On Wed, Feb 17, 2010 at 02:38:04PM -0500, Miles Nordin wrote: > copies=2 has proven to be mostly useless in practice.
I disagree. Perhaps my cases fit under the weasel-word "mostly", but single-disk laptops are a pretty common use-case. > If there were a real-world device that tended to randomly flip bits, > or randomly replace swaths of LBA's with zeroes As an aside, there can be non-device causes of this, especially when sharing disks with other operating systems, booting livecd's and etc. > * drives do not immediately turn red and start brrk-brrking when they > go bad. In the real world, they develop latent sector errors, > which you will not discover and mark the drive bad until you scrub > or coincidentally happen to read the file that accumulated the > error. Yes, exactly - at this point, with copies=1, you get a signal that your drive is about to go bad, and that data has been lost. With copies=2, you get a signal that your drive is about to go bad, but less disruption and data loss to go with it. Note that pool metadata is inherently using ditto blocks for precisely this reason. I dunno about BER spec, but I have seen sectors go unreadable many times. Sometimes, as you note, in combination with other problems or further deterioriation, sometimes not. Regardless of what you do in response, and how soon you replace the drive, copies >1 can cover that interval. I agree fully, copies=2 is not a substitute for backup replication of whatever flavour you prefer. It is a useful supplement. Misunderstanding this is dangerous. -- Dan.
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