ZFS or UFS for 4TB hardware RAID6?

Maxim Khitrov mkhitrov at gmail.com
Mon Jul 13 18:03:20 UTC 2009

On Mon, Jul 13, 2009 at 1:46 PM, Richard Mahlerwein<mahlerrd at yahoo.com> wrote:
>> Your mileage may vary, but...
>> I would investigate either using more spindles if you want
>> to stick to RAID6, or perhaps using another RAID level if
>> you will be with 4 drives for a while.  The reasoning
>> is that there's an overhead with RAID 6 - parity blocks are
>> written to 2 disks, so in a 4 drive combination you have 2
>> drives with data and 2 with parity.
>> With 4 drives, you could get much, much higher performance
>> out of RAID10 (which is alternatively called RAID0+1 or
>> RAID1+0 depending on the manufacturer and on how accurate
>> they wish to be, and on how they actually implemented it,
>> too). This would also mean 2 usable drives, as well, so
>> you'd have the same space available in RAID10 as your
>> proposed RAID6.
>> I would confirm you can, on the fly, convert from RAID10 to
>> RAID6 after you add more drives.  If you can not, then
>> by all means stick with RAID6 now!
>> With 4 1 TB drives (for simpler examples)
>> RAID5 = 3 TB available, 1 TB worth used in "parity".
>> Fast reads, slow writes.
>> RAID6 = 2 TB available, 2 TB worth used in "parity".
>> Moderately fast reads, slow writes.
>> RAID10 = 2 TB available, 2TB in duplicate copies (easier
>> work than parity calculations).  Very fast reads,
>> moderately fast writes.
>> When you switch to, say, 8 drives, the numbers start to
>> change a bit.
>> RAID5 = 7TB available, 1 lost.
>> RAID6 = 6TB available, 2 lost.
>> RAID10 = 4TB available, 4 lost.
> Sorry, consider myself chastised for having missed the "Security is more important than performance" bit. I tend toward solutions that show the most value, and with 4 drives, it seems that I'd stick with the same "data security" only pick up the free speed of RAID10.  Change when you get to 6 or more drives, if necessary.
> For data security, I can't answer for the UFS2 vs. ZFS.  For hardware setup, let me amend everything I said above with the following:
> Since you are seriously focusing on data integrity, ignore everything I said but make sure you have good backups!  :)
> Sorry,
> -Rich

No problem :) I've been doing some reading since I posted this
question and it turns out that the controller will actually not allow
me to create a RAID6 array using only 4 drives. 3ware followed the
same reasoning as you; with 4 drives use RAID10.

I know that you can migrate from one to the other when a 5th disk is
added, but RAID10 can only handle 2 failed drives if they are from
separate RAID1 groups. In this way, it is just slightly less resilient
to failure than RAID6. With this new information, I think I may as
well get one more 2TB drive and start with 6TB of RAID6 space. This
will be less of a headache later on.

- Max

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