path checker open() flags, devmapper ioctl()

[Christophe Varoqui <christophe.varoqui@free.fr>]

Notice to reader : real questions are hidden in this obfuscated
message ! I'll mark them with ** to augment the chances for them being
answered :)

Path checker maintainers,

** would you object to my changing the checkers open() flags from O_RDWR
to O_RDONLY ? ** Directio and readsector0 would not object, and all
other checkers use the file descriptor to submit SG_IO ioctls which do
not require Write capability.
Re-reading the SG documentation at http://sg.torque.net/sg/sg_io.html, I
understand it should be ok. In real life, scsi_id does exactly that for
example.

The particular issue I'm facing is that on a RHEL5 box presented with
EMC Symmetrix R2 Logical Units (write-protected LU receiving SRDF
updates from R1 LU), the checker bails out on open(/dev/sd?, O_RDWR|
O_NONBLOCK) returning EROFS. opening with O_RDONLY grants us a usable
fd, which give the checker a chance to report the actual path state.

I'd like to fix the checker inconsistency, but we'll hit another wall
short after that : because the device mapper refuses to honor the
R2-multipath-tables loading without the read-only devmap option.

This behavior is new to RHEL5. R2 LU could perfectly be multipathed in
RHEL4.
** Does someone have insight about this new kernel behaviour, and about
the multipath layer desired behaviour in this context ? ** I personnaly
would rather not care for promoting a devmap from RO to RW upon R1-R2
split, and the even more tricky demoting from RW to RO (what about io
flush ?) upon un-spliting.

Regards,
cvaroqui