Hardware Reference
In-Depth Information
To find out the I/O scheduler option being used and the other available choices (in
your kernel), we can consult the
/sys
pseudo file system:
$ cat /sys/block/mmcblk0/queue/scheduler
noop [deadline] cfq
$
The name
mmcblk0
is the name of the device that your root file system is on. The
output shows in square brackets that the
deadline
I/O scheduler is being used. The other
choices are
noop
and
cfq
. These I/O schedulers are as follows:
Name
Description
Notes
noop
No special ordering of requests
cfq
Completely fair scheduler
Older
deadline
Cyclic scheduler, but requests have deadlines
Newest
The
deadline
I/O scheduler is the newest implementation, designed for greater
efficiency and fairness. The
deadline
scheduler uses a cyclic elevator, except that it
additionally logs a deadline for the request. A cyclic elevator is one where the requests are
ordered according to sector numbers and head movement (forward and backward). The
deadline
scheduler will use the cyclic elevator behavior, but if it looks like the request is
about to expire, it is given immediate priority.
rootwait=
This option is used when the device used for the root file system is a device that is started
asynchronously with other kernel boot functions. This is usually needed for USB and
MMC devices, which may take extra time to initialize. The
rootwait
option forces the
kernel to wait until the root device becomes ready.
Given that the root file system is on the SD card (a MMC device), the Raspbian image
uses the following:
rootwait
nfsroot=
The
nfsroot
option permits you to define a kernel that boots from an NFS mount
(assuming that NFS support is compiled into the kernel). The square brackets show
placement of optional values:
nfsroot=[server−ip:]root−dir[,nfs−options]
