Compression may also be used for reducing the size of images on disk. The most obvious effect is the decrease in storage space. A fully-mapped mouse brain will take approximately 24 terabits of data, while a rat brain will take approximately 80 terabits of data. A 70% compression rate would reduce the required storage space to about 7.2 terabits and 24 terabits respectively.
Compressed disk images will also increase the disk flying throughput. With an image compression of 70%, disk drives can supply over 3 times more frames/second. Using the disk bandwidths obtained from [1], we can determine the disks required for meeting the requirements for flying throughput. Recent studies at the Army High Performance Computing Research Center at the University of Minnesota (AHPCRC) have measured the performance of single disk arrays. When going through a file system, normal I/O runs at about 6 Mbits/second. Direct I/O which bypasses the file system buffer cache and beams data directly to the user's application buffer, runs at 14.5 Mbits/second on a non-fragmented file. Direct I/O and striping across multiple disk arrays can achieve up to N times 14.5 Mbits/second where N is the number of arrays striped over. The AHPCRC currently stripes over 4 or 8 arrays, producing transfer rates around 40 Mbits/second for 4 arrays. In the 3-5 year time frame they expect to see the single array speed go to 100 Mbits/second and possibly 1 Gbits/second for large file transfers. If the physical design matches users' needs effectively, the database retrieval rate may approach the maximum disk rate, but applications cannot exceed this retrieval speed.
Figure 5 shows the disk drive throughputs compared to the increase in bandwidth as simultaneous users increase. At least 8 direct I/O disk arrays are required to meet the flying requirements for one user. However, with compression, one user's flying need can be met with 4 direct I/O disk arrays.
Figure: Bandwidth versus Simultaneous Users. The
Mbits/second axis is a log 10 scale. The increasing curves are
bandwidths without compression and with compression. The horizontal
lines are all disk throughput rates.
Figure 6 shows the disk rates compared to the decrease in bandwidth as servers increase. A 1 disk array will not satisfy even a single user's flying requirements. An 8 disk array will satisfy user flying requirements only with compression and 40 servers. Without compression, an 8 disk array will need 140 servers to satisfy user flying requirements. A 4 disk array will need compression and 100 or more servers to meet flying requirements.
Figure: Bandwidth versus Number of Servers. The
decreasing curves are bandwidths with compression and without
compression. The horizontal lines are disk rates.
A drawback of compression is that it will decrease CPU frame throughput. Compressed images on disk must be uncompressed before the CPU can perform image computation to generate the 2D frame. And compressing before sending takes additional CPU time.
