Introduction to High-Availability Clusters On Linux And Other Systems

Most SCSI disks support a 'SCSI reservation' command.
If a machine sent such a command to the disk, it works as a "lock" against I/O coming from other machines.
If a machine sends an I/O (read/write) request to a SCSI Disk reserved by another machine, it will get an error, with a code of "reservation conflict".
If the reserving machine crashes, another machine may send a "break reservation" (or a "reset target") command, to brutally break the lock.
Of-course, now the second machine needs to send a SCSI reservation, before sending any I/O to the device.
This might cause file-system consistency problems (e.g. if the two machines play too much with 'break reservation').

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