Solved Striped mirror vs raidz2 for reliability

mefizto said:
Furthermore, even with multiple controllers, if one fails, the entire stripped mirror will become unusable.

Or, am I missing a point?
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You are missing the point, completely.

Consider a single mirror, with each disk connected to a different controller. Now cause a single disk, single data connector, single cable, or single controller to fail. The mirror survives because it has a second redundant "side" which is still working.

Now construct a second, and successive, mirrors, identically.

Now cause one disk in each mirror, one splitter cable, or one controller to fail. Each mirror, individually, will survive because each has a second redundant "side" which is still working. Because each mirror survives such a scenario, a stripe of such mirrors survives.

My ZFS server has four SATA controllers and many mirrors (one mirror for the "root" on SSD, and many striped mirrors to form the "tank"). Each "disk" in each mirror is connected to a different controller. I can lose up to two controllers (or their associated splitters, cables, and connectors), and the system will still be running 100% correctly. RAID-Z is much more vulnerable to data loss under these circumstances -- because you would need a separate controller for each disk (or, to be fair, every two disks for RAID-Z2) to be able to guarantee similar redundancy.
 
Well, if everything is on a single controller and that controller dies the array is never marked degraded.

Of course you should have a second controller on the shelf.
 
Hi gpw928,

thank you for the explanation, I have learned something. In view of that, I re-checked, but the motherboard (Supermicro X9CSM) has, indeed, only a single controller.

Kindest regards,

M
 
Hi gpw928,

thank you for the card recommendation. Now that you mentioned it, I remember that I looked at such a solution in the past, when I was going to expand the number of disks. But, I vaguely remember something about a need to re-flash such a card, since ZFS needed a direct access to the ports, or something like that.

Let me do some further research since I like your idea now that I have a false impression that I understand it.

Kindest regards,

M
 
mefizto said:
But, I vaguely remember something about a need to re-flash such a card, since ZFS needed a direct access to the ports, or something like that.
Click to expand...

The linked (LSI 9210) is *just* a host bus adapter, which is what your want for ZFS — direct access to the devices.

The 9260 series were the same-generation (6Gb/s) RAID adapters.

You can re-flash some RAID cards (popular with cheap recycled Dell cards, I recall) to IT (initiator target / vanilla HBA) mode.
 
Hi Eric A. Borisch,

thank you for the information. I will get one, once I figure out the strange connectors on it.

Kindest regards,

M
 
mefizto said:
I will get one, once I figure out the strange connectors on it
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The cables are usually sold separately. You would need two "SFF-8087 to 4xSATA Adapter Cables".

There's plenty of options for purchase when you google "LSI SAS 9210-8i".

Re-flashing used RAID cards is a good low cost approach (or even using un-flashed RAID cards in JBOD mode will usually work). You have to do a bit of homework to travel down that path. But there's plenty of people around who can offer advice.
 
Hi gpw928,

yes, I did google for the card, and, indeed, found plenty of them; however, without cables. I found "Mini SAS to 4 SATA", but was not sure whether it was the correct one. Now with your identifier, I am sure it is the correct one.

I do not think that for the price of the "real" HBA as per Eric A. Borisch's post it makes any sense to re-flash a RAID card.

Kindest regards,

M
 
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