Seagate 1 TB SAS (not SATA) hard drive on Smart Array?

[jknowles]

I'm looking to purchase a DL180 G5 with Smart Array E200/128. I noticed that Seagate now makes SAS versions of their 1 TB, 750 GB, and 500 GB 7200rpm enterprise level drives. The SAS version is roughly $90 more than the SATA version of the same drive, depending on the capacity of the drive. Smart Array controllers are limited to 1.5 Gb/s transfer rate with SATA drives, but it doubles to 3 Gb/s with SAS drives. The seek time, latency, and spindle speed of these drives is roughly twice as slow their 15k SAS drives, but 6 or 8 of them in RAID 10 should yield pretty good performance and huge capacity for a low GB per dollar cost. Does anyone know about or have any experience with using these drives with a Smart Array controller - maybe even the SA E200?

[Scott mC]

SAS is Serial attached SCSI. The largest (affordable SAS drive you can buy these days is 300GB which is 15k).

This 1TB disk you are talking about would be sata only. The reason it will work on a smart array is because SAS and SATA have the same connection (interface) i.e. Serial

Further to this if a 1TB SAS drives existed, I guarantee a 1TB SAS drive would be atleast $900 if not $9000 more than a 1TB SATA Drive.

I havn't configured a 1TB drive on a E200 but am planning to install 5 x 1TB drives on a P400 in a DL180 G5.

This means having to purchase hot plug cases for the drives as to my knowledge HP do not yet sell 1TB SATA.

[Patrick Terlisten]

Hello,

a SAS Version of a SATA drive would be possible, but I don't know for which customer circle that drive should be. SAS and SATA share the same physical interface, so I don't know which benefit those drives should have. The performance can't be the answer, one drive on it's own isn't able to fill a 1,5GB link, whether if SAS or SATA

If you want to add those drives to a DL180 take care about the drive cage! The drives in a DL180 are hot-pluggable if you're using a Smart Array controller.

And remember: This solution won't be a supported config!

Best regards,
Patrick

[jknowles]

Scott, you should have at least checked before you wrote. Do a search for Seagate PN ST31000640SS. It runs about $370 + tax and ship. As I wrote in my first post, it is an SAS version of their SATA drive - comes with a different board with dual SAS ports. They also make a 500 and 750 GB model in SAS now too.

[jknowles]

Hi Patrick,

I am assuming that in combination with a Smart Array controller, the benefit will be to double the allowed transfer rate to 3 Gb/s, but I don't know that for sure. I also don't know if the real performance gain is worth the extra $80 or $90 per drive and under what applications it would be cost effective. I'm likely to invest in the SAS version for that "just in case" factor where a little more performance might help.

One SATA drive should be able to transfer at 150 MB/s (not GB/s) and one SAS should be able to transfer at 300 MB/s. Each channel of the controller is independant and the controller should aggregate the transfer of each drive. So 2 SAS drives should be able to max out at 600 MB/s (minus controller overhead), 3 SAS = 900 MB/s, 4 SAS = 1200 MB/s.

The SA E200 is a 2.5G 4x PCIe card capable of 2 GB/s - that is full duplex. Each 2.5G link can do 250 MB/s, x4 = 1000 MB/s, and double that for full duplex. Once you figure in overhead, 4 SAS drives could max out the transfer capacity of the card if you were doing nothing but reads or writes. That would be really unusual, so I could still see a big benefit of going SAS on those if they do work at 3 Gb/s.

[Patrick Terlisten · Accepted solution]

Hello,

no SAS will transfer 3GB/s, even no SATA drive will get up to 1,5GB/s. These are the bandwidth of the channel, not the transfer rate of the drive.

Think about that:

IOPS = (1 / [Rotation Latency + Average Seektime]) x 1000

A 7200upm drive has a rotation latency of round about 4m. Lets assume a average seek time of 5ms.

IOPS = (1 / [4 ms + 5 ms]) x 1000
IOPS = (1 / 9) x 1000
IOPS = 0,11 x 1000
IOPS = 110

Bandwidth = IOPS x transfer size

If you're using Exchange, every IO has a size of 4KB.

If you have 8 drives, you will get

MB/s = (IOPS x Transfer Size) / 1024
MB/s = (880 IOPS x 4 KB) / 1024
MB/s = 3520 KB / 1024
MB/s = 3,44 MB

On top the overhead which depends on the RAID level (only 25% usable IOPS for a write in a RAID 5, 50% usable IOPS in a RAID 1+0). Okay, these values are without any caches or anything else, but it will give you a really good overview what you can expect.

Best regards,
Patrick

[jknowles]

Hi Patrick,

I see what you mean. Not to be picky, but you were throwing me at first with the GByte/s rather than Gbit/s. The drive interface is capable of 3 Gb/s as is each of the controller's port, but each drive cannot transfer 300 MB/s. The Seagate spec sheet shows the max sustained transfer rate is 105 MB/s which doesn't even max out the 150 MB/s limit of SA E200 port. So even in a perfect scenario, where all data is perfectly sequential, the drive can't max out the port with either interface. The IOPS of the Seagate drives is right around 75 in case anyone cares. :-)