FireWire Chain Loop

[zombitronic]

I'm wondering about drawbacks or benefits of creating a chain of two FireWire drives looped back to one computer. I intend to setup two external 1TB drives as either a single 2TB concatenated RAID array or a striped RAID 0 array to be used as a Time Machine backup volume. All ports and cables are FireWire 400.

The setup in question would be:

Setup 1 (loop)
Mac FireWire Port 1<--->External Drive A FireWire Port 1/External Drive A FireWire Port 2<--->External Drive B FireWire Port 1/External Drive B FireWire Port 2<--->Mac FireWire Port 2

vs.

Setup 2 (one long chain)
Mac FireWire Port 1<--->External Drive A FireWire Port 1/External Drive A FireWire Port 2<--->External Drive B FireWire Port 1

vs.

Setup 3 (two short chains)
Mac FireWire Port 1<--->External Drive A FireWire Port 1
Mac FireWire Port 2<--->External Drive B FireWire Port 1

<---> = FireWire 400 cable

According to Apple, (regarding FireWire 800):

In fact, you can even loop your FireWire 800 chain back to your Mac for redundancy while performing live.

However, according to FireWire Depot:

When you are connecting devices to your computer, don't make a second connection between two devices that are already part of the tree. The devices will not work properly because the transmission of the second connection is blocked and forms a loop within the tree arrangement.

Unless I'm not understanding this, these two statements contradict each other. I tend to believe Apple since they are responsible for much of FireWire's development but such a blatant contradiction makes me wonder. Would anyone be kind enough to shed some light on this?

 

[madog]

According to a Western Digital:

FireWire devices can be connected in any combination of branching and chaining, as long as no loops are formed.

http://www.wdc.com/en/library/firewire/2079-001012.pdf

Then according to some random site:

Better user experience: No matter how you connect devices together, FireWire 800 just works. In fact, you can even loop your FireWire 800 chain back to your Mac for redundancy while performing live.

http://www.2ndchancepc.co.uk/firewire-faq.html

So as I see it, FW devices may have the potential to be looped, however, that doesn't necessarily mean that all are capable of it. Unfortunately, due to laziness or slopiness of the FW interface not all manufacturers create it equal. I only recently heard of this, but some external FW enclosures are incapable of being booted to even though that is an explicit feature of all Macs with FW. Also, it looks like both the info we found regarding looping directly references FW 800 and not 400, which may be the reason for the misinformation from the other sources as they just mention FW in general as looping may be exclusive to 800.

So I believe it just depends if your particular device is capable of it (who knows, they might not know and therefore tell everyone to just avoid looping to err on the safe side). I would also go and check out Apple's discussion boards to see if you can find a more definitive answer.

Edit: Also, there is no initial drawback of chaining FW devices together although as I mentioned before, some manufacturers are lazy/stupid and therefore their FW chipsets may run slower than others; so various types of FW devices chained together may result in one not transferring at its full potential if at the end of a chain with slower ones in front. On the other hand, if they are all from the same manufacturer than it won't be an issue. (The differing speeds are typically a non-issue anyway depending on your intent, but you can find various independent tests of different manufacturer's FW devices on the webs).

 

[zombitronic]

Thanks for the reply. That quote from the "random site" was directly copied from Apple's FireWire 800 page. That's fine, but then they put a copyright at the bottom... I guess they're small enough to get away with that. But whatever.

I'm just going to try a FireWire loop and see what happens. I've got a 1TB Maxtor OneTouch 4 Plus and another one coming in the mail tomorrow. I think I'll RAID 0 them. With each drive connected to its own port on the Mac, my Setup 3 (two short chains), I should get the speed advantage of simultaneously writing to each drive. If I then complete the loop by connecting the drives to each other, creating my Setup 1 (loop), I may get some sort of redundancy benefit, although I'm just doing this as a shot in the dark. I really don't know what or how I'd stand to gain. If anyone has any ideas about this, I'd love to hear them.

The worst scenario for a RAID 0 configuration would seem to be my Setup 2 (one long chain). I don't think I would get the benefit of simultaneous read/write speeds, as it would only be communicating to the Mac through one port.

If I made a concatenated RAID array, any of these setups would be fine, since the second drive isn't used until the first one is full. I wouldn't be gaining any simultaneous read/write speed benefit, anyway. Not only this, but I've read that with a concatenated RAID array on Mac OS X, if one drive goes bad you lose the data on both drives, anyway. That means that if you've got two of the same sized drives in a RAID array in Mac OS X, you should always use a RAID 0 array over a concatenated array. This way, you get the benefit of simultaneous read/write speeds with no more and no less risk of data loss.

I'll make a concatenated RAID array out of two of my internal disks for my ever growing iTunes Library. I'll do this instead of RAID 0 because the drives are not the same size. This way, the drives can be combined into one large volume (750GB + 500GB = 1.25TB). With a RAID 0 array, I'd lose 250GB of drive space since each drive will only support striping up to the size of the smallest drive (500GB + 500GB = 1TB). I know I would gain the read/write benefit of striping in RAID 0, but I think that extra 250GB of space may end up being more beneficial to me than speed, especially as I start to add more HD content to my Library.

Of course, no backup method is 100% safe, but with this method I would have to lose a drive from the concatenated array and the RAID 0 array at the same time to actually lose my data. It's possible, but unlikely.

 

[BlueRevolution]

So, RAID 10?

 

[zombitronic]

So, RAID 10?

No, RAID 10 would be several RAID 1s within an overall RAID 0, or several mirrored RAIDS within an overall striped RAID.

I'm setting up an external FireWire striped RAID (RAID 0) for backups and a separate internal concatenated RAID (kind of the opposite of one drive with multiple partitions; instead, multiple drives with one partition) for my iTunes Library.