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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

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Specifically what card/s are you looking at –  Chopper3 Sep 17 '12 at 9:39
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And which servers, which controllers, which external enclosures are in use? –  ewwhite Sep 17 '12 at 15:25

2 Answers 2

up vote 1 down vote accepted

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?

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Thank you for the help. The h/w RAID is for ESXi; the JBOD for ZFS on a Solaris VM. I'm stuck for now with a DL145 G3 & useless on-board SATA. I'd rather have the ZFS on a non-RAID HBA, but slots and $ may force me to do everything via one card. The HDDs normally respond fine but some don't have TLER and so may freeze up trying to fix flaky sectors. At first the HDDs will be in a makeshift enclosure & connected straight to external SAS ports via breakout cables. Once we pass 8 HDDs we'll get an enclosure $ allows (maybe MD1000). Can one SAS expander not mix RAID & JBOD? –  Kevin Sep 18 '12 at 5:49
    
Good advice. Ended up switching to a 4U with plenty of slots, a SAS bay, and integrated Smart Array. Used the internal RAID for the VMFS, and a Dell SAS 6/e for external storage for ZFS, and it's been working very well and way easier to maintain. –  Kevin Feb 14 '13 at 15:19

The PERC series of RAID cards do not directly support JBOD, but it can be simulated by setting up each drive as a RAID 0 containing only one drive.

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Same for HP SmartArray cards. –  Chris S Sep 17 '12 at 12:54
    
Thank you for the tip. I haven't committed to a particular RAID card yet so the card might not end up being a PERC - that was just an example.I figured worst-case the JBOD could be simulated as such but that has some disadvantages. For one the metadata will mean the disks are useless without that particular card. Also - my original concern - if one of the drives that doesn't support TLER (time-limited error recovery) hits a flaky sector and decides to take upwards of a minute trying to fix it, a RAID card will fault the drive if it's in an array and it takes more than ~8 sec to respond. –  Kevin Sep 18 '12 at 5:58

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