Storage strategy. RAID 1 or 5? Backup? Both?

Started Oct 28, 2012 | Questions thread
kelpdiver Veteran Member • Posts: 3,426
Re: How's RAID Z1 working for you?

Jim Cockfield wrote:

Raid Z1 and Raid 5 both scare me, because I've seen lots of "horror stories" about another drive failing after the rebuild starting when replacing a failed drive, resulting in the loss of the entire array.

Apparently, many modern drives just don't cope with RAID 5 (or, RAID Z1 if using ZFS) very well, as it's very common to have timeout issues with another drive being removed from an array during that rebuild process from what I can find out about those setups.

So, I've been wondering if a RAID Z2 config (equivalent to RAID 6) may be a better option, since it allows for more than one drive failure. Sure, you lose some of the available space with a given number of drives. But, reliability would be a more important consideration from my perspective.

For the concerns you have, R6/Z2 doesn't actually change matters, and introduces a new point of failure - what if the two parity stripes are in conflict?  Which one is authoritative?   If you believe in how zfs solves the write hole problems and if you believe that weekly scrubbing will fix bad blocks and identify a silent failing drive, then Raid Z1 gets the job done.  Z2 just lets you suffer an outage without falling into degraded performance and running without a safety net.

The TLER issue has been around for a while.  I've never had to do a full rebuild, so I don't know how serious it really is.  Again, scrubbing would identify bad blocks in advance of a rebuild event.  You can mitigate it by biasing towards the drives that have settable parameters, or in the case of WD, the Red line of drives.  I have two as part of a Z filer and they seem to be working out fine, though they do command a premium over the EARX which also seem to be working fine.

How many drives will you put into this enclosure?  For 4 drives, raid 10 would be better than Z2.  While read performance is the same, write performance is much better with the mirrored setup.

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