I know that mounting the same disk with an ext4 filesystem from two different servers (it's an iSCSI vloume) will likely corrupt data on the disk. My question is will it make any difference if one of the servers mounts the disk read-only while the other mounts it read-write?I know OCFS2 or the likes could be used for this and that I could export the disk with NFS to be accesible to the other server, but I would like to know if the setup I propose will work.
Can the same ext4 disk be mounted from two hosts, one readonly?
linux;mount;ext4
No.  It won't give consistent results on the read-only client, because of caching.  It's definitely not designed for it.  You could expect to see IO errors returned to applications.  There's probably still some number of oversights in the code, that could cause a kernel crash or corrupt memory used by any process.But most importantly, ext4 replays the journal even on readonly mounts.  So a readonly mount will still write to the underlying block device.  It would be unsafe even if both the mounts were readonly :).