Similar to any other SCSI-based RAID system, Alexander presents host systems with a linear logical block address space. Internally, Alexander must place blocks so as to facilitate graceful degradation. Thus, to control placement, Alexander introduces a transparent level of indirection between the logical array used by the file system and physical placement onto the disks via the indirection map (imap); similar structures have been used by others [12,46,47]. Unlike most of these other systems, this imap only maps every live logical file system block to its replica list, i.e., all its physical locations. All unmapped blocks are considered free and are candidates for use by D-GRAID.