"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
roy ----- 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 roy -- Roy Sigurd Karlsbakk (+47) 98013356 r...@karlsbakk.net http://blogg.karlsbakk.net/ GPG Public key: http://karlsbakk.net/roysigurdkarlsbakk.pubkey.txt -- I all pedagogikk er det essensielt at pensum presenteres intelligibelt. Det er et elementært imperativ for alle pedagoger å unngå eksessiv anvendelse av idiomer med xenotyp etymologi. I de fleste tilfeller eksisterer adekvate og relevante synonymer på norsk. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
