May 25, 2018

Using RAID in Linux - page 4

The Mysteries of RAID

  • August 1, 2002
  • By Alexander Prohorenko

Linux supports these RAID levels very well. It also supports their combinations, for example, the popular 0+1, which sometimes can be called RAID 10. Also, there is Linear-mode--also known as "paritioned volumes."

We should mention there are a lot of different pieces of literature and documentation about the different RAID levels, so we will browse them only briefly.

  • Linear-mode. Some would argue that this is not true RAID, because there is no "redundancy." In this mode the crash of any drive will destroy the whole array. It is used for the unification of small paritions of disks into one big-sized partition. Sometimes it can increase productivity. For example, when we have a lot of small files, with the ext2 file system trying to place them all over the partition, and of course, the files are all going to different disks. Good choice for news servers and other servers with similar functionality.
  • RAID 0. Again, nothing impressive about reliability. It's a classic "fast" RAID--speed, speed, and more speed, especially with reading/writing big data streams. Of course, this is done at the expense of reliability. It's similar to Linear-mode, but paritions are separated by (so-named) stripes or chunks, which will later be uniformly placed on devices included in array. The best choice for big database files, multimedia, streaming video, and so on. With medium file sizes and medium size chunks, we will see almost no effect.
  • RAID 1. Famous and very popular since the advent of Novell NetWare "mirrors." Two and more partitions of same size--information will be mirrored to each other. This is the first of "reliable" RAID levels, which achieved because of the accuracy of keeping information, which is about 100% * (N - 1), where N is number of disks in array. When one fails (or even a few of them, if N > 2), it's usually possible to work with the others. RAID Linux drivers can balance the top-level load on reading, separating requests to other "mirrors." Sometimes this helps to increase the speed of reading by almost N times. During writing, RAID 1 usually slows down because the same information could be copied on multiple disks at once. It happens that sometimes productivity of writing could be N times worse than with just one disk. RAID 10 in reality is just Linux using two or more RAID 0 arrays combined into a single RAID 1. This is possible because the RAID-array can be treated just like a normal block device or hard disk. Everything you can do with a disk, you see, you can do with an array. So, RAID 10 appears to be a good option when you have a lot of disks and controllers.
  • RAID 4. It's much like RAID 5, but the reliability is worse. As such , it's not popular, since anyone learning twoard the relable end of the RAID spectrum is just as likely to "bump up" to RAID 5. Any additional information you can find in documentation files.
  • RAID 5. Neutral in many cases. It supports the necessary reliability in cases of disk crash and supports rather good speed for reading/writing an is very economical on disk space. Total disk space equals (N - 1) * S, where N is the number of disks and S is the size of one partition. Be sure not to use RAID 5 if your the number of disks is less than three--such a configuration doesn't have any advantages over RAID 1. You'll just get "dummy" productivity and problems during recovery. RAID 5 is created with a number of chunks like in RAID 0, but part of these chunks consist of odd data blocks uniformly on all of the existing partition of the array. In the case some disk crashes, the array continue to work, however with less speed. And information from the broken disk will be restored according of contents of odd blocks. By adding a spare disk, information can be reconstructed and the workability of the array will be fully restored. It goes without saying that there is a way to do all that "on the fly" without turning off system. However, because it is necessary to reorganize all odd data blocks, it's impossible to change the size of an existing RAID 5 array without removing all data on it. It is possible to support more reliability--for example, organizing RAID 5 over three three-disk RAID 5 arrays, which supports workability in the case of a simultaneous crash of any of the three disks. And so on. Of course, relability can always be improved by losing additional disk space.

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