SAS Drive Technology - Large or Small?

I have had quite a lot of questions from channel partners and customers alike around hard disk drive technology. There is quite a bit of confusion in the market right now 1st of all around the new SAS (Serial Attached SCSI) hard drives and secondly since they are currently available in 2 form factors, Large Form Factor (LFF) and Small Form Factor (SFF); which drive type is best? Most hardware vendors have not made it any easier by making both types of drives available for their servers which means end users are faced with multiple options when trying to decide on a configuration.

First of all I should point out that SAS is the successor to SCSI. SAS is a serial technology where every drive can have it's own dedicated 'pipe' or bandwidth lane to the SAS RAID controller as opposed to traditional Parallel SCSI whose drives share a 'pipe' to a single channel on the SCSI RAID controller. For example the P400 controller in the HP ProLiant DL380 G5 is an 8 port SAS controller i.e. 1 port for every drive in the DL380 G5 which can house 8 Small Form Factor (SFF 2.5") drives.

HP have had a lot of success in the past with our Universal SCSI drive which meant that the same SCSI drive would go in our ML, DL or BL servers and MSA storage i.e. 1 drive for all servers and DAS (Direct Attached Storage) and shared storage solutions. As a result of this strategy which our customers have overwhelmingly applauded, HP has decided to adopt a new single universal drive type for the latest SAS and indeed SATA technologys based on the SFF 2.5" disk meaning our customers will only have to endure a single drive type transition. The industry is rapidly moving towards SFF 2.5" disks and LFF 3.5" disks are only a temporary stop gap to fill areas where capacity is more important than performance or even price/performance.

As stated HP has transitioned all of our enterprise servers to the new universal drive type; Small Form Factor 2.5” SAS (Serial Attached SCSI). We did this primarily to avoid our customers going through 2 transitions i.e. from U320 3.5” Parallel SCSI hard drives to Serial Attached SCSI (SAS) 3.5” Large Form Factor Drives and then to Small Form Factor 2.5” SAS drives.

HP currently offers SFF 2.5” SAS drives in 36GB and 72 GB capacities at 15K speeds. We also offer SFF 2.5” SAS 10K drives in 36GB, 72 GB and 146 GB. It is important to note that no vendor offers a 146GB 15K 2.5” SFF disk drive as this is still has not been launched by the hard disk vendors. This is not a major issue as the performance difference between SFF 2.5” 10K SAS and 15K LFF 3.5” SAS is very low. In a RAID 5 set of 4 drives this can be as low as 5% – see diagram below.




SFF Drive - Physical Advantages
The HP SFF 2.5” SAS drives are 2nd generation SAS hard drives compared to 3.5” LFF SAS drives which still use U320 parallel SCSI mechanics. The SFF drives use less power (about half that of LFF drives), are quieter, have a better MTBF (Mean Time Between Failure) rating increasing availability and reliability and the server supports more spindles which ultimately has the greatest impact on performance.

Sample Scenario
To achieve a 400GB RAID 5 disk partition we could use 4 x 146GB 2.5” SFF SAS 10K Hard Drives. This would give much better price/performance than the 3.5” LFF 15K disk solution, however the 15K 3.5” LFF drive array would be approximately 5% faster performing.

The HP list pricing (Feb. '07) for the 2 drive types are as follows
€355 for a 146GB 10K SFF 2.5” SAS Drive
€505 for a 146GB 15K LFF 3.5” SAS Drive

For a 400 GB RAID array based on 4 Hard Drives with both drives types in terms of price and relative performance. (See Diagram Below for performance data)

€1420 – SFF 2.5” 10K for 1200 IOPs approximately
€2020 – LFF 3.5” 15K for 1260 IOPs approximately

The price/performance for the 10K array would be far superior to the 15K solution while the performance is approximately 5% less.

The advantage of using SFF 2.5” 10K drives is that due to their smaller size you can have a total of 6 per DL360 G5 1U server, 8 per DL380 G5 class 2U server and 16 in both the DL580 G5 and ML370 G5. This means we can add up to 2 more drives a typical 2U server (8 as opposed to 6 with 3.5") to the RAID Array increasing not only the available disk space in the RAID array but also dramatically increasing performance. By adding a 5th 146 GB 2.5” 10K SFF SAS drive to the array we exceed the performance of 15K SAS (see diagram below) while still being a more cost effective solution.

At 5 drives IOPS approx.
15K LFF gives 1260 IOPS – limited to 4 drives in a 2U server.
10K SFF gives 1490 IOPS and costs less.
You are still left with the option to add a 6th drive to the RAID set at a later date if you would like to further increase the performance and/or size of the array.

At 6 drives IOPS approx.
15K LFF gives 1260 IOPS - limited to 4 drives in a 2U server
10K SFF gives 1650 IOPS and costs fractionally more.

Conclusions
In conclusion Small Form Factor 2.5” SAS drives are the new universal drive type in the industry. HP have adopted the standard universal drive for all of our servers meaning our customers will not have to transition again in 12-18 months time away from legacy 3.5” drive technology. Small Form Factor 2.5” drives are not available in capacities greater than 72GB in 15K disk speed.

SFF drives provide more advantages than a 15K 3.5”SAS drives in terms of power, cooling, reliability, and density and we have shown that the overall solution can beat 15K in terms of not only price/performance but also raw performance.