Linear (Append) Mode
Linear mode requires a minimum of two disks, but does not require that member disks be the same size or type. Since the system writes to each disk until it is full, the speed and size of individual disks is largely irrelevant, in terms of aggregate RAID performance.
The following is a very simple linear array using /etc/raidtab:
# A linear array with two member disks raiddev /dev/md0 raid-level linear chunk-size 64 persistent-superblock 1 nr-raid-disks 2 device /dev/sdb1 raid-disk 0 device /dev/sdc1 raid-disk 1
raiddev begins the definition of a new array,
/dev/md0 in this case. All entries that follow
apply to the previously defined array, until another
raiddev entry is parsed.
raid-level, as the name implies, sets the array
type for the current array.
As I mentioned earlier, chunk-size would
normally define the number of kilobytes to write to each member disk
for arrays that support disk striping. However, when working with
linear mode arrays, the chunk-size defines the
rounding factor. Each component disk is sized so
that it is a multiple of the rounding factor. Because of the way the
RAID superblock is placed on each array member, rounding factors of
less than 64 KB are effectively equal to 64 KB.
The chunk-size, regardless of what type of
array is used, must be defined as any power of two. In fact, for the
rest of this chapter, we’ll create arrays with a 64 KB
chunk-size. You might notice some errors in the system boot messages and your log files, which warn ...
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