Raid10
RAID10 provides a combination of RAID1 and RAID0, and is sometimes known as RAID1+0. Every datablock is duplicated some number of times, and the resulting collection of datablocks are distributed over multiple drives.
When configuring a RAID10 array, it is necessary to specify the number of replicas of each data block that are required (this will normally be 2) and whether the replicas should be 'near', 'offset' or 'far'. (Note that the 'offset' layout is only available from 2.6.18).
When 'near' replicas are chosen, the multiple copies of a given chunk are laid out consecutively across the stripes of the array, so the two copies of a datablock will likely be at the same offset on two adjacent devices.
When 'far' replicas are chosen, the multiple copies of a given chunk are laid out quite distant from each other. The first copy of all data blocks will be striped across the early part of all drives in RAID0 fashion, and then the next copy of all blocks will be striped across a later section of all drives, always ensuring that all copies of any given block are on different drives.
The 'far' arrangement can give sequential read performance equal to that of a RAID0 array, but at the cost of reduced write performance.
When 'offset' replicas are chosen, the multiple copies of a given chunk are laid out on consecutive drives and at consecutive offsets. Effectively each stripe is duplicated and the copies are offset by one device. This should give similar read characteristics to 'far' if a suitably large chunk size is used, but without as much seeking for writes.
It should be noted that the number of devices in a RAID10 array need not be a multiple of the number of replica of each data block; however, there must be at least as many devices as replicas.
If, for example, an array is created with 5 devices and 2 replicas, then space equivalent to 2.5 of the devices will be available, and every block will be stored on two different devices.
Finally, it is possible to have an array with both 'near' and 'far' copies. If an array is configured with 2 near copies and 2 far copies, then there will be a total of 4 copies of each block, each on a different drive. This is an artifact of the implementation and is unlikely to be of real value.
