Novell LINUX ENTERPRISE SERVER 11 - STORAGE ADMINISTRATION GUIDE 2-23-2010, Administration Manual, Page: 95 / 140

Managing Software RAIDs 6 and 10 with mdadm 95
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Near Layout
With the near layout, copies of a block of data are striped near each other on different component
devices. That is, multiple copies of one data block are at similar offsets in different devices. Near is
the default layout for RAID10. For example, if you use an odd number of component devices and
two copies of data, some copies are perhaps one chunk further into the device.
The near layout for the
mdadm
RAID10 yields read and write performance similar to RAID 0 over
half the number of drives.
Near layout with an even number of disks and two replicas:
sda1 sdb1 sdc1 sde1
0 0 1 1
2 2 3 3
4 4 5 5
6 6 7 7
8 8 9 9
Near layout with an odd number of disks and two replicas:
sda1 sdb1 sdc1 sde1 sdf1
0 0 1 1 2
2 3 3 4 4
5 5 6 6 7
7 8 8 9 9
10 10 11 11 12
Far Layout
The far layout stripes data over the early part of all drives, then stripes a second copy of the data
over the later part of all drives, making sure that all copies of a block are on different drives. The
second set of values starts halfway through the component drives.
With a far layout, the read performance of the
mdadm
RAID10 is similar to a RAID 0 over the full
number of drives, but write performance is substantially slower than a RAID 0 because there is more
seeking of the drive heads. It is best used for read-intensive operations such as for read-only file
servers.
The speed of the raid10 for writing is similar to other mirrored RAID types, like raid1 and raid10
using near layout, as the elevator of the file system schedules the writes in a more optimal way than
raw writing. Using raid10 in the far layout well-suited for mirrored writing applications.
Far layout with an even number of disks and two replicas:
sda1 sdb1 sdc1 sde1
0 1 2 3
3 5 6 7
. . .
3 1 2 3
7 4 5 6
Far layout with an odd number of disks and two replicas: