HP Xw8400 Software RAID in Linux Workstations - Page 15

Tip: You can use mdadm as a daemon to monitor a RAID array, by running: mdadm --monitor --mail=root@localhost --delay=1800 /dev/md0 This will poll the array in intervals of 1800 seconds, and critical events and failures will be emailed to the system administrator. There are many other monitoring systems available for Linux software RAID as well. Multiple Disk Failure In the case of a temporary failure of multiple disks, such as a disk controller failure or cable coming loose that affects multiple disks, the RAID superblocks will afterwards be out of sync and the RAID array can no longer be initialized. Using mdadm, you can run: mdadm --assemble ñforce to try and recreate the array. If that method doesn't work, you can run: mkraid --force to rewrite the RAID superblocks. In order for this to work, you will need to have a completely up-todate /etc/raidtab file, otherwise, if the ordering of the disks is different than expected, data on all disks could be lost. Additional Configuration Information The Persistent Superblock Previously, the raidtools, which are included with most major Linux distributions, would read your /etc/raidtab file, and then initialize the filesystem. This required that the filesystem on which /etc/raidtab resided was mounted, which was unfortunate if you wanted to boot on a RAID. The persistent superblock solves these problems. When an array is initialized with the persistentsuperblock option in the /etc/raidtab file, a special superblock is written to the beginning of all disks participating in the array. This allows the kernel to read the configuration of RAID devices directly from the disks involved, instead of reading from the /etc/raidtab configuration file that might not be available at all times. You should still maintain a consistent /etc/raidtab file, since you may need this file for later reconstruction of the array. The persistent superblock is mandatory if you want auto-detection of your RAID devices upon system boot. Chunk Sizes The chunk size is defined as the smallest amount of data that can be written to a device. You can never write completely in parallel to a set of disks. If you had two disks and wanted to write a byte, you would have to write four bits on each disk, with every second bit going to disk 0 and the others to disk 1. Hardware doesn't support that, so chunk size is used instead. A write of 16kB with a chunk size of 4kB will cause the first and the third 4kB chunks to be written to the first disk, and the second and fourth chunks to be written to the second disk, in the RAID-0 case with two disks. Thus, for large writes, you may see lower overhead by having fairly large chunks, whereas arrays that are primarily holding small files may benefit more from a smaller chunk size. 15