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6 Posts
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4549
Clariion CX3-40 Mixing SATA and FC shelves on the same Bus
Morning All,
I was hoping someone could clarify a query I have had from one of our storage guys. We have a CX3-40 with a number DAE3P shelves. If we had 1 Shelf with FC disk and another shelf with SATA II disk on the same Bus and loop would the SATA disks effect the performance of the FC disks due to the slower rotational speeds and transfer times of the SATA disk on the same FC-AL.
Thanks
Bren
I was hoping someone could clarify a query I have had from one of our storage guys. We have a CX3-40 with a number DAE3P shelves. If we had 1 Shelf with FC disk and another shelf with SATA II disk on the same Bus and loop would the SATA disks effect the performance of the FC disks due to the slower rotational speeds and transfer times of the SATA disk on the same FC-AL.
Thanks
Bren
DaveZ1
75 Posts
0
May 1st, 2008 10:00
"Writing to a 7200RPM disk will take longer than writing to a 15K disk. So the slower disk will hold the bus longer."
The latter statement is not true. I can categorically state that we have never made that statement in either guide. But I understand that the misconception is widely held.
Here's the lowdown:
There have been times when we suggested using a different bus for certain drives - when they were at a lower interface rating, 2 Gb/s interface vs. 4 Gb/s. If you had the original 10K rpm drives for the CX3 series, for example, they were 2 Gb/s. You would not want to put them on a bus with 4 Gb drives as then the bus arbitrates down to 2 Gb/s. But our SATA drives are 4 Gb/s -- they will work on the bus just as well as any FC 4 Gb/s drive. So, why do folks think they 'slow down' the bus.
The confusion stems I think from 2 things:
1) statements that guided the user to separate high-bandwidth disk traffic (which is often but not always on SATA) from random. You don't want your backup to disk traffic to soak up all the bus bandwidth, affecting your online apps like Exchange. This has NOTHING to do with the disk transfer rate - it is about the workload you are asking the disks and the bus to absorb. If your BU2D is close to or more than 300 MB/s, you are using up most of one bus's bandwidth. You should put your exchange drives on another bus.
2) the misunderstanding that SATA drives somehow 'slow down' the bus. Nothing could be further from the truth. There are 3 factors at work here:
2-a) The disk transfer rate is immaterial - the transfer onto the bus is made from buffers in the HADA electronics, long (in computer terms) after the transfer from the platter is made, and it is controlled by the interface electronics.
2-b) The arbitration for the bus is handled by the interface electronics which, in the SATA II product we have shipped for over a year now, is 4 Gb/s capable. So, no slowdown from that.
2-c) The transfer PATTERN is really controlled by the SP. Those manufacturer specs for MB/s transfer rates are really just pie-in-the-sky (e.g., marketing) and only applicable to a single disk on a busmastering PCIe controller on a server (doing nothing else on that bus). In the context of a storage system, where you have multiple drives arbitrating & exchanging information in a controlled manner on a FC bus, those transfer times are immaterial. The transfer rates of the bus will only be matched by multiple drives, which will hit maybe 30 MB/s each with a good IO profile. Whether it is 10 drives or 20, it's all the same to the bus. There is a cost to having MORE drives on the bus, but that is a small effect.
Lastly, the proof is always in the pudding. If SATA drives 'slowed down' the bus then you'd wonder why we hit some of our higherst bandwidth numbers WITH SATA DRIVES. And, we do.
So, please do the reading yourself. Place drives on buses based on general aggregation of workload - if you have very different workload profiles then you might separate groups of disk by bus. Certainly do so if you have disks rated at different interface speeds (e.g., 2 Gb/s vs. 4 Gb/s). But in the majority of cases, mixing SATA and FC in an of itself has no effect in the real world.
flip200
6 Posts
0
April 30th, 2008 03:00
If this is the case then the slower SATA drives will hold up the bus while it transfers data.
Is this true with the Ultrapoint shelves?
Kiran3
410 Posts
0
April 30th, 2008 03:00
it is not advisable to share the disks in a single raid group but
Kiran3
410 Posts
0
April 30th, 2008 03:00
even though SATA is said to be slower, its still fast enough.
flip200
6 Posts
0
April 30th, 2008 04:00
What i am trying to understand is the architecture of the shelves (FC-AL) and how the use of SATA and FC disks effect the performance on the same bus. The SATA II disks have a 4Gbps paddle card on them so there is no difference in bus speed between the disks.
