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Re: RAID levels not intuitive in anaconda GUI



On Dec 20, 2012, at 8:25 AM, Máirín Duffy <duffy fedoraproject org> wrote:
> 
> On Thu, 2012-12-20 at 05:00 -0700, Chris Murphy wrote:
>> 1. I suggest exchanging "Redundancy (mirror)" for "Mirroring".
>> Redundancy applies to RAID 1, 4, 5, 6 so using it as a primary term is
>> confusing. If this checkbox means RAID 1, then it really ought to say
>> just "Mirroring" or "Mirroring (RAID1)".
> 
> Swapping 'Redundancy (mirror)' for 'Mirroring' seems like a reasonable
> change to me, especially if it makes the label less confusing. Would you
> be willing to file a bug for this under F19?

Yes.


> 
>> 2. "Optimize performance (stripe)" I think it's more clear if this
>> "Performance optimized striping" if this is meant just for RAID 0.
> 
> It's not just meant for RAID 0.

There's a considerable write performance penalty for RAID 5 and 6. In certain cases there's also one for RAID 4.


> 
>> 3. "Error detection (parity)" is a confusing label. Parity applies to
>> md RAID 4, 5 and 6 and I can check this option by itself with nothing
>> else checked which then implies RAID 4 which is so uncommon that
>> supporting it doesn't make sense. Once selected I can't deselect it;
>> and I can't select either option below. But I can select the two
>> options above. So upon checking this, I have RAID 4, and can choose
>> either RAID 41 or RAID 40, which are even more rare than RAID 4.
> 
> While I understand your concern that the arrangement of checkboxes
> allows for more rare RAID configurations, a flat list of RAID levels
> also treats more rare levels at the same level as the more common ones
> so the base problem here is the same.

While I'm not advocating a return to a flat list, the advantage is the rare/unadvisable RAID options are simply not displayed. It's immediately clear what's possible. Presently I have to play with the interface to discover what is and isn't possible, and there's actually some hidden discovery in that I can check the boxes "Redundancy + Redundant" for 61, but when I click Apply Changes, this is refused.


>> 
>> Error detection itself is misleading because in normal operation RAID
>> 4, 5, 6 themselves do not detect any errors above what the drive
>> firmware detects (which is the same for RAID 1 and RAID 0). In order to
>> get error detection the user must initiate or schedule a scrub or
>> repair. Conversely, Btrfs does have error detection which is active
>> during normal operation regardless of the profile used.
> 
> How would you suggest changing the label?

No easy answer.

A user who wants redundancy while using most of their drives for their stuff, would be served by this option. So some way of conveying "Redundancy using X data drives, Y parity drives, some performance loss for writes" with an additional checkbox for Dual Parity. So that would mean collapsing the three parity options into two options. 

If that's in the ball park of agreeable, then I'd actually change my mind on the first checkbox Redundancy vs Mirroring, to have some way of conveying "Redundancy using X data drives, X mirror drives, minimal performance loss for writes".

RAID 0 would be "No redundancy, using X data drives, performance optimized".

I think I understand the goal, which is to get away from secret decoder ring UI. "Distributed" is better than "RAID 5" in that sense, but I wonder if "distributed parity" and "error detection" and even the word "parity" itself, which I used earlier, are helpful or just another term people would have to look up to understand what to do.

I wouldn't mind seeing this functionality made even more aggressive based on layered use cases, even using a use case based UI. Something based on this:

http://blog.linuxgrrl.com/wp-content/uploads/2011/12/partitioning-personas.png

But put into a separate application that makes it easier to configure RAID for general use, not just installing an OS, would be seriously very useful.

>> 
>> Error correction is true, but it's also true for RAID 1.
>> 
>> 4. "Distributed" parity applies to RAID 5 or 6. It's unclear which I'll
>> get, or why I can select it all by itself with no other options
>> including without Error detection (parity) selected.
>> 
>> 5. "Redundant" under Error detection is confusing. RAID 4 is redundant,
>> RAID 5 is redundant, RAID 6 is redundant. I'd guess it means RAID 6
>> except…
> 
> It's indented under parity, meaning redundant parity. Marian had this
> confusion as well, so I think we need to tighten up the visual design
> there and perhaps grey out those two sub checkboxes unless error
> detection (parity) is active, perhaps even change the labels to
> 'redundant parity' and 'distributed parity'

I think also what's happening is part of the UI changes live as I check boxes (graying out or checkboxes check or uncheck automatically) – but in other cases they change or an option is refused once I click on Apply Changes.


>> 
>> Items 3-6 I think need to be consolidated into just RAID 5 or 6, and
>> skip RAID 4. As for what plain language to use to describe them, it's
>> difficult. "Single drive fault tolerance" is maybe OK, but that applies
>> to a two disk RAID 1.
> 
> If we drop RAID 4 I'm sure we'll get more self-righteous hate mail. Can
> I blame you and provide your email address if we do this? :)

Yeah sure, for all two people who install an OS to RAID 4? Haha. It doesn't hurt anyone to leave it in, unless their use case is heavy small file write and then the parity disk gets bogged down well before the data disks. So either the user still needs to know such things about the RAID level they've effectively chosen, or the UI instead needs to draw out the usage context, and make a choice for the user based on that. And I think that actually would help both pros and non-sophisticated users alike.

> 
>> And possibly the best multidisk options, which are also the easiest and
>> fastest to grow when more space is needed: linear, and RAID 1+linear,
>> combined with XFS for parallelization. But this isn't an option at all.
> 
> If XFS is a supported fs type (I don't know if it is offhand) you could
> select RAID as a partition type and XFS as the filesystem, using RAID
> for the technology dropdown. 

XFS is, but unfortunately there isn't a UI option for RAID linear/concat. Exposing this with checkboxes probably clutters the UI and would add to confusion, even if it presents a good option for a number of use cases. It's better to stuff this behind a use case UI, and if the usage/context fits, then this becomes the suggested storage tech to use.




Chris Murphy


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