Drive speed and "Beyond RAID"

Does anyone know what happens with individual drive speed if you mix drive speeds in a Pro.

Lets say you had a mix of 5400 and 7200 drives - does the drobo make use of the extra speed of the 7200’s while read/writing or does the unit drop all drives to the same speed - i.e. 5400.

If each drive is treated individually then I guess the overall (spin) speed of the RAID is going to be an average of all of the drives together…

Drobo - would you be able to answer? Is there anything in a document that might explain this?


drobo cant change the speed of the drives (nothing can!) - thats fixed in their motors at the time of manufacture and nothing can change that, 5400 rpm drives will ALWAYS run at 5400 rpm and 7200rpm drives will ALWAYS run at 7200rpm[hr]
and the limiting speed of all drobos (except the elite) is their processors - not the drive’s speeds

Thanks Chris.

Actually I think some drives spin at different speeds depending on the operation - or at least you used to be able to get drives with different spin speeds - the 5400/7200/10k and 15k speeds were always the maximum speeds from what I remember when buying drives years gone by. Maybe this is not the case now?

Anyway, all of that above is not relevant. My questions is - if the unit is undertaking a read operation of lets say 700MB, and that 700MB is (obviously) split across multiple disks, how does the Drobo deal with the different speeds - does the data contained on the faster drives get read and sent over the network before the 5400’s has finished or is the unit caching information so it’s sent in a certain order. I.e. how is the overall performance working while there is a mixed drive speed scenario.

Bare in mind how a standard RAID works…and then this is the basis of my question - how does Beyond RAID work with the drive speeds - how does the BUS itself handle the different drive spin speeds. I’m not even bothered about how the Beyond RAID works with different factors other than the spin speed - manufacturer, drive cache etc.

If I was watching a movie stored on the Drobo - the data has to be read in a certain way else the video won’t play correctly. I.e. there could be delays waiting for information stored on the slower drives, so the faster drives in this instance don’t gain you any performance over-all, as they are having to wait for IO to happen AFTER the slower drives have caught up.

Does this make sense?

there has never been a drive sold at retail which can vary its spin speed

i think hitachi did experiment with trying to develop them (it was for laptops - 7200 rpm on mains and 5400rpm on battery) but the design of the motors means that they are almost impossible to vary their speed on the fly and they gave up

the confusion with modern day drives is because of WD’s marketing crap on their greenpower drives - which they described as “variable speed”. they then had to later issue a press statement explaining that they mean that each drive has a different FIXED INVARIABLE speed (which it also turns out is not true - you can relatively easily work out the rotational speed of the drive and as yet they haven’t shipped one which isnt 5400rpm) - the speed varied between drives, not during the drive’s operation.

i get what you are asking, but the drive speed doenst really factor into it - dont forget the drobos tend to read at 40-80MB/sec MAX depending on model (never above about 30-35MB/sec if you are on USB). even slow GP drives never really drop below 60MB/sec PER drive - so with a drobopro (8 drives) you are looking as a sustained/continuous read off the drives of 480MB/sec - 1GB/sec total.

the limiting factor is usually the slower of the drobo’s processor or the interface to the host computer - eitherway the drives will probably be sat there idle for a period of the time, even if they are very old and slow

OK so a small update - the current flock of “Green” drives have variable spin speed. This doesn’t really change my question but it does add to it - as if you have a 7200 “green” drive and a 5400 “green” drive - how does the drobo deal with the differences when fetching and filing data, and what speed will the drive spin at based on what instruction it receives from the Drobo…?

Maybe this is a question for support. I’ll try them and post back here when I hear back.

as i explained quite clearly they do NOT have variable spin speed - they CANNOT change their rotational speed.

when WD say VARIABLE - they mean that it MAY vary between batches of the drives - NOT that the drives can change their speed.[hr]
you can even test this yourself if you happen to have a CRO handy - just hook a mic up to it and listen to the frequency of the hum of the drive and you can then work backwards and calcuation the rotational velocity - some websites have actually done this and the drives DO NOT change their rotational speed - this would be ridiculously complicated for the drive to do![hr]
The biggest problem is fly height. Read/write heads fly over the platters by use of a slider… it’s a ceramic “wing” that produces lift that keeps it from touching (and damaging) platter surface but also prevents it from flying so high that read/write capability would suffer.

Actually Chris I think you and I may have posted at the same time as I had not read your second post before posting MY second post - I was not debating what you were saying (in your second post) as I had not in fact seen it. I.e. you hadn’t “quite clearly explained” prior to me posting my 2nd post.

I’m not sure what’s going on with the time/date stamping of these posts on here either - there was not 1 hour between any of these posts.

I see what you’re saying about the over-all performance and the read/write capabilities of the unit itself. Arguably then, there is no gain in using faster drives, 5400’s will still perform above the capability of the actual network transfer rate of say 40Mb’s…

And again, not to debate what you’re saying, WD do state that their intelli technology is an algorithm which calculates the drive spin speed based on the specific operation being undertaken. It would be hard to debate that this reads like the spin speed of the drive is “variable” - though I am not aware how it would be possible to then change the rate at which the heads move across the disk and read the data etc. Anyway, again, this is not relevant.[hr]
Actually the issue is/was that I didn’t get notified about your second post Chris, and I updated the thread without having refreshed my page - so had not seen your post.

DRi have said that there is a SLIGHT (i.e. measurable in benchmarking but not noticable in the real world) speed improvment using 7200 rpm drives in the pro, for “lesser” drobos there is no difference between 5400 and 7200rpm drives (and conversely for elites there is quite a difference between 5400 and 7200)

WD have had to tighten up their language before they get sued for being misleading, their current GP page says:

Calculates optimum seek speeds to lower power consumption, noise and vibration.

A fine-tuned balance of spin speed, transfer rate and caching algorithms designed to deliver both significant power savings and solid performance. Additionally, WD Caviar Green drives consume less current during startup allowing lower peak loads on systems as they are booted.

you’ll note that the “power” no longer mentions spin speed being variable

intelliseek it simple common sense in that there is no point in whipping the heads over to the correct track if your sector isnt going to come around for 3/4 of a revolution - so it calculates how quickly it has to move the heads so that the head gets to the right track just in time to read the data and doenst waste power getting there too early.

OK, so this will save me some money as the current crop of higher capacity drives are much cheaper at 5400.

Yeah I was just reading about that press statement you mentioned about the who variable thing. I’m surprised they still go as far as to say what they DO say. Very misleading.

On the basis that I use my Drobo for home use and that I’m about to have 4 more 4 more drive and all those additional platters - I don’t see any need for 7200’s. I might boot some VM’s off of an NFS share on the unit eventually so I will see how that performs - could be interesting!

Thanks Chris, have a good 2011 !

Great thread, very informative! Thanks for sharing your thoughts as I had wondered the same thing about my Drobo FS.

of course aside from the initial outlay - the 7200 rpm drives run hotter and draw more power which will add $$$ to your electric bill over time too! all good reasons to stick with the 5400 drives

and DYNAMIC was the word i was thinking of - the drives have a variable (isnt that same on all the drives), static speed. but most people reading that interpreted it to mean it was dynamic (i.e. could be changed at will)

hope you have a good 2011 too (with your cool & quiet 5400rpm drives!)