xref: /openbmc/linux/Documentation/admin-guide/device-mapper/striped.rst (revision 0898782247ae533d1f4e47a06bc5d4870931b284)

=========
dm-stripe
=========

Device-Mapper's "striped" target is used to create a striped (i.e. RAID-0)
device across one or more underlying devices. Data is written in "chunks",
with consecutive chunks rotating among the underlying devices. This can
potentially provide improved I/O throughput by utilizing several physical
devices in parallel.

Parameters: <num devs> <chunk size> [<dev path> <offset>]+
    <num devs>:
	Number of underlying devices.
    <chunk size>:
	Size of each chunk of data. Must be at least as
        large as the system's PAGE_SIZE.
    <dev path>:
	Full pathname to the underlying block-device, or a
	"major:minor" device-number.
    <offset>:
	Starting sector within the device.

One or more underlying devices can be specified. The striped device size must
be a multiple of the chunk size multiplied by the number of underlying devices.


Example scripts
===============

::

  #!/usr/bin/perl -w
  # Create a striped device across any number of underlying devices. The device
  # will be called "stripe_dev" and have a chunk-size of 128k.

  my $chunk_size = 128 * 2;
  my $dev_name = "stripe_dev";
  my $num_devs = @ARGV;
  my @devs = @ARGV;
  my ($min_dev_size, $stripe_dev_size, $i);

  if (!$num_devs) {
          die("Specify at least one device\n");
  }

  $min_dev_size = `blockdev --getsz $devs[0]`;
  for ($i = 1; $i < $num_devs; $i++) {
          my $this_size = `blockdev --getsz $devs[$i]`;
          $min_dev_size = ($min_dev_size < $this_size) ?
                          $min_dev_size : $this_size;
  }

  $stripe_dev_size = $min_dev_size * $num_devs;
  $stripe_dev_size -= $stripe_dev_size % ($chunk_size * $num_devs);

  $table = "0 $stripe_dev_size striped $num_devs $chunk_size";
  for ($i = 0; $i < $num_devs; $i++) {
          $table .= " $devs[$i] 0";
  }

  `echo $table | dmsetup create $dev_name`;