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
