/proc/mdstat
/proc/mdstat provides a way to examine the state of the md driver, including information about active software arrays. When no arrays are running, displaying /proc/mdstat simply shows which RAID levels the kernel supports.
# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5]
read_ahead not set
unused devices: <none>In this example, there are no active arrays, but this kernel
supports linear, RAID-0, RAID-1, and RAID-4/5. The
read_ahead value is not currently set, because no
arrays are active. read_ahead defines the number of
sectors the kernel should cache during sequential reads. Finally,
unused devices is also empty, because there are no
devices in the md subsystem that are not
currently in use by an array.
If arrays are defined, /proc/mdstat provides detailed information about them, as shown in the following code:
# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5]
read_ahead 1024 sectors
md2 : active raid1 sde1[1] sdd1[0]
17920384 blocks [2/2] [UU]
md1 : active raid0 sdc1[1] sdb1[0]
35840768 blocks 64k chunksFirst, note that read_ahead is now set to
1024 sectors. That means that during sequential reads, the kernel will
attempt to cache a maximum of 1024 sectors worth of data, or about 512
K (1024 sectors, with approximately 512 bytes per sector). The default
value of 1024 sectors is a hard limit set by the
md driver. Next, each array is listed, with the
most recently activated array first. In this case,
/dev/md2, a RAID-1, is listed first ...