On Aug 26, 2011, at 4:02 PM, Brandon High <bh...@freaks.com> wrote:

> On Fri, Aug 12, 2011 at 6:34 PM, Tom Tang <thomps...@supermicro.com> wrote:
>> Suppose I want to build a 100-drive storage system, wondering if there is 
>> any disadvantages for me to setup 20 arrays of HW RAID0 (5 drives each), 
>> then setup ZFS file system on these 20 virtual drives and configure them as 
>> RAIDZ?
> 
> A 20-device wide raidz is a bad idea. Making those devices from
> stripes just compounds the issue.

Yes... you need to think in the reverse. Instead of making highly dependable
solutions out of unreliable components, you need to make judicious use
of reliable components. In other words, RAID-10 is much better than RAID-01,
or in this case, RAID-z0 is much better than RAID-0z.

> The biggest problem is that resilvering would be a nightmare, and
> you're practically guaranteed to have additional failures or read
> errors while degraded.

I'm getting a but tired of people designing for fast resilvering. This is
akin to buying a car based on how easy it is to change a flat tire. It
is a better idea to base your decision on cost, fuel economy, safety,
or even color.

> You would achieve better performance, error detection and recovery by
> using several top-level raidz. 20 x 5-disk raidz would give you very
> good read and write performance with decent resilver times and 20%
> overhead for redundancy. 10 x 10-disk raidz2 would give more
> protection, but a little less performance, and higher resilver times.

A 20 x 5-disk raidz (RAID-z0) is a superior design in every way. Using 
the simple Mean Time To Data Loss (MTTDL) model, for disks with 1 million
hours rated Mean Time Between Failure (MTBF) and a Mean Time To Repair 
(MTTR) of 10 days:
   5 disk RAID-0 has MTTDL of 199,728 hours
   20-way raidz of those disks has a MTTDL of 437,124 hours
as compared to:
   20 x 5 raidz has MTTDL of 12,395,400 hours

For MTTDL, 12,395,400 hours is better than 437,124 hours
QED

  -- richard


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