"copies" won't help much if the pool is unavailable. It may, however, help if, 
say, you have a RAIDz2, and two drives die, and htere are errors on a  third 
drive, but not sufficiently bad for zfs to reject the pool


----- Opprinnelig melding -----
> 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
> situation?
> 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?
> Thanks,
> //Jim Klimov
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss@opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss

Vennlige hilsener / Best regards

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