Fix hot spares
The issue with hot spares in ZoL is because it opens all leaf
vdevs exclusively (O_EXCL). On Linux, exclusive opens cause
subsequent exclusive opens to fail with EBUSY.
This could be resolved by not opening any of the devices
exclusively, which is what Illumos does, but the additional
protection offered by exclusive opens is desirable. It cleanly
prevents you from accidentally adding an in-use non-ZFS device
to your pool.
To fix this we very slightly relaxed the usage of O_EXCL in
the following ways.
- Functions which open the device but only read had the O_EXCL flag removed and were updated to use O_RDONLY.
- A common holder was added to the vdev disk code. This allow the ZFS code to internally open the device multiple times but non-ZFS callers may not.
- An exception was added to make_disks() for hot spare when creating partition tables. For hot spare devices which are already opened exclusively we skip creating the partition table because this must already have been done when the disk was originally added as a hot spare.
Additional minor changes include fixing check_in_use() to use
a partition instead of a slice suffix. And is_spare() was moved
above make_disks() to avoid adding a forward reference.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #250