RAID-10: Two great RAIDs that go great together
By Rick Cook
About this tip: The differences between RAID1+0 and RAID0+1, and when to use each.
Combining RAID levels 0 and 1 confers benefits in recovery and reliability, albeit at the cost of halving the storage capacity of the RAID array.
RAID-0 breaks the data into segments and stripes those segments across the available disks with no redundancy or parity checking. RAID-1 completely mirrors the contents of one disk onto one or more additional disks, providing complete redundancy of the data, and cutting the storage capacity in half because all the data is duplicated. Combining the two is often referred to as RAID-10, and it comes in two different forms.
In RAID0+1, the data is organized as stripes across multiple disks and then the striped disk sets are mirrored. In RAID1+0, the data is mirrored and the mirrors are striped. Both versions will survive a single disk failure without the loss of data. However, RAID1+0 will recover faster because only the failed drive has to be rebuilt rather than the entire stripe set.
If a second drive fails before the failed drive is rebuilt or replaced, things are quite different. Assuming a single mirror copy, a RAID0+1 array is more likely to lose data than a RAID1+0 array in the event of two drive failures. The reason is that a single failure in stripe set knocks out the entire stripe set, leaving you with no redundancy. There is a 50% chance that the second stripe set will be affected by the second failure. If that happens, you have lost data.
With RAID1+0, the disk is broken into what are in effect multiple mirrors, each mirror (in this case with two drives) holding part of a stripe set. For the second failure to take down the array, the mirror of the failed disk would also have to fail.
Because it is more reliable, RAID1+0 is the preferred method of organizing a RAID-10 array. However, note that data loss if two drives fail is only significantly less likely with RAID1+0, but not impossible. For additional security, you can use a RAID level that supports parity as part of your storage scheme.
Most RAID vendors support the RAID1+0 configuration and many also support RAID0+1. RAID arrays that do not support 1+0 are quite rare.
For more information:
Checklist: Seven RAID configuration essentials
About the author: Rick Cook has been writing about mass storage since the days when the term
meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For
the last 20 years, he has been a freelance writer specializing in storage and other computer
11 Feb 2005
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