2 - Raspberry PI Community Projects

RAID-5 RAID-5 addresses the asymmetry issue of RAID-4: parity blocks are spread over all of
the N+1 disks, with no single disk having a particular role.
Read and write performance are identical to RAID-4. Here again, the system stays functional
with up to one failed disk (of the N+1), but no more.
RAID-6 RAID-6 can be considered an extension of RAID-5, where each series of N blocks involves
two redundancy blocks, and each such series of N+2 blocks is spread over N+2 disks.
This RAID level is slightly more expensive than the previous two, but it brings some extra
safety since up to two drives (of the N+2) can fail without compromising data availability.
The counterpart is that write operations now involve writing one data block and two
redundancy blocks, which makes them even slower.
RAID-1+0 This isn't strictly speaking, a RAID level, but a stacking of two RAID groupings. Starting
from 2×N disks, one first sets them up by pairs into N RAID-1 volumes; these N volumes
are then aggregated into one, either by “linear RAID” or (increasingly) by LVM. This last
case goes farther than pure RAID, but there's no problem with that.
RAID-1+0 can survive multiple disk failures: up to N in the 2×N array described above,
provided that at least one disk keeps working in each of the RAID-1 pairs.
GOING FURTHER
RAID-10
RAID-10 is generally considered a synonym of RAID-1+0, but a Linux
specificity makes it actually a generalization. This setup allows a system
where each block is stored on two different disks, even with an
odd number of disks, the copies being spread out along a configurable
model.
Performances will vary depending on the chosen repartition model and
redundancy level, and of the workload of the logical volume.
Obviously, the RAID level will be chosen according to the constraints and requirements of each
application. Note that a single computer can have several distinct RAID arrays with different
configurations.
12.1.1.2. Seing up RAID
Setting up RAID volumes requires the mdadm package; it provides the mdadm command, which
allows creating and manipulating RAID arrays, as well as scripts and tools integrating it to the
rest of the system, including the monitoring system.
Our example will be a server with a number of disks, some of which are already used, the rest
being available to setup RAID. We initially have the following disks and partitions:
• the sda disk, 4 GB, is entirely available;
• the sde disk, 4 GB, is also entirely available;
• on the sdg disk, only partition sdg2 (about 4 GB) is available;
Chapter 12 — Advanced Administration
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