Next, we enable Volume Write Back Cache on the ICH8R. then ran the same test again.
(click the picture for a clearer graph)
| J-Micron | Intel (Volume Write Back Cache Enabled) | |||
| Deskstar 160 GB - Array | Caviar 160 GB - Array | Deskstar 160 GB - Array | Caviar 160 GB - Array | |
| Random Access (ms) | 13.5 | 13.5 | 13.3 | 13.1 |
| CPU Utilization (%) | 4 | 5 | 5 | 6 |
| Average Read (MB/s) | 112.0 | 111.8 | 125 | 128.5 |
| Average Write (MB/s) | 88.0 | 86.5 | 64.7 | 60.6 |
The fluctuations are less frequent when Volume Write Back Cache is enabled on the ICH8R. Like the number on the table suggest, the Caviar has upper hand here, offering higher average reads on the first parts of the benchmark. But further down, the two arrays behave similarly - except for that one drastic drop on the Caviar. If you look at average write results, the Deskstar has a slight edge over the Caviar. The reason becomes clear when we look at the graph - the Caivar array dropped to below 40 MB/s at some points. As far as writes are concern, its the Hitachi Deskstars that seems to be stronger. Look closely and you'll see the Deskstar array can mostly maintain writes above 40 MB/s (only dipping three times on the last parts of the benchmark) while the Caviars repeatedly drop below 40 MB/s - about 9 times pretty evenly through the latter half of the benchmark..
Taken from the HTML help file from the Intel's Matrix Storage Console:
"Multiple I/O requests from the host are grouped into fewer requests and written from the cache to the volumes at defined intervals. The Write Back Cache can be enabled or disabled at any time without the need to reboot the system."
Now, if you compare the results to the previous ones, you'll see one glaring trend - according to HD Tach, average write results on both arrays are lower with Volume Write Back Cache enabled. We saw a drop from 103 MB/s to 64.7 MB/s on the Deskstar array and from 97.5 MB/s to 60.6 MB/s on the Caviar's. We did get a boost in average read though.This is interesting, after all, enabling Volume Write Back Cache is supposed to boost performance, not lower it.
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