Another issue that D-GRAID must address is performance. Fault-isolated data placement improves availability but at the cost of performance. Data accesses to blocks of a large file, or, with directory-based grouping, to files within the same directory, are no longer parallelized. To improve performance, Alexander performs access-driven diffusion, monitoring block accesses to determine which are ``hot'', and then ``diffusing'' those blocks via replication across the disks of the system to enhance parallelism.
Access-driven diffusion can be achieved at both the logical and physical levels of a disk volume. In the logical approach, access to individual files is monitored, and those considered hot are diffused. However, per-file replication fails to capture sequentiality across multiple small files, for example, those in a single directory. Therefore we instead pursue a physical approach, in which Alexander replicates segments of the logical address space across the disks of the volume. Since file systems are good at allocating contiguous logical blocks for a single file, or to files in the same directory, replicating logical segments is likely to identify and exploit most common access patterns.
To implement access-driven diffusion, Alexander divides the logical address space into multiple segments, and during normal operation, gathers various statistics about the utilization and access patterns to each segment. A background thread selects logical segments that are likely to benefit most from access-driven diffusion and diffuses a copy across the drives of the system. Subsequent reads and writes first go to these replicas, with background updates sent to the original blocks. The imap entry for the block indicates which copy is up to date.
The amount of disk space to allocate to performance-oriented replicas presents an important policy decision. The initial policy that Alexander implements is to reserve a certain minimum amount of space (specified by the system administrator) for these replicas, and then opportunistically use the free space available in the array for additional replication. This approach is similar to that used by AutoRAID for mirrored data [47], except that AutoRAID cannot identify data that is considered ``dead'' by the file system once written; in contrast, D-GRAID can use semantic knowledge to identify which blocks are free.