11. Performance - Write

Figure 19 looks at write performance across all the systems evaluated to date. The light blue line is Windows XP Pro that we decided to test when FreeNAS would not boot on the test system. The key qualitative conclusions of this chart are:

XP Pro has a bigger write cache than the Linuxes. I would guess bigger than 32 MB because that is the one database file size at which XP Pro dominated the Linux operating systems. The variation in write performance across competitors is just as big as the variation in read performance. Look at the scale of the graph in Figure 19 (0 to 120,000) and compare it to Figure 16 (4,000 to 49,000). The astronomical score of XP Pro on 32 MB file sizes has the effect of visually distorting the performance of the other operating systems, making them look slower than they are. To make it easier to see the relationships in the data, Figure 20 breaks out just the Ubuntu results

Figure 19: All System Write Performance (click image to enlarge)

In Figure 20, by dropping XP Pro we can more easily compare the three implementations of Ubuntu. The red line is no-RAID on Sempron 2200+, the green/blue line is no-RAID on the Duron 1.6 GHz and the blue line is hardware RAID in the Sempron 2200+. By adding linear trend lines, the relationships on Figure 24 show how each of these implementations behaves as the database size doubles.

Figure 20: Ubuntu Write Performance (click image to enlarge)

One practical implication of this analysis is that if you collected data on file sizes of the files you will use on your NAS, you can select a summary statistic, say the median file size, and then you can use the trend lines on this chart to tell you which implementation of Ubuntu would give you the best speed. 

For example, if you knew that the median file size is 32 MB, you could use either XP Pro with no RAID, or Ubuntu with a hardware RAID card to get very good performance. If your median file size is larger than a gigabyte, then you would be smart to go with the red line in Figure 24 (no-RAID on Ubuntu with as fast a processor as you can get). You tell me how big the most common file is, I'm beginning to be able to tell you which RAID implementation will provide the best speed.

Figure 21 shows the strangest relationship uncovered in this analysis. As files get bigger, the faster Sempron's performance crosses the line of the slower Duron's performance. This means for very big files, the Duron's write performance is better! Go figure. Ifyou know why this is the case, please post in the Forumz and share your experiences with the rest of the RAID geek community.

Figure 21: Ubuntu Sempron vs. Duron Write Performance (click image to enlarge)