarithmic curve to just under two-tenths of a second. The data dies at that point, but given the way these machines work, the response time was going to get a lot worse from that point with the addition of more computing requirements. On the new iSeries machine using the feature 2757 PCI-X RAID card and the same 18 GB, 10K RPM disks, system response times start out at 0.025 seconds, which is half that of the older set up, and the curve is flatter for longer. The response time is well under one-tenth of a second until about 4,300 TPM, at which point the curve bends up sharply, and the response time quickly climbs to 0.35 seconds, as a throughput rate of 5,000 TPM is reached. If I were going to be fair, I would say that with one-tenth of a second as a response time, the old equipment could do about 2,500 TPM and the new equipment could do about 4,300 TPM, which is an improvement in throughput of 72 percent. That's a lot, but it is not a factor of three, so be careful in your planning assumptions.

The second test IBM ran compared the performance of the iSeries on the CPW test comparing the performance of the feature 4778 PCI RAID card with 15 feature-6719, 35-GB, 10K-RPM disks in a feature 5074 tower against a feature 2757 PCI-X RAID controller in three different configurations--with the same 35 GB, 10K RPM disks in the same feature 5074 tower, with the same setup in a 5094 tower, and with the new feature 4326 disks in a 5094 tower. As you might expect, the 35 GB, 10K RPM disks had essentially the same performance as the 18 GB, 10K RPM disks, with performance hitting a knee in the curve at a throughput of about 2,500 TPM. Using the same 35 GB, 10K RPM disks, the feature 2757 RAID card hit the one-tenth of a second knee at about 4,400 TPM in the 5074 tower and did a little better, at about 4,600 TPM, in the 5094 tower. Response times with the new card were very low, and stayed that way until the card's extensive cache was full up. With the new feature 4326 disks, performance was increased to about 5,600 TPM with the feature 2757 RAID card and the disks stored in the 5094 tower. So the extra 5,000 RPMs of disk speed--an increase of 50 percent--yielded about only 18 percent more performance on the CPW test. Still, the tests show that 15 35-GB, 15K disks on the feature 2757 PCI-X RAID card can deliver more than twice the throughput of 15 10K-RPM disks (in either 18 GB or 35 GB capacities) attached to an old 2778 or 4778 PCI RAID card. This implies that customers can consolidate their disks by a factor of two and still meet the same performance characteristics--if their workloads and I/O requirements resemble CPW. Whether they could consolidate down by a factor of four, by using the new 70 GB disk drives, is unclear, but not likely for high-I/O OLTP environments.

The last new disk controller IBM added to the iSeries line is feature 5705, a non-RAID PCI-X disk/tape controller that supports six disk drives and one or two tape or optical drives. This controller is only supported on the new iSeries Models 800 and 810. It costs $950. IBM also announced that feature 5702 is a PCI-X tape controller that supports up to two external tape drives, and that costs $950 as well.

And, finally, as I told you three weeks ago, IBM has announced the feature 4326 35-GB, 15K RPM disk drive and the feature 4327 70-GB, 15K RPM disk drive for the new iSeries lines. These drives can be installed inside the new iSeries Model 800, 810, 825, 870, and 890 server chassis, or they can be inserted in the new PCI-X I/O towers, which is detailed (along with pricing, obtained before the announcement) in the January 20 issue. The new feature 4326 35-GB, 15K RPM disk drive has a list price of $2,350, or about $67 per GB, which is about 20 percent more costly than the feature 4319 35-GB, 10K RPM disk, which had its price reduced to $1,960 (or $56 per GB) as part of the iSeries revamping. This stands to reason, since 20 percent is about the same ratio of performance improvement that the new 35 GB disk offers over the old 35 GB disk on IBM's own performance tests, as explained above. The feature 4327 70-GB version of the 15K RPM disk drive costs $3,600, or $51 per GB. That's a 24 percent price cut on raw capacity for the fatter 70 GB, 15K RPM drive compared with the skinnier 35 GB version. Fatter disks always cost less per unit of capacity, mainly because they are not much more difficult to make than skinnier ones based on the same technology. Incidentally, IBM has kept the price of its 18 GB, 10K RPM disks the same: at $1,400, or $80 per GB. Yes, IBM has priced the others more attractively to get customers to move to new drives, in case you are wondering. The days for 18 GB SCSI disk drives are numbered.