On Sat, Mar 28, 2009 at 11:12 AM, Bob Friesenhahn < bfrie...@simple.dallas.tx.us> wrote:
> On Sat, 28 Mar 2009, Tim wrote: > >> >> #1: yes, there is harm as he may very well run into inconsistent >> performance >> which is a complete PITA to track down when you've got differing raidtypes >> underlying a volume. >> > > Inconsistent performance can come from many things, including a single > balky disk drive. The small difference between RAID types does not seem > like enough to worry about. If it was a mix between raidz2 and mirrors then > there is more cause for concern. > > It is true that if the performance of the vdevs is not well balanced, then > some vdevs could fill up faster than others when the system is under > extremely heavy write loads. An extra parity disk is hardly a "small difference". You pay your penalty at some point for the extra parity, and it will come back to bite you in the ass. It's why NOBODY including Sun, supports it on enterprise arrays. > > > #2: raidz2 isn't always "wise" to choose. It's a matter of performance, >> space, security requirements. 7+1 is fine for raidz1. If he was pushing >> 10 >> data disks that'd be another story. >> > > Many in the industry have already declared RAID5 to be "unsafe at any > speed" with today's huge SATA disk drives. The data recovery model for > raidz1 is similar to RAID5. If the user can afford it, then raidz2 offers > considerably more peace of mind. > > If you are using 750GB+ SATA drives then your "7+1 is fine for raidz1" > notion does not seem so bright. > Many in the industry make their money selling you disk drives, of course they're going to declare you need to buy more. The "math" you ask people to do points towards a 7+1 being more than acceptable if you have a hot-spare. --Tim
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