NAS - Why So Slow?

[Sentinel]

Hello,

I've been considering a NAS solution instead of using my soon to be replaced PC as a pseudo-NAS. Looking at reviews of several consumer NAS solutions I am confused as to why the data transfer rates to these units is so poor considering a direct wired gigabit connection. My external 2.5inch USB HD operates as fast as the best of the consumer NAS's out there as far as data transfer is concerned (around 35Mb/s), so why with double the bandwidth of a USB 2.0 connection are these devices so slow?

Why not simply use my old PC as a NAS as far as performance goes?

Any insights appreciated.

[glc]

In order for a NAS to be faster than USB 2.0, it has to be gigabit, and all network devices using it must be gigabit - network adapters, cables, and router/switch.

[mojo]

Consumer NAS devices are nice because they are easy for most people to use, but if you care about performance and know (or are willing to learn) what you're doing then you really want a file server. I have a linux fileserver running on an old althon 2500+ (32 bit) that runs like a dream. I have about 3TB in it and file transfers are usually 40-50 MB/s.

[Sentinel]

Originally Posted by glc In order for a NAS to be faster than USB 2.0, it has to be gigabit, and all network devices using it must be gigabit - network adapters, cables, and router/switch.

Right - so why when tests with these NAS devices via a direct gigabit wired connection (PC gigabit NIC via CAT5 cable to gigabit NIC on the NAS) yield such slow transfers?

[Sentinel]

Originally Posted by mojo Consumer NAS devices are nice because they are easy for most people to use, but if you care about performance and know (or are willing to learn) what you're doing then you really want a file server. I have a linux fileserver running on an old althon 2500+ (32 bit) that runs like a dream. I have about 3TB in it and file transfers are usually 40-50 MB/s.

Hmmm ... perhaps I'm expecting more from a gigabit connection. I must confess that I cannot remember how to convert what a megaBIT per second equates to in terms of megaBYTES per second.

Perhaps the limitations of a gigaBIT connection are around 40Mb/s or so?

[glc]

Perhaps you need CAT6 cables.

Theoretical limit of a gigabit connection is 125 megabytes per second. I doubt that you will ever see it.

[mojo]

Cat6 cables won't make any difference. If the gigabit lights come on on the nic and the switch, that means they had a quick chat and determined they could talk at gigabit speeds just fine. There's 8 bits in a byte, so like glc said, gigabit is 125 MB/s, theoretically. In reality, several things slow you down, including harddrive speed and IO processing. You can get in the 50 MB/s range with a server, maybe even as high as 60-70 MB/s with a smokin' hot server that's finely tweaked and has very new, fast harddrives, but you'll never see anything close to 125 MB/s. Consumer NAS devices are cheap and made for use by, well, consumers, so they're just not built for speed.

[Sentinel]

Originally Posted by mojo Cat6 cables won't make any difference. If the gigabit lights come on on the nic and the switch, that means they had a quick chat and determined they could talk at gigabit speeds just fine. There's 8 bits in a byte, so like glc said, gigabit is 125 MB/s, theoretically. In reality, several things slow you down, including harddrive speed and IO processing. You can get in the 50 MB/s range with a server, maybe even as high as 60-70 MB/s with a smokin' hot server that's finely tweaked and has very new, fast harddrives, but you'll never see anything close to 125 MB/s. Consumer NAS devices are cheap and made for use by, well, consumers, so they're just not built for speed.

I still don't quite see why you have to have expensive "smokin'" fast servers with "fast" HDD's. Even the average HD today can sustain 60+ Mb/sec (I've just purchased one that reaches around 90Mb/sec) so I cannot see how the HDD is any kind of bottleneck. I don't dispute that you need expensive hardware to achieve even 50% of the available gigabit bandwidth, but I just don't understand why. If you have a reasonable HD and a quality gigabit NIC and cable, where else do you lose so much speed? OK, the RAID controller might slow things up a bit, but when you consider that the RAID controllers thrown in on most motherboards will achieve near maximum transfer rates of the HDD then I'm baffled why only high-end file servers can reach most reasonable transfer speeds.

[mojo]

I'm not entirely sure myself, but experience and reading lots of reviews tells me that it's a fact. When I said smokin' hot, I should have clarified it a bit. You don't need some crazy expensive beast that would put a nice gaming rig to shame, just something reasonable. I guess I really meant that for top speeds, the "hey I cobbled together this collection of 5 year old parts" method isn't the best idea. I think a lot of the speed loss comes from network processing. I know I experience much better speeds when I use jumbo frames vs. regular ethernet frames. This isn't possible with a regular NAS device, and there are many other tweaks you can do with a full server that just aren't available on a NAS device.

For much more information than I could ever give you, I suggest poking around this site:
http://www.smallnetbuilder.com/content/view/85/93/

My link is to the NAS section, but the whole site is gold mine if you like learning about networks.