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Testing - iozone

So if file copying with Vista can show > 100 MB/s speeds, why can't iozone? Well first, we need to realize that my Vista file copy was done between two machines that each had 2 GB of RAM and the largest file in my test file copy folder is only 1 GB. So that means that memory cache effects on both machines is in play.

But the major difference is that my iozone testing is usually done with a 64 KB record (block) size, to reflect the 64 KB (actually 60 KB) network transfer limit of Windows XP. iozone is perfectly capable of testing with larger record sizes, so let's see what happens when we do!

Figure 5 shows the iozone write throughput vs. file size plot extended to 4 GB file size (to make sure that cache is busted on both the iozone and Test NAS) and for record sizes of 64 KB, 128 KB, 256 KB, 512 KB and 1024 KB.

iozone throughput vs. file vs. record size - write
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Figure 5: iozone throughput vs. file vs. record size - write

Remember that all the results that exceed maximum LAN throughput (in this case 113 MB/s, which was actually measured between two machines with PCIe gigabit NICs) are cached results. So looking at the plots up to 512 MB file size, we see that the best cached performance was when using a 256 KB block size, followed by 128 KB, 64 KB, 512 KB and 1024 KB. This surprised me, given that the Vista SP1 file copy used the 1024 KB record size.

Figure 6 expands the Y scale and zooms in on the non-cached area of the plot, so that we can more clearly see what happens there. 256 KB still appears to be the winner when cache runs out. But the larger record sizes of 512 and 1024 KB seem to do better than they did while cache was in effect.

iozone throughput vs. file vs. record size - write
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Figure 6: iozone throughput vs. file vs. record size - write detail

The read performance plot (Figure 7) shows that results generally improve with increased record size, at least once file sizes are larger than 32 MB. This explains why Microsoft probably made the choice to use larger record sizes when possible in Vista SP1.

iozone throughput vs. file vs. record size - read
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Figure 7: iozone throughput vs. file vs. record size - read

If you look at the 1024 KB record size plot line at a 1 GB file size, you can see that iozone's read results match up pretty well with the Vista SP1 file copy. But for writes, not so much. Why?

The answer is found in the Russinovich article, which describes how pipelined I/O can be used by Vista when it is doing a network transfer to another Vista or Windows Server 2008 system. This is due to the use of an improved SMB protocol—SMB2. iozone, on the other hand, always issues only one I/O transfer at a time.


I now have a better understanding of why some readers have been reporting higher throughput than I get when using iozone, when using Vista SP1 file copies to measure NAS performance. Vista SP1 still reserves its best performance for when it is reading and writing files to another Vista SP1 or Windows Server 2008 system, since only those platforms support SMB2. But the use of large block / record sizes doesn't require SMB2; just a network connection and client system capable of dealing with the larger sizes.

So will running Vista SP1 on your LAN computers bring you blazing new speed from your NAS? Perhaps. First, you'll need to be running a gigabit Ethernet LAN and using PCIe NICs in your systems. Otherwise, even with NASes (or servers or even other computers) capable of 100+ MB/s throughput, PCI NICs will limit throughput to somewhere between 60 and 70 MB/s.

You'll also need systems capable of running a gigabit connection at or near full capacity. I haven't done exhaustive testing on this. But from what I have seen in my experiments, any Core 2 Duo system (or its equivalent) should have plenty of power to meet this requirement.

Once you have ensured that your computers and network have the required throughput, then you can turn to your NAS. So far, the only NASes that I have seen that are capable of running anywhere near 100 MB/s are powered by Intel CPUs and have at least 1 GB of memory, i.e. the QNAP 5-bay TS-509 Pro and NETGEAR 6-drive ReadyNAS Pro. The larger memory certainly helps, but the performance gains, especially when running RAID 5, come from the higher compute power.

It's also important to note that even if you are running Vista SP1, you might not get high file transfer performance from all the applications that you run. As Mark Russinovich noted in his article's closing Summary, applications need to use the CopyFileEx API to get improved file copy performance. So if you are running multiple users against a NAS-hosted database, or doing video editing on a NAS-hosted file, you will probably not see the same throughput as you get with a Vista Explorer file copy.

These experiments have lead me to believe that it will be valuable to add Vista SP1 Filecopy write and read benchmarks to the NAS Charts when I switch over to using the new NAS Test Bed machine. This decision isn't carved in stone at this point, however. So let me know what you think.

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