I read in the Ultrapoint Technology guide that the new shelves have a loop switch which may get round this?
I have read all the documentation that i can find and i can't seem to get a definitive answer.
Thanks for you help
DGM3
238 Posts
1
April 30th, 2008 13:00
Regards,
DGM
flip200
6 Posts
0
May 1st, 2008 01:00
You have mentioned that when data is transfered to the disk it is transferred at bus speed and then the disk releases the bus. Would the time it takes to physically transfer the data to the platters effect the lenght of time the disk holds the bus for. i.e the time it takes to transfer data to a SATAII (7200)disk that holds the bus would be alot slower than a FC (15k) disk even if the the bus is connected to both disks at 4Gbit/sec over Fibre Channel.
Maybe onboard cache on the disk and the NCQ releases the bus quicker?
Cheers
Bren
flip200
6 Posts
0
May 1st, 2008 08:00
Cheers
B
kelleg
4.5K Posts
1
May 1st, 2008 08:00
Writing to a 7200RPM disk will take longer than writing to a 15K disk. So the slower disk will hold the bus longer.
On-disk write cache is disabled for data protection - the Write cache on the storage processors is protected by the SPS battery backup, the disks are not. If you have write data in an on-board disk cache, you could lose this data if you have a power outage.
EMC CLARiiON Best Practices for Fibre Channel Storage: FLARE Release 26 Firmware Update - Best Practices Planning
http://powerlink.emc.com/km/live1/en_US/Offering_Technical/White_Paper/H2358_clariion_best_prac_fibre_chnl_wp_ldv.pdf
EMC CLARiiON Fibre Channel Storage Fundamentals - Technology Concepts and Business Considerations
http://powerlink.emc.com/km/live1/en_US/Offering_Technical/White_Paper/H1049_emc_clariion_fibre_channel_storage_fundamentals_ldv.pdf
regards,
glen kelley
kelleg
4.5K Posts
0
May 1st, 2008 09:00
To take the time to configure for Best Performance up-front is the most important step you can take to ensure that you have the best possible performance, at least until you change your operations.
glen
kelleg
4.5K Posts
0
May 1st, 2008 10:00
regards,
glen
Allen Ward
2.1K Posts
0
May 1st, 2008 14:00
Are you the one that wrote my all time favorite line?
"Many EMC personnel believe that the CLARiiON RAID optimizations work only with a 4+1 or 8+1 stripe, which is not true..."
The first time ever (and one of few ever found) that I saw an EMC employee admitting that their fellow employees were not infallible. And trust me, the number of times I pulled that document out to help support my case (both with my co-workers AND with EMC Support folks) I would have paid for a permanent subscription to the Best Practices guide.
Sorry, I know I'm getting a bit off topic here, but I just wanted you to know how much those documents are appreciated!!!
Kiran3
410 Posts
0
May 4th, 2008 23:00
So, why do folks think they 'slow down' the bus.
Just because most are used to reading in the books that SCSI HDD is FAST and combined with FC its FASTEST
flip200
6 Posts
0
May 6th, 2008 03:00
Thanks DaveZ for the post that explains alot about the architecture of the Clariion and the FC / SATA disks. Believe me I have spent days reading the documentation on the Clariion and have not seen anywhere an explanation on how the disks transfer data to the Bus (eg HADA Electronics). Maybe this is a lack of my understanding at the hardware level but what i was trying to do is get a definitive answer either way of how data was transferred.
When I posted: "Writing to a 7200RPM disk will take longer than writing to a 15K disk. So the slower disk will hold the bus longer." I was not saying that you had made that statement in the guide, what i was doing was making a statement on the facts that I had to hand on FC-AL and my lack of knowledge of the drive and interface electronics.
When someone in my comapny stated that mixing SATA and FC on the same bus dragged down the performance, I was very sceptical as this would mean a fundemental oversight on the part of EMC ..... and so my I started my marathon document hunt to prove that this was not the case.
Anyway I hope this thread helps other people out there....... thanks to DaveZ for the answer and the excellent fundementals and best practices guides.
Cheers
Bren
JasonZ2
2 Posts
0
January 5th, 2009 08:00
I just want to make sure of something before I continue on with adding another DAE3P. I currently have 3 DAE3P hooked up and one of them contains SATA II drives. I was told by an "EMC engineer" that when I wanted to add additional DAE3P's with FC drives, that I will always have to put the SATA II DAE at the end of the loop or I will incur performance issues.
Is this true or does the engineer not quite grasp the technologies involved?