sata - Preventing h/w RAID cards from dropping slow JBOD disks

I'm considering buying a used SAS h/w RAID card for externally attaching HDDs to an HP ProLiant I'm setting up.

However, I only require RAID functionality on some of the drives. Theoretically it should be simple to JBOD the other drives, but some of them are inexpensive SATA disks and probably cannot have TLER disabled.

I'd like to know, prior to actually ordering a RAID card, whether typically RAID cards would still enforce dropping of disks that do not respond within a few seconds, even if the disk is in a JBOD, and whether there is any way to disable this.

Ideally it would be nice to be able to select certain SAS ports that will be pass-through, bypassing the RAID engine entirely and just acting as an HBA for those ports. I know I could buy a separate SAS HBA but that seems like a waste of $ and is also impractical as it's a 1U server so space is extremely limited.

My question then is whether the functionality I'm looking for (pass-through on certain ports or at least JBOD drives not getting themselves dropped due to slow response) is typical of proper h/w RAID cards such as PERC 5/E etc. I've browsed through the latter's manual but unfortunately, as with most user manuals, it states the obvious and doesn't state the unobvious.

Thanks for any info,
Kevin

Answer

This is a little confusing. You're asking for too many disparate qualities.

• If you don't need functionality, use a proper SAS HBA. I presume this is for a software disk management solution like ZFS, Linux RAID or maybe something more esoteric, like HDFS...




• If you DO need hardware RAID functionality, leverage a hardware RAID controller (possibly the onboard controller available on the server you have).




• As far as dropping disks due to slow response, what are you planning? SAS controllers will downshift speeds if there's an issue, but you shouldn't be using disks that don't respond within normal thresholds.




• All of this is normally possible, but it sounds like you're using a 1U HP ProLiant server (DL160? DL165? DL360?), which is probably the least flexible server you could ever buy for disk drive bay and PCIe flexibility. If it's a modern ProLiant, you'll have an onboard controller available. Even a 2U ProLiant DL380 G6/G7 would give you the flexibility of having multiple controllers to address one of two 8-bay drive cages.




• It sounds like you're looking at external enclosures as well. Many controllers connect to HBA's using a SAS SFF-8088 connector. It's a 4-lane connector. Many enclosures have an internal SAS expander. That could impact the approach of having RAID for some disks and JBOD for others on the same controller. Why not keep the HW RAID disks inside the server and use the external enclosure for JBOD? Does your server have an onboard RAID controller? Which enclosure are you using?