Hello all,

  Over the past few years there have been many posts suggesting
that for modern HDDs (several TB size, around 100-200MB/s best
speed) the rebuild times grow exponentially, so to build a well
protected pool with these disks one has to plan for about three
disk's worth of redundancy - that is, three- or four-way mirrors,
or raidz3 - just to allow systems to survive a disk outage (with
accpetably high probability of success) while one is resilvering.

  There were many posts on this matter from esteemed members of
the list, including (but certainly not limited to) these articles:
* https://blogs.oracle.com/ahl/entry/triple_parity_raid_z
* https://blogs.oracle.com/ahl/entry/acm_triple_parity_raid
* http://queue.acm.org/detail.cfm?id=1670144
* http://blog.richardelling.com/2010/02/zfs-data-protection-comparison.html

  Now, this brings me to such a question: when people build a
home-NAS box, they are quite constrained in terms of the number
of directly attached disks (about 4-6 bays), or even if they
use external JBODs - to the number of disks in them (up to 8,
which does allow a 5+3 raidz3 set in a single box, which still
seems like a large overhead to some buyers - a 4*2 mirror would
give about as much space and higher performance, but may have
unacceptably less redundancy). If I want to have considerable
storage, with proper reliability, and just a handful of drives,
what are my best options?

  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.

  To simplify the matters, let's assume that this is a small
box (under 10GB RAM) not using dedup, though it would likely
use compression :)

  Question to theorists and practicians: is any of these options
better or worse than the others, in terms of reliability and
access/rebuild/scrub speeds, for either a single-sector error
or for a full-disk replacement?

  Would extra copies on larger disks actually provide the extra
reliability, or only add overheads and complicate/degrade the

  Would the use of several copies cripple the write speeds?

  Can the extra copies be used by zio scheduler to optimize and
speed up reads, like extra mirror sides would?

//Jim Klimov

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