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The current state of udev and devicer-mapper devices makes it difficult to construct a mapping of DM partitions and their underlying DM device. For example, with a /dev directory with the following contents: $ ls -d /dev/dm-* /dev/dm-0 /dev/dm-1 /dev/dm-2 /dev/dm-3 it is not immediately apparent if these are completely separate devices, or partitions and real devices intermixed. In contrast, SCSI devices would appear as so: $ ls -d /dev/sd* /dev/sda /dev/sda1 /dev/sdb /dev/sdb1 Here, one can immediately determine that there are two devices (sda and sdb), each containing a single partition. The lack of a predictable and consistent mapping from DM devices to DM device partitions makes it difficult for user space to process these devices the same way it does SCSI devices. As a result, the ZFS utilities do not partition DM devices, and instead set the "vdev_wholedisk" label to 0 and treat them as partitions. This has the side effect that, even if ZFS has sole ownership of the device, the IO scheduler will not be modified because it is treated as a partition. This change adds an exception for DM devices in vdev_elevator_switch, allowing the elevator to be modified even though the "vdev_wholedisk" property is not set. Signed-off-by: Prakash Surya <surya1@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1149 |
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