DSM-G600, DNS-3xx and NSA-220 Hack Forum

Ok - there's JBOD and there's JBOD - and by that I mean that is that the term JBOD has different connotations depending on who is using it.

I've seen the term used to refer to multiple individual disks - which on the DNS-323 is referred to as "standard (individual) disks" and which to my mind is what the term implies - Just a Bunch Of Disks - and I've also seen it used to refer to concatenated or spanned disks, as it does on the DNS-323.

Now - the theory with concatenated or spanned JBOD is that if a disk fails, only the data on that particular disk will be lost - but in practise, this does not appear to be the case - I invite you to configure your DNS-323 for JBOD, fill the disks, pull one and tell me how much of your data were you able to access - if any - who knows - your results might be different to mine.  I'm also not aware of any device on which concatenated/spanned JBOD actually works as per the theory above - but I'm sure I haven't tested them all, so I could be wrong there also.

I personally see no valid reason - on this device - to use either RAID0 or JBOD - and I suspect these features are there because the marketing department thinks that users expect them on a NAS (Network Attached Storage). 

RAID0 does a terrific job at boosting throughput on DAS (Direct Attached Storage) but on a NAS, the disk interface is no longer the bottleneck, the network is - so the RAID0 implementation offers nothing other than a little less data security - similarly JBOD (concatenated/spanned) might have had value back in the days of lower capacity disks, but as capacity has increased and prices dropped, the need for JBOD has diminished.

Both features could be dropped without - at least in my opinion - diminishing the value of the device.

sjmac

You may have noticed I like to explore - research the options (the internet is a wonderful thing), not necessarily accepting what I find as gospel, especially when the concept doesn't sound right to me, create my own theories (some right hopefully, and other wrong) and then test, test and test again.

Like I said earlier RAID0 works in a direct attach, but on this device, and most network attach devices, it doesn't - JBOD - well, I haven't tried recovering the data from a JBOD with a direct attach to a PC, although I have done that with "standard" disks, using the ex2ifs driver (not that I needed to, but I had to test it, didn't I - how else would I know if it worked)

The RAID on this device is software RAID, as for reading the disks on a linux system - RAID1 no problem, RAID0 is beyond my limited linux knowledge, and I haven't bothered to learn how, since I won't be using it.

Large drives are cheap - unlike ten years ago when I created a 10GB array from 5x2 GB UltraSCSI disks - more because I had them lying around than anything else - I see no reason why I would want to use "JBOD linear" as D-Link calls it.

I do not have 2 disks to try. my guess will be JBOD is using LVM to join 2 partitions linearly, and raid 0 is ... well raid 0 stripping

Viracocha wrote:

As far as I know, RAID 0 has a restriction in that if you have two hard drives with different capacities, on the bigger disk you'll only be able to use the same capacity as the smaller one. JBOD doesn't have this restriction, you can use the full capacity of both drives.

Quite correct

Viracocha wrote:

With gigabit ethernet, I'm not so sure that the network will be such a bottleneck. At least not compared to USB or firewire. There is some overhead, of course, caused by IP layers that doesn't exist with those, but it will still be very comparable. Granted it still doesn't reach the theoretical transfer rate of a SATA bus, but I'm not sure the drives themselves can keep up with that yet.

Gigabit ethernet has a maximum theoretical throughput that exceeds that of both fireware and USB 2.0, however, that theoretical maximum is rarely achieved in practice, and my tests have shown that maximum throughput on the DNS-323, without jumbo frame hovers around ~170 mbps - regardless of drive configuration - I understand with jumbo frame you can squeeze another 30% or so out of the device - seems a little much to me, but I'm not in a position to confirm or deny

You can decide for yourself whether increased throughput with jumbo frame points to the network, or the silicon and/or the efficiency of the ip stack as being the bottleneck - I think it's a moot point - what is evident is the device delivers a practical maximum throughput of around ~170 mbps when used in conjunction with a Windows server on a gigabit network known to be capable of accomodating throughput topping 300 mbps, and that changing the disk configuration did not significantly affect the throughput.

The theoretical advantage of RAID0 is not realized by this device.

Here's a picture I made when I did some experiments with frame size stuff:


Its a 1000Mbps NIC, so 10% is 100Mbps. The disk configuration is probably "standard", but may have been RAID-0 at that time. I thought the fact that the throughput changed so much with frame size was a strong indication that the disks were not the bottleneck in this case.

It was fun to play, but big frame sizes caused problems on my local network, and are best avoided in my opinion.

I read around jumbo frame issues, and this link covers most of them well: http://www.smallnetbuilder.com/content/ … 01/54/1/2/

edit: sort of worked out how to put an image inline ...

Last edited by sjmac (2008-04-22 09:34:36)

I just checked the dlink product description pages and it reads: "RAID 0 combines both drives in a 'striped' configuration, which provides the highest performance, enabling speeds up to 23MBps-Read and 15MBps-Write (MBps: Megabytes per second)". With transfer rates like that, raid 0 shouldn't really prove any benefit as long as you have a decent HD (are there even any sata hds that with lower read speeds?).

Maybe I was just naive in assuming that if it supports gigabit ethernet, it should be able to attain those speeds as long as the HD you used could support those transfer rates.

Levis wrote:

So, in conclusion, with the same HDs how much is the difference in transfer between JBOD and Raid 0 with a gigabit lan?

Not much, if any.