I am looking at the specs and manual for the IBM System x3650. According to the specs a ServeRAID-8k-l hardware RAID comes standard, but there's an option to upgrade to ServeRAID-8k. If I understand this correctly, this system will always have some h/w RAID.

I have found that h/w RAID firmware often assumes that all disks attached should be part of a RAID configuration managed by h/w, and of the same make of h/w at that. This won't always be the case, either because one is using s/w RAID and/or because HDDs are being brought in from other systems with data on them.

My question is whether this limitation applies to the x3650 or whether the on-board SAS ports can 'pass-through,' allowing the OS to 'see' an attached physical disk directly.

(I'm quite cautious because I once bought an Adaptec RAID whose specs and manual claimed it had this capacity, only to find out it didn't.)

I'm also wondering of the same on-board controller, notwithstanding the usual cautions on using disks that big, whether it is able to talk to 2TB disks (I've seen some that don't).

  • Are you trying to build a ZFS or Windows Storage Spaces system? – ewwhite Jan 22 '13 at 5:54
  • ZFS would be one of the main uses. The h/w RAID isn't needed. The question boils down to how the "integrated ServeRAID-8k-l controller" and "integrated eight-port SAS controller" are related and whether the former can be disabled/removed to use just the SAS controller (HBA). If not then installing a standalone HBA would likely be required. – Kevin Jan 22 '13 at 14:37

Both of the RAID cards (8k or 8k-l) you are considering are able to configure the drive as a "Simple Volume". This is done in the ServeRaid Manager management software bundled with the server. documentation here

This simply exposes the drive as a single LUN. This approach is often used when recovering damaged RAID arrays for example. (However, see the comments below.)

It seems that these controllers however have only been updated to provide support for 2TB drives. There is no mention of 3TB device support in the most recent firmware update.

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    When configured as a "simple volume" doesn't the server whipe all the disks as a part from initialization? – Str82DHeaD Jan 22 '13 at 7:14
  • Yeah, in my experience RAID controller "Simple Volumes" normally write metadata to the disk which will in effect delete whatever was on it, which is my main concern. The system's spec sheet says "All x3650 models include an integrated eight-port Adaptec AIC9580W Serial-Attach SCSI (SAS) controller" but then goes on to mention the "integrated ServeRAID-8k-l controller" that comes standard, so I'm not sure if this means all attached devices are under h/w RAID or if there is a way to remove/bypass the ServeRAID to get straight HBA behaviour or if I'd have to add an HBA card. – Kevin Jan 22 '13 at 14:15
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    If you want to completely bypass the RAID functionality, then, as @ewwhite has pointed out, you'll need to get a straight SAS HBA. The onboard raid controller handles all of the backplane ports, and the only way to get a single drive to work without RAID on that is the 'Simple Volume' method. – James Hebden Jan 22 '13 at 22:28
  • Thanks for the confirmation! I notice that the x3650 specs state standard support of the external 4-port SAS connector, but the ServeRAID-8ks are designated internal SAS while the optional MegaRAID is designated external SAS/SATA. This vaguely suggests the external port is a straight HBA, at least if a MegaRAID isn't installed. Is this so? (I don't have one to test right now so I only have IBM's wishy-washy docs to go on.) – Kevin Jan 23 '13 at 2:35
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    The x3650 normally only has one external SAS port, which is handled by the 8k. So the same limitations apply on that port. The four external port reference likely refers to an expansion card, such as the LSI MegaRAID card, which I've needed to install in the past to hook up things like tape libraries to the server. The MegaRAID is basically a PCIe SAS HBA, and doesn't have the same limitations as the 8k which is a dedicated RAID controller. – James Hebden Jan 23 '13 at 5:30

If you want to do this the right way, you'll need a standalone SAS HBA.

See: ZFS SAS/SATA controller recommendations

However, sometimes, I will configure ZFS on top of hardware RAID arrays. This is typically when I'm looking for ZFS filesystem flexibility or specific features (like compression). Also, if you have a high-end RAID controller (like an HP Smart Array), leveraging its battery or flash-backed cache is more convenient and affordable than a proper ZIL device (STEC ZeusRAM or DDRDrive) and still gives you the low-latency write speed.

Remember, without either of the above options, you won't have a real write cache...

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