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| Q & A (How to Maximize SATA Value with SAS) |
- Is there a enterprise customer that is deploying SAS?
Yes, there are some OEM customers we are designing their next generation products using SAS but not all using SAS drives
- Doesn't putting different rotational speed drives together in the same raid cause noise problems?
So far there is no noise problems reported but vibration problem is a challenge, since SATA drives are not designed to be robust enough to be placed next to a 15K RPM drive
- Can a SAS controller be connected to a SATA backplane?
Yes, when connecting a SAS controller to a non-expander based backplane, whether SAS or SATA, it makes no difference to the controller.
- What are the performance differences between SATA add SAS controllers?
If the comparisons are done with the exact configuration using same SATA drives, there should not be any differences
- SATA RAID Controllers can also support multilane, how does it compare to SAS multilane cabling?
Cabling is the same for both SAS and SATA, but SAS data is streamed across the 4x lanes where SATA is still treating the 4x lanes as 4x individual lanes
- What is the major performance difference between 2U 24 bays and 2U 12 bays SAS?
The difference is basically the result of combination of spindle count and rotational speeds. If comparison is done using 10K RPM 3.5" HDD and 10K 2.5" SFF HDD, you will see the performance doubled. Having said that, you need to check if the link to initiator is wide enough to sustain the throughput provided by 24x 2.5" SFF drives. You might see the same performance if the link bandwidth is mixed out with 12x spindles.
- What is the largest drive supported on the Back planes and Adaptec Controllers?
Theoretically there should not be any limitation. So far we have tested the Seagate 750GB HDD without problem.
- When you have 128 drives connected to one controller, that need 128x60MB/s =7.6GB/s. Can one controller handle that?
The limitation is really on the PCI-X or PCI-E bus on the motherboard. With a PCI-X 133MHz slot you will only see less than 800MB/S throughput and at least double on a PCI-E 8X slot. Therefore, when you use 128x drives off one single controller your max throughput will be the bus limitation. To improve performance you will have to use multiple cards
- Are their any special cable standards that we should adhere to in order to maintain the best performance?
All the cables used today whether it's the multilane or single lane, they are designed to sustain over 3G and in some case 10G. Performance should not be affected by a well-shielded cable no matter which type.
- What's the difference between a multiplier and an expander?
Multiplier is always working as 1:X (15 max) multiplication, so no matter how many drives behind the multiplier the link back to host is still one lane at 300MB/S. The expander can combined the lanes into N:X ratio (N being 4 or 8 in our design), so one can design the product based on needed bandwidth and choose the expander size accordingly.
- Are the AIC controllers specific to an one Server Box?
AIC does not offer controllers at this present time, but we offer expander I/O modules. Our expanders are designed to support any controller such as Adaptec and others. We have being working with various controller manufacturers on firmware compatibility and all the known issues have been worked out.
- I see large data availability- what about RAID redundancy?
RAID redundancy can be done using two dedicated servers using Adaptec controllers connecting to the dual expander modules on the JBOD. However, for cache coherency between the RAID controllers there must be a high speed interconnect between the servers.
- Is there a size limit that the drives can be?
No, any size of drives can be used.
- What problems are expected by having different rpm drives in the same raid module?
Vibrations coming from the higher-speed SAS drives might make the less robust SATA drives to fail, so vibration isolation must be done well in the enclosure.
- What's the difference between active-active failover and passive active fail-over?
Active-Active is referring to full redundancy all the way through and each HDD is actively connected to both initiators at all times, and replacement can be made any time with zero down time. Passive-Active means the replacement on the failed component can only be made system shutting down, and the fail-over path is not active while the other path is still working.
- Any redundancy (in case that one daisy cable gets disconnected)from daisy chain cables? If I have 8 daisy chain chassis, JBOD1 is offline, will this affect the rest 7 to be seen by my main chassis?
Depending how JBOD1 is offline. If JBOD 1 is completely blown up, then you will not be able to see the rest of the daisy-chained boxes. In the case if you are using dual expander modules for redundant paths, you will still see the daisy-chained boxes if one of the module failed in JBOD1. The second path will keep the link active to the rest of the daisy-chained boxes.
- What is the cable length to daisy chain each external JBOD?
The connection length is limited to 8~10M today from expander to expander, but is being considered to move up in the future.
- What are the programmable options in the staggered spin up for the expander backplane?
Staggered spin up delay settings are programmed in the firmware on the expander board, so it's not a user changeable setting. We use 6 seconds as the interval between every column of drives, as this is how Vitesse is supporting Staggered Spin Up. We cannot set delays between every HDD.
- What is the performance of SAS using expanders? What type of transfer rate is expected with SAS?
SAS expander is designed to support 3Gb/S per lane, so depends on how many wide-port lanes are there that is the maximum throughput you will see. For example, for 4x wide-port lane the max throughput is 1.2GB/S, and 2.4GB/S for two 4X lanes. The bottleneck will be the PCI-X or PCI-E bus which determines how much data can pass through the bus. Also the performance is determined by the actual application and environment. For the best case scenario you will see 90+MB/S per HDD under 100% sequential read applications.
- Who do we contact about reselling this solution?
For AIC/Xtore products please contact Bell Micro products or Avnet Applied Computing Solutions.
- Do you have any customers using 128 daisy-chained devices in production environments?
Yes, for archive applications.
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