> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Jim Klimov
>    I wondered if the "copies" attribute can be considered sort
> of equivalent to the number of physical disks - limited to seek
> times though. Namely, for the same amount of storage on a 4-HDD
> box I could use raidz1 and 4*1tb@copies=1 or 4*2tb@copies=2 or
> even 4*3tb@copies=3, for example.

The first question - reliability...

"copies" might be on the same disk.  So it's not guaranteed to help if you have 
a disk failure.

Let's try this:  Take a disk, slice it into two partitions, and then make a 
mirror using the 2 partitions.  This is about as useful as the copies property. 
 Half the write speed, half the usable disk capacity, improved redundancy 
against bad blocks, but no better redundancy against disk failure.  ("copies" 
will actually be better, because unlike the partitioning scenario, "copies" 
will sometimes write the extra copies to other disks.)

Re: the assumption - lower performance with larger disks...  rebuild time 
growing exponentially...

I don't buy it, and I don't see that argument being made in any of the messages 
you referenced.  Rebuild time is dependent on the amount of data in the vdev 
and the layout of said data, so if you consider a mirror of 3T versus 6 vdev's 
all mirroring 500G, then in that situation the larger disks resilver slower.  
(Because it's a larger amount of data that needs to resilver.  You have to 
resilver all your data instead of 1/6th of your data.)

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