What does Drobo tell OS X when its full and waiting for a drive for expansion?

Forum History General Discussion
1

So my setup was:

4x2TB, 4x1TB, dual-disk redundancy and about 6% free space.

Bought two more 2TB drives. Popped out the yellow lit one, Drobo Dashboard says I have 0 bytes free because it’s waiting for a new drive to expand (though with dual-disk redundancy it should have free space, shouldn’t it?).

In the meantime, I have a bunch of iTunes downloads going and they just pause. No error message from iTunes such as, your drive is full, or anything like that. Double click a QuickTime movie on the DroboPro, nothing happens.

Pop the new 2TB drive in, as soon as the Drobo recognizes it, I jump to 1.37TB free space and my iTunes downloads resume and QuickTime pops open with the movie file.

My question is, what is the Drobo telling OS X during this waiting for a drive period? Because OS X doesn’t end up freaking out or anything, it seems to calmly pause all disk activity. But doesn’t this require some kind of OS-level drivers for this behavior? If Dashboard were closed, would it have freaked out? How does Windows handle this?

2

when drobo gets above 95% full it starts to slow down (as in deliberately throttle itself) to stop you from ever actually reaching the limits of what can be stored… you would have seen it at 100% full - when its essentially stopped…

it tells the OS (windows or OSX) I’m writing… im just an INCREDIBLY slow drive!!! hence why your downloads seemed to have stopped without timing out.

it very clever as it doesn’t require dashboard to be running or special drivers/interaction with the OS

3

Goodcow, that sounds normal.

Drobo will try to keep whatever level of redundancy you set it to, so if you told it you want dual-disk redundancy, that’s what it will do.
If you had 6% free and removed a disk, Drobo will resize itself using the remaining disks and see that it has less than enough space for redundancy, so it’s now more than full. Once you add a disk, it again resizes to use the new disk.

At >100% full, Drobo will almost completely stop writing to the disk. I see a few bytes per second in writes, with a LOT of I/O wait in between. I suspect it’s doing just barely enough to keep the OS from freaking out and thinking the Drobo died. Remember, Drobo has no special drivers, so all the OS sees is “good” or “bad,” not “hold on, a drive just failed and suddenly I’m too full.”

Dashboard is not required for any of this to work. I like to use it to see what Drobo is thinking, that’s all.

4

Oh I know it’s normal, I’ve expanded my DroboPro four previous times, I was just curious as to how it worked in a technical sense.

5

it just gets very very very slow to stop you from ever actually hitting that hard wall

6

This is a somewhat related question.

Under OS X, let’s say you’ve set yourself a 16TB volume but have less actual space, for example four 2TB drives only. And with 1TB of free space left, you copy a 2TB file to your Drobo. The system should let you copy to it as it thinks it’s a 16TB volume but how does the Drobo handle this?

Is this when the self-throttling comes into play? You are simply denied the write to protect the system?

7

It would never be able to finish copying as the Drobo would throttle its speed so severely it would take forever.

8

So that is when the throttling jumps in? It’s pretty much a mechanism the Drobo uses to force the user to cancel the operation before the disks are ever full?

Why is there no popup message that is a little bit more obvious than this?

9

Not really. The very very slow write is a mechanism the Drobo uses to keep the OS from failing the volume completely, as would happen if writes ceased working, even though the Drobo cannot actually take more data and remain redundant.

I don’t understand this question.
On my PC, Drobo Dashboard does give popups. And turns red. And sends email if I configure it to. Are you asking why the OS doesn’t understand the internal state of the Drobo? That hardly seems fair to Apple. :slight_smile:

10

johnny - because there is no mechanism built into windows for a drive to signal that is thin provisioned and that it is running out of real space (even tho the virtual space looks like there is lots left). it simply isnt in the sata/usb protocols, the only way for drobo to stop you from hitting the hard wall is to slow down.

of course if dashboard is running then that can tell you that you are low on space - but some people dont run dashbaord, and even then - dashboard can stop the writes, thats a basic windows/osx function and th eonly way drobo can interact with that is to alter the speed at which it is accepting new data.

11

Got it, for the throttling.

And for the popup message, obviously we get the lights and popups from Drobo Dashboard but I meant, a more obvious message that actually says to not put any more files or that throttling is in effect past 95% of used capacity. But I guess they don’t want to be that transparent about it.

12

As previously explained in this thread, Drobo Dashboard will alert you with popups, red warnings, and email, and of course the lights on the Drobo itself indicate when it’s getting full.

At first it sounded like you wanted more from your Drobo, but since the Dashboard and the Drobo lights do what you ask, yet fail to answer your request, then I guess you want Mac OS itself to understand thin provisioning and to…interpret slow performance as indicating a volume is getting full? Include hooks in the filesystem for products such as Drobo to report capacity?

As previously explained in this thread, Mac OS does not understand the concept of “thin provisioning.” A volume has a fixed size, and cannot get “full” before all extents in the volume have been allocated, or there are no free inodes.

Frankly, even thinking about how to handle thin provisioning at the filesystem level gives me a headache. The bitmap would have to be extended with a new flag for “unallocated but unavailable” blocks. The filesystem and all fs utils would have to be modified to understand the new bitmap format. Where would the backup superblock be written for a dynamically resizeable volume?

In any case, this is really not the right forum to request features from Apple.

13

i wouldn’t be surprised if there will be a more elegant way to handle thin provisioning in the future (or perhaps a FS which easily resizes - a dynamic rather than fixed volume size - negating the need for thin provisioning)

14

[quote=“rdo, post:12, topic:2112”]As previously explained in this thread, Drobo Dashboard will alert you with popups, red warnings, and email, and of course the lights on the Drobo itself indicate when it’s getting full.[/quote]Yes, obviously it will warn you that it is full. But unless I’m mistaken, it will not say outright “Do not put any more files as filling me up any more will be a bad thing for the system”.

The average user will not understand why Drobo suddenly slows to a crawl when it’s past 95% full. It does sort of say that is the case in the manual and in the knowledge base but that is often overlooked for random Joe expecting his Drobo to behave like conventional drives.

I’m just saying it’d be a better user experience that along with the “alert (…) with popups, red warnings and email (…) when it’s getting full” to also say why and to not go over the limit.

I’ve never actually hit the limit on any of my drives so maybe it does tell you that. If that is the case, everyone can ignore what I’m saying.

[quote=“rdo, post:12, topic:2112”]At first it sounded like you wanted more from your Drobo, but since the Dashboard and the Drobo lights do what you ask, yet fail to answer your request, then I guess you want Mac OS itself to understand thin provisioning and to…interpret slow performance as indicating a volume is getting full? Include hooks in the filesystem for products such as Drobo to report capacity?[/quote]You’re looking into what I said way too deep.

I’m just asking that instead of a simple “Drobo is getting full” message or whatever it actually says to a more obvious explanation to the user that might say “Drobo is getting full, you will be experiencing slow down to the drive because ___ and so on” instead of having the user go online or look at his manual for answers when this is normal behavior. It would avoid some confusion.

Again, I haven’t seen the actual message Drobo Dashboard will display so ignore what I’m saying if it does explain the situation better than a simple “Drobo is getting full”.

And it’s not a feature request or anything. Just a random question sidestepped from the original thread.

15

It might work if a future Drobo “faked” SMART messages to the OS, then it might be able to provide some more useful information without requiring some OS-specific coding or installation.