With Linux, the open source OS [Operating System] becoming more popular every day with various flavors [distributions], Bright Enterprise Labs decided that it would be a good idea to point out one excellent feature that Linux offers for free - in all versions on the market. Software based RAID (Redundant Array of Independent Disks). No fancy add on with driver support needed, all you need is two or more storage connectors on your mainboard. Not only SATA is what you can use, even a mix of IDE and SATA drives is possible, although we don't recommend it due to the very difference in bandwidth that both interfaces offer. Also, two IDE drives on the same port will suffer performance when they have to share the available bandwidth of that link.
Most of our readers will know the difference between RAID0, RAID1, RAID (1+0) aka. RAID10, RAID4, RAID5 and RAID6, but we'd like to briefly introduce them to you. The definition 'n' means the total capacity of a disk drive.
RAID0 a.k.a. Drive Span or Stripping (Total capacity = n * number of drives)
Requires two drives and will extend one drive to the next one. This will give you a maximum of capacity, where you reach 100% of your total capacity in one drive, but if one drive fails, all your data will be lost. Also, the reason why it is very popular is the fact that it speeds up your performance quite a bit because it can read / write to two disks at a time, thus doubling the performance of a single drive.
RAID1 a.k.a. Drive Mirror or Mirroring (Total capacity = total capacity / 2)
Requires two drives. All your data on drive #1 will be written instantly to drive #2. This will give you only 50% of your total capacity, but a 50:50 chance if a single drive fails; the remaining one will simply continue to handle your I/O requests.
RAID10 a.k.a. 0+1 (Spanned Mirror) (Total capacity = total capacity / 2)
Requires at least four drives [any number of drives that can be divided by two]. It adds the functionality of both RAID0 and RAID1. Two or more drives will be accessed as a span, but at the same time, requiring the same number of drives for a second span where all the data is also mirrored to. Also, you get the speedup that RAID0 offers. This will also give you 50% of your total capacity, but enables you to use more than 2 disks in one array.
RAID4 (Total capacity = n * number of drives - n)
Requires at least three disk drives. The major difference to other RAID levels is the fact that there is one dedicated drive containing parity data. Thus on every write among the data disks, there's a write on the parity disk, causing a lot more write accesses to this disk and limiting the maximum RAID performance to the maximum performance of that parity disk. Also, due to the higher number of accesses, this drive is most likely to fail.
RAID5 (Total capacity = n * number of drives - n)
Requires at least three drives, but has no limits if you want to add more drives. The capacity of one drive is removed from the total capacity. The extra capacity is needed for the extra parity data that is being stored among all drives. If one drive fails, the RAID will become 'degraded' but will still let you read data off this drive. If another drive fails, all your data will be lost.
RAID6 (Total capacity = n * number of drives - 2 * n)
Requires four or more drives and is similar to RAID5, simply writing the parity data to two different drives and thus is able to handle a drive failure and still allows you to write - if a second drive fails, you'll only be able to read from this drive. Think of it as RAID5+1.
There are many more RAID levels available. Sometimes vendor specific (in other words, Netapp has modified some RAID levels within its own product lineup and Intel uses RAID1 & RAID0 on its drive matrix option found in today's chipsets) or simply not found frequently on the market, so we'll stick to the most used RAID levels.
Linux RAID modules (md) also offers you to add one or more 'spare' drive to any RAID other than RAID0, where you do not have any safety and a spare drive doesn't make sense at all. This spare drive will jump in and replace a failed disk immediately, so your system doesn't run with a problem for too long.
A downside of Linux RAID is that while it is not impossible to make it work, it's pretty tricky to boot from a RAID drive setup, so you should have a single drive to boot from, after that, the rest of the system can run off a RAID. But we're sure that this will be solved in the future, where you'll have the option of setting up a RAID at the time of Linux installation.
CONTINUED: Creating a RAID, Adding a drive, MDAM Growing RAID, Conclusion.
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