topic Re: SAS vs. U320 Performance with RAID5 in Disk

SAS vs. U320 Performance with RAID5

sgunelius — Fri, 19 May 2006 13:50:34 GMT

I understand that SAS should be able to provide improved performance based on the stated throughput of 3GB/sec compared to U320 at 320MB/sec, but wanted to know if anyone out in the field has had the opportunity to compare performance of the two when they are configured in a RAID5 array?

We may be making the shift to SAS, but with 146 & 300 GB drives sizes available on U320 (I know performance is best with a large number of small drives versus these large drives) I might be hard-pressed to move to SAS without some valid performance data. The ability to address twice as many devices on SAS as you can on U320 isn't a factor for us at this time, but the performance differences is. Thank you very much for any information you can provide.

Re: SAS vs. U320 Performance with RAID5

raadek — Fri, 19 May 2006 16:32:37 GMT

First of all SAS is 3Gbit, not GByte per second. But it's point to point architecture - one 'bus' per drive.

As U320 drives are on the same bus they share 320MB/sec bandwidth. Every drive has certain limitations & you could roughly assume that up to 4 drives on U320 bus will not fill the bandwidth anyway.

And you have multi-channel U320 controllers, 2 or 4, so in theory you could have e.g. 16 U320 drives in the same array not being 'bus-constrained'.

SAS is definitely the way of the future, but taking into account current capacities it's still some sort of a hype rather than real performance benefit.

Rgds.

Re: SAS vs. U320 Performance with RAID5

sgunelius — Mon, 22 May 2006 09:01:15 GMT

Thank you for your response and sorry about mistakenly using GB instead of Gb. Have you deployed any of the ProLiant SAS models? If so, were there any immediate performance improvements/degradations that you noted?

As you mentioned, the limited capacities for the SAS drives and the availability of dual/quad-port array controllers for U320 make it difficult for me to abandon the proven U320 technology for SAS, but our HP rep mentioned that HP is going SAS and the next versions of SAS promise even better throughput (6Gb/sec).

Re: SAS vs. U320 Performance with RAID5

raadek — Mon, 22 May 2006 09:11:00 GMT

I do not have hands-on experience with SAS.

But again - don't let yourself be brain-washed with the throughput figure. Your system is always as fast as your NARROWEST part of the entire data pipe. Yes, SAS is cool, etc. But look e.g. at the PCI-X slot - you will not pump more through it than 1GB/s roughly. Hence only three 3Gbit/s SAS lines will saturate this (assuming you have hard drives fast enough to do this).

On the top of that in real world we often need random, not sequential performance (e.g. in a database environment) - and for that a bus bandwith is not the limiting factor.

Rgds.

Re: SAS vs. U320 Performance with RAID5

sgunelius — Mon, 22 May 2006 10:58:53 GMT

So, even if I could realize better throughput from selection of the SAS model of a ProLiant server, I might end up saturating the bus and may not achieve an appreciable increase in performance. Dang it!

Thanks again for your response.

Re: SAS vs. U320 Performance with RAID5

Leif Halvarsson_2 — Tue, 23 May 2006 05:59:56 GMT

Hi,

If you check this document, you can find that the same disk model can be delivered with different interfaces (LVD/FC/SAS/). But the performance specifications is exact the same, regardless of the type of interface.

So, the interface will have very little impact, the type of disk is more iportant.

Re: SAS vs. U320 Performance with RAID5

sgunelius — Tue, 23 May 2006 08:08:13 GMT

Leif,

Thank you for your response. Yes, I had forgotten that the devices I would be connecting to will be the most important factor regardless of the connection method. I suppose I'll make sure that if SAS is selected that we employ 15K drives as well.

By the way, I noted in your profile that you are from Sweden. My father's family emigrated from Halsingland to northern Minnesota back in the late 1800's. My father is still doing the research on the rest of the family tree.

Tack sÃ¥ hemskt mycket!

Scott

Re: SAS vs. U320 Performance with RAID5

raadek — Tue, 23 May 2006 15:04:27 GMT

Small update here: HP just announced new, PCI-Express x8 based SAS RAID card - P400.

Hence in theory you could stream up to 2GB/s through the slot.

But again - it's rather hard drive spec which will limit the overall performance, than it's interface (especially with random read / writes)