RAID 1 is mirroring and is generally the fastest and most reliable way to protect your online data. The most common...
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concern with RAID 1 is that it requires 100% more disk than if you don't mirror your disks. To create a mirror of 20 GB of space, you need 40 GB of disk. The performance impact of RAID 1 is that writes will generally be negligibly slower (because you are constrained by the system's speed in writing to the slower disk), and reads will be negligibly faster, because the read can be performed against the faster disk.
There are, in fact, two variations of RAID 1, RAID 0+1 and RAID 1+0. Without going into too much detail here (this is going to wind up being a long answer), RAID 0+1 involves creating a RAID 0 disk stripe first, and then mirroring that stripe's contents to an identical stripe. In RAID 1+0, the disks being allocated are mirrored in pairs first and then those pairs are striped. Looks and sounds similar but there are some serious technical differences. The bottom line is that 1+0 is better than 0+1.
RAID 5 is a form of parity RAID, where data is striped across all the disks in the RAID stripe (the collection of disks that make up RAID 5), plus one extra disk. This extra disk contains calculated values that are generated by applying Boolean arithmetic to all of the data on the other disks. (What I have just described is actually RAID 4. RAID 5 works the same way, except that it takes that parity data and stripes it across all of the disks in the RAID 5 stripe to improve performance.) Any lost disk in a RAID 5 stripe can be recovered through the use of the parity information.
RAID 5 writes slower than RAID 1 for several reasons, including all of the arithmetic that must be done every time a write is generated. What's more, in order to do the calculations, in some cases, data must be read from all the disks so that the calculations can be made. RAID 1 does not require any math or extra reading. The rule of thumb is that if your disks are going to do less than about 15-20% writing, then RAID 5 may be OK. Any more than that, and you should probably not do RAID 5.
When a disk is lost, and isn't that why you are looking at RAID in the first place, replacing a disk in RAID 1+0 requires copying all the data from the surviving copy of the failed disk onto the replacement disk. In RAID 5, all the data on all the disks must be read and the appropriate calculations made, before the data can be written to the replacement disk.
If you lose two disks at the same time in RAID 5, all the data is lost, and must be recovered from backup tapes. If you lose two disks in RAID 1+0, unless they happen to be both sides of the same mirror, the system will be able to recover your data without having to resort to backup tapes.
Please note that if your disks are in a hardware array, then the performance comparisons are likely invalid, since those arrays generally cache their data, hiding those performance issues.
Hope this helps...
Evan L. Marcus
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