From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <2c2226061eedb01d1a6fd20d2e42a947@collyer.net> To: 9fans@cse.psu.edu Subject: Re: [9fans] now for something completely devfs :) From: Geoff Collyer In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Date: Wed, 9 Jul 2003 15:52:51 -0700 Topicbox-Message-UUID: f1a79b60-eacb-11e9-9e20-41e7f4b1d025 I think the paragraph on the wiki would be clearer like this: If you already have a fossil and want to add another disk to mirror it, you can setup the mirror as said before, with the original fossil device (possibly a partition) being mirrored by the new disk, start your fossil and while you are using it, use dd(1) to copy the fossil device to the mirror device or partition. Since writes go to all devices in the mirror but reads are performed just by the first device (as long as it has no read errors), the copy can be made this way with relative safety. Races are still possible, since dd might read a block from the fossil device, fossil might change it and write it to both devices (original fossil and mirror) and then dd might write the now-stale block to the mirror, so it's safer to make the copy when fossil (and venti, if you mirror it) are idle. This might require booting your file server off another standalone Plan 9 file & cpu server. How do you know if a disk in a mirror has died? The kernel should print I/O-error messages on the console. devfs should cope until you can shut the system down and replace the dying disk. With modern disks, I/O errors are generally a sign of degradation: the disk has forwarded as many bad sectors as it can and now the reserved bad sector areas are full.