dm-queue-length

dm-queue-length 是一个用于设备映射器目标（device-mapper targets）的路径选择器模块，它选择飞行中 I/O 数量最少的路径。此路径选择器的名称是“queue-length”。

每个路径的表参数：[<repeat_count>]

<repeat_count>: The number of I/Os to dispatch using the selected
                path before switching to the next path.
                If not given, internal default is used. To check
                the default value, see the activated table.

每个路径的状态：<status> <fail-count> <in-flight>

<status>: 'A' if the path is active, 'F' if the path is failed.
<fail-count>: The number of path failures.
<in-flight>: The number of in-flight I/Os on the path.

算法

dm-queue-length 在 I/O 分派/完成时分别增加/减少“in-flight”计数。dm-queue-length 选择“in-flight”计数最小的路径。

示例

当使用两条路径（sda 和 sdb）且 repeat_count == 128 时。

# echo "0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128" \
  dmsetup create test
#
# dmsetup table
test: 0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128
#
# dmsetup status
test: 0 10 multipath 2 0 0 0 1 1 E 0 2 1 8:0 A 0 0 8:16 A 0 0