On Mon, April 26, 2010 17:21, Edward Ned Harvey wrote: > Also, if you've got all those disks in an array, and they're MTBF is ... > let's say 25,000 hours ... then 3 yrs later when they begin to fail, they > have a tendency to all fail around the same time, which increases the > probability of exceeding your designed level of redundancy.
It's useful to consider this when doing mid-life upgrades. Unfortunately there's not too much useful to be done right now with RAID setups. With mirrors, when adding some disks mid-life (seems like a common though by no means universal scenario to not fully populate the chassis at first, and add more 1/3 to 1/2 way through the projected life), with some extra trouble one can attach a new disk as a n+1st disk in an existing mirror, wait for the resilver, and detach an old disk. That mirror is now one new disk and one old disk, rather than two disks of the same age. Then build a new mirror out of the freed disk plus another new disk. Now you've got both mirrors consisting of disks of different ages, less prone to failing at the same time. (Of course this doesn't work when you're using bigger drives for the mid-life kicker, and most of the time it would make sense to do so.) Even buying different (mixed) brands initially doesn't help against aging; only against batch or design problems. Hey, you know what might be helpful? Being able to add redundancy to a raid vdev. Being able to go from RAIDZ2 to RAIDZ3 by adding another drive of suitable size. Also being able to go the other way. This lets you do the trick of temporarily adding redundancy to a vdev while swapping out devices one at a time to eventually upgrade the size (since you're deliberately creating a fault situation, increasing redundancy before you do it makes loads of sense!). > I recently bought 2x 1Tb disks for my sun server, for $650 each. This was > enough to make me do the analysis, "why am I buying sun branded overpriced > disks?" Here is the abridged version: No argument that, in the existing market, with various levels of need, this is often the right choice. I find it deeply frustrating and annoying that this dilemma exists entirely due to bad behavior by the disk companies, though. First they sell deliberately-defective drives (lie about cache flush, for example) and then they (in conspiracy with an accomplice company) charge us many times the cost of the physical hardware for fixed versions. This MUST be stopped. This is EXACTLY what standards exist for -- so we can buy known-quantity products in a competitive market. -- David Dyer-Bennet, d...@dd-b.net; http://dd-b.net/ Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/ Photos: http://dd-b.net/photography/gallery/ Dragaera: http://dragaera.info _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
