Discussion - DNS-323

Question: Does hard drive speed matter in the D-Link DNS-323?

Answer: No.

None of the hard drives was able to distinguish itself from the pack in the DNS-323. The D-Link was rated pretty highly in performance when Craig Ellison reviewed it, but it can't compete with the 256 megabytes of cache, and Celeron processor of the Thecus 5200. At least to me, since both units use the same Ethernet chips (Marvell 88E1111 "Alaska"), it looks like the difference comes down to cache and CPU.

The effect of the cache can be seen I think on the vertical separation between the D-Link and Thecus 5200 in the 32 MB (32768), 64 MB (65536), and 128 MB (131072), file sizes. The 256 MB of cache have the effect of pulling the Thecus disk performance up significantly (approximately 35,000 Kbytes/sec). Once the file size fills the cache, however, performance falls off. But even when out of cache, the Thecus is faster than the D-Link.

This, I think, is the result of the faster processor. Even for the 256 MB (262144), 512 MB (524288), and 1 GB (1048576) file sizes, the Thecus is significantly faster than the D-Link. At these file sizes, the Thecus's performance is significantly better, but it is no longer anything to get excited about (approximately 2,000 to 8,000 Kbytes/sec).

Aside On Cache:

You don't measure disk throughput for long before you get a vague suspicion that RAM anywhere in the system is trying to undo your efforts to measure raw hardware performance. Motherboard makers, operating system makers, and NAS makers are all trying to squeeze as much performance as they can from the lowest cost designs they can come up with. One cheap way to make a design perform better is to add cache and to work hard to utilize the cache effectively.

The effect of cache on throughput creates two camps among SNB readers (at least the ones I've heard from). One camp wants us to take all the RAM out of the test computers so that they can see how fast the bare disk hardware performs. In answering the proverbial question: Which came first, the chicken or the egg? I see the lose-the-cache people as egg advocates.

The other camp wants us to put all the RAM that is practical into the test computer so that they can get an estimate of the maximum real-world performance they can expect from a piece of equipment. These readers look to me, like chickens.

We at SNB see both points. We love to open up the case and see the hardware ("Oh baby, look at them eggs!"). But, we also like understand the value users will receive in real life from the products we review ("Is this chicken going to be worth the money? Is there another chicken that would make me happier of the lifetime of this product?"). So we try to strike a balance in our reviews.

For the Thecus 5200, that balance is struck between 128 MB (131072) and 256 MB (262144) file sizes in the Iozone test suite. On the left hand side of that line, "chickens" can see what ideal performance is like if they load up the RAM on their client machines and NASes. (I asked, but Tim Higgins would not let me hot-rod the Thecus 5200's RAM above 256 MB).

Once file sizes go beyond 256 MB, the "egg" advocates get to see what happens when the Thecus NAS cache runs out of gas and the hardware starts crawling along on its hands and knees.

Tags: NAS,

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