Please help to understanding speeds and bottlenecks of SAS, STP, HBAs and PCIe

E

enigmah

n00b
Joined
May 13, 2010
Messages
23
Hi guys, I've read and learned quite a lot in this amazing forum :).
But there are still some basic questions regarding SAS, STP, HBAs and PCIe left...

I would like to understand the recent limitations which occur on the way from the harddisk, through the HBA (with RAID 6), to the PCIe Ports. Which speed can be achieved now and where is the bottleneck? Where will the bottleneck be with SAS 2?

On the PCIe side most HBAs use PCIe v1.x with 250 MB/s per lane. Generally 8 lanes are used so the speed should be 2 GB/s in total. This will double with newer cards supporting PCIe v2.x. -> Nothing to worry. Even with PCIe v1.x you can nearly satisfy two 10 Gbit/s NICs.

Regarding the HBAs (in my case RAID 6) I've no idea about the limits. When there should be a bottleneck it has to be while writing. What's about the max read speed? -> Please clarify.

And last what's about the limitations on the way from the HBA's to the harddisks? The HBA has a peer-to-peer connection to each harddisk (without extender). SAS 1 offers 3 Gbit/s and SAS 2 6 Gbit/s. Net this is round about 300 for SAS 1 and 600 MB/s for SAS 2. When you think about a typicall 8 port controller the speed should sum up to 2400 MB/s. But I've read in a post from odditory that this is limited to 600-650 MB/s (SAS 1) because of the STP protocoll when you're using SATA drives. I can't understand why it should be 600-650 MB/s because I haven't found much information about the STP protocoll.
Right now you'll be limited to about 600-650MB/s which is a limitation of the STP (Serial Tunneling Protocol) when you're attaching more than 8 drives and using an expander. With SAS-2 that should double.
Click to expand...
This seems to be the real limitation but I've no idea why exactly.

Thanks for your help in advance!
 
This is a good question. My understanding previously was that the throughput on parity RAID was usually limited by the processor on the RAID controller. Parity calculations are double on a RAID6 over RAID5 so that may be a factor. I don't know about the SATA Tunneling Protocol, maybe there is a hard limit somewhere, if anyone here knows I would be interested. If that's the case it would be nice if the drive makers would put SAS interfaces on some of the 7200 RPM drives.

Dustin
 
PCIe 2.0 x8 can have 4GB/s

But a 9260-8i's 8 ports can have at most 4.8GB/s, so the actual throughput is limited by PCIe to 4GB/s
 
But what about a two 9260-8i + two SAS expander setup to serve 24 drives??? Can we have 8GB/s throughput???
 
No. Limited by chipset.

Benefits
Superior performance
o Maximum reads: 2,875MB/s
o Maximum writes: 1,850MB/s
PCI Express 2.0 provides faster signaling for high-bandwidth applications
Support for 3Gb/s and 6Gb/s SATA and SAS hard drives for maximum inside-the-box flexibility
SafeStore Encryption Services for more secure data protection
Low-profile MD2 form factor for space-limited 1U and 2U environments
 
aznx said:
No. Limited by chipset.

Benefits
Superior performance
o Maximum reads: 2,875MB/s
o Maximum writes: 1,850MB/s
PCI Express 2.0 provides faster signaling for high-bandwidth applications
Support for 3Gb/s and 6Gb/s SATA and SAS hard drives for maximum inside-the-box flexibility
SafeStore Encryption Services for more secure data protection
Low-profile MD2 form factor for space-limited 1U and 2U environments
Click to expand...

You mean limited by mobo chipset???

If you are talking about RAID card chipset, then since you have two RAID cards now, supposedly you can get double the throughput.

By the way, that 2.875GB/s read rate limit is bogus. Tom's Hardware could get it to 3.4GB/s with RAID0 SSDs.
 
Thanks guys for the beginning. But could anybody give me a hint where I can learn more about STP?
 
You must log in or register to reply here.
Back
Top