SQL Server Performance Forum – Threads Archive

Controllers & Channels – Odds and Ends

I’m trying to get a better understanding of how controllers work and how that translates to a solid SQL Server configuration. I’ve read Joe’s posts, which help quite a bit, but was hoping for some answers to these questions. – What is the relationship between a SCSI port and a channel? For example, if I have an HP controller with 4 ports, does that mean I also have 4 channels? – Is it possible to assign multiple channel to the same set of disks? For example, I have an HP controller with 4 ports. Can I dedicate that controller to our data drive and assign all available channels to the drive or is there only a 1:1 relationship between drives and channels? – If I’m picturing a controller correctly, it is a physical card typically containing 2 – 4 ports. Each port communicates to a disk through one or more channels. Each disk connects to a channel through a PCI slot. Is this correct? I’m a bit fuzzy here. Thanks, Dave

1. same thing
2. no, not for scsi
3. more or less, be aware that many 4 channel SCSI cards actually two 2 channel scsi-pci chips, then use a pci bridge to get the second pair of scsi channels.
this is still good for getting random IOPS, but does not scale for sequential IO. so the preference was to use dual channel scsi controllers, unless the need for disks exceeds the available PCI-X slots.
try to use SAS going forward

Thanks Joe. Will the vendor’s specs indicate if their 4 port cards are actually using two 2 channel scsi-pci chips? How do I determine the performance impact of bridging the two cards? Dave

i would not waste time looking through the detailed documents and just assume it is so,
there is no reason you cannot build a perfectly good system with the dual channel cards
so there is no need to pursue this matter

Hi Joe, I have no problem with the assumption, however it’s the server team that is questioning why I think a 4 port controller is not a good option. They wanted to install tempdb, log files and backups on one 4 port controller. I asked them to move the logs to the internal controller shared by the OS. I was hoping to show them some information indicating the issues associated with their approach. Thanks, Dave

if you have one 4 port, configure 4 disks to each of the 4 scsi ports create a database spread across all 16 disks,
write a query that will do a forced table scan that returns very few rows, compare with two 2 port cards, also with 4 disks per port,
you will find that the 2 port card can do 260MB/sec per scsi channel, or 520MB per card and the combined throughout is 1040MB/sec if each card is on a separate PCI-X bus the 4-port card might do 600-700MB/sec

Thanks Joe. I’ll give it or something similar a try. I was told our external storage unit contains only two ports, so it looks like my tempdb and backup files will be sharing the same port. My data is on its own controller attached to the other port in the eternal box. The sharing of the same port looks like another reason I want my logs moved to the internal. Dave

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