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Most discussions of ZFS suggest that the hardware RAID be turned off and that ZFS should directly talk to the disks and manage the RAID on the host (instead of the RAID controller).

This makes sense on a computer with 2-16 or even more local disks, but what about in an environment with a large SAN?

For example, the enterprise I work for has what I would consider to be a modest sized SAN with 2 full racks of disks, which is something like 400 spindles. I've seen SAN shelves that are way more dense than ours, and SAN deployments way larger than ours.

Do people expose 100 disks directly to big ZFS servers? 300 disks? 3000 disks? Do the SAN management tools facilitate automated management of this sort of thing?

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4 Answers 4

up vote 4 down vote accepted

FWIW I have experience with up to 92 disks in a single ZFS pool and so far it works fine.

But if you're really talking about several hundreds of disks I would consider partitioning them into a small number of disjunct (but still large) pools. I don't want to know how long e.g. a zpool scrub runs on a 3000 disk pool (but you want to scrub regularly). Also the output of commands like zpool status would be unwieldy with such a large number of disks. So why put all eggs into a single basket?

(Side note on dedup: Notice that although dedup can be controlled at the dataset level it will find duplicates at the pool level. I.e. you'll probably get worse dedup results if you're partitioning as suggested. On the other hand you'll need much more memory to hold the dedup hashes of a single giant pool which might not fit into ARC+L2ARC if the pool's too big. So if you are using dedup the amount of available memory is probably a good indicator for the maximum practical pool size.)

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+1 agree. You don't have to put all disks in a very big pool. –  Pier Jun 27 '10 at 9:56
    
I was thinking...does any san (the most common from hp, ibm, emc and so on) expose all disks directly to the boxes? Or you must (as far as i saw) create luns and then associate them to servers? Or as chris is intending, it's more like some DAS? –  Pier Jun 27 '10 at 13:17
    
Please note that Sun/Oracle Support recommends that dedup to be disabled even on their own OpenStorage product series as the performance hit is quite drastic. –  pfo Nov 3 '10 at 1:00

Here is a website you may want to look at to consider size and configuration of the pool(s) for probability of data loss.
https://blogs.oracle.com/relling/entry/zfs_copies_and_data_protection

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If you don't give ZFS redundant data to work with (e.g. mirrors, RAID-Z), then you lose many of the benefits of using it. The number of disks involved won't change that fact. However, whether that matters really depends on your environment. You have to determine what storage features you need (a potentially labor-intensive analysis) , and then go hunting for the least expensive solution (you can afford) that meets your needs. That may mean using ZFS everywhere along with specialized Oracle storage devices (some people do that and have many disks exposed to ZFS without problem, and use Oracle tools to do management), it may mean using only enterprise SAN products, or it may mean using some hybrid (in which case you'll probably have to develop some tools and processes on your own to manage the environment). Don't forget that your analysis needs to account for the human element as well (e.g. maybe you have a team of Storage people who have highly useful and specialized training with a SAN product which you would lose if you went to an all ZFS solution).

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We let our SANs manage the RAID. Why spend money on all that battery backed NVRAM and those dedicated processors and then offload the work onto the server, whose CPUs I want doing something other than RAID checksums?

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I think this conundrum is referred to as "the wheel of reincarnation" where there is a constant cycle between offloading tasks to a specialized CPU, then rolling the tasks back onto the CPU as the general purpose CPU gets fast faster than the specialized CPUs. –  chris Jun 28 '10 at 15:42

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