May 18, 2005, 10:24 AM — Imagine you've just received shipment of a new Sun server, pre-installed with Solaris 9 and ready for setup as a web server for your small company. Now imagine that you want to take advantage of Solaris Volume Manager (previously called DiskSuite) to configure your new server to be resistant to disk failures. How do go about configuring the system and how do you set up its disks to provide fault resistent data storage?
One choice is to set up your application partition as a RAID 5 volume. Why might you want to use RAID 5? Well, RAID 5 allows your system to continue running when any one of its component disks or partitions fails. It does this by calculating parity values that allow it to reconstruct the content of any single disk that might fail from the content of the other volume components.
RAID 5 works by computing parity values -- usually generated using a series of simple XOR calculations. On a four-disk RAID 5 volume, four pieces of data would be XORed with each other to generate a fifth value to serve as the parity. If any of the four values or the parity were unavailable due to a component failure, the remaining values could reconstruct the missing one. To understand how this works, let's look at some simple examples written in Perl.
In the following line of Perl, we've XORed four values together (Note that the parentheses are needed to preserve the order of operations) and computed a parity value of 2. Note that ^ represents a bitwise XOR.
> perl -e '$parity = ((( 6 ^ 7 ) ^ 8 ) ^ 11 ); print "$parity\n"' 2
Now let's pretend that one of the values has become unavailable. Pick any of the four values listed in the original expression and replace it with the computed parity.
> perl -e '$missing = ((( 6 ^ 2 ) ^ 8 ) ^ 11 ); print "$missing\n"' 7
Notice how the value of $missing is the same as the value you dropped. RAID 5 devices used a calculation of this variety (though not necessarily with such small pieces of data) to construct parity values and recover data when a volume component fails.
RAID 5 is also relatively conservative with respect to disk space. Whether you have a three-disk system or a five-disk system, for example, only one disk is used to provide this redundancy. The space equivalent of the others serves as usable storage. If you built a RAID 5 volume on a four-disk system, for example, you would have the space of three of your disks for data storage and still be able to lose any one of them without going down (although you would run at reduced efficiency because your missing data would have to be recomputed for every operation).
Since our hypothetical system has a nice pre-install of Solaris 9 on disk 1 and an acceptable partition map, we can even set up our RAID volumes without having to reinstall. Let's see how this can be done.
Partitioning
The partitioning on the original disk seems to be quite acceptable. We have 10G+ root (system disk containing /usr etc.), a separate /var of roughly 6 GB with which to keep log files and such from overrunning important application space and a large /space partition into which we can load our applications. We also have 2 MB of swap.
Assuming we want to keep this partitioning, the first thing we need to do is partition the other two drives. Since components in a RAID array should all be the same size, we probably want to reflect the partitioning of the first drive on the other two -- at least for those partitions we plan to put into a RAID volume. Let's say we decide to partition the second two drives identically with the first.
Once all three disks are partitioned, we will have something that looks like this:
+-------+ +-------+ +-------+ | 0 / | | 0 | | 0 | +-------+ +-------+ +-------+ | 1 swap| | 1 | | 1 | +-------+ +-------+ +-------+ | 5 var | | 5 | | 5 | +-------+ +-------+ +-------+ | 7 | | 7 | | 7 | | space | | | | | +-------+ +-------+ +-------+
Putting /space into a RAID 5 array, therefore, requires only that we claim all three instances of partition 7 and issue the Volume Manager commands to create our RAID volume.
The first Volume Manager command that must be run to create RAID volumes is metadb. This command sets up the database that will maintain information about the RAID volume. This database is sometimes stored in its own partition, but let's say we decide to store it in partition 7. We also decide to store the database in several locations, so we use a command like this:
# metadb -a -f c0t0d0s7 c0t1d0s7 c0t2d0s7
