Re: block/dk devices and lseek()

[mlelstv%serpens.de@localhost (Michael van Elst)]

rvp%SDF.ORG@localhost (RVP) writes:

>Are block and dk* (wedge) devices supposed to support lseek()?

When a disk device is opened, the DIOCGPARTINFO ioctl is used to
query the size of the disk and cache it in the vnode.

The dk driver doesn't represent a disk and doesn't support DIOCGPARTINFO.
It does support DIOCGWEDGEINFO, DIOCGDISKINFO and DIOCGSECTORSIZE +
DIOCGMEDIASIZE that would reveal the information.

The same is true for the dm driver.


But that's only half of the story.

>$ sudo stat -f '%N: %z' /dev/rsd0
>/dev/rsd0: 7849115648			# works
>$ sudo stat -f '%N: %z' /dev/sd0
>/dev/sd0: 0				# is this correct?

The stat command uses the lstat() system call, it does not
open a device, but only returns the cached size information
in the vnode.

A device that is open or where the vnode is still cached after an open
will reveal the size, otherwise you get a zero result. E.g.

# stat -f "%N: %z" /dev/raid0b
/dev/raid0b: 968884224
# stat -f "%N: %z" /dev/rraid0b
/dev/rraid0b: 0
# dd if=/dev/rraid0b of=/dev/null count=10
10+0 records in
10+0 records out
5120 bytes transferred in 0.014 secs (365714 bytes/sec)
# stat -f "%N: %z" /dev/rraid0b
/dev/rraid0b: 968884224

The root block device can also be special as it usually isn't opened
(with spec_open) and cannot be opened later as it is always busy, so
the size is never cached.