Re: UBI fastmap updates

[Richard Weinberger]

Am Thu, 02 Aug 2012 20:40:00 +0300
schrieb Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>:

> Hi Tim,
> 
> On Thu, 2012-08-02 at 10:03 -0700, Tim Bird wrote:
> > I'm don't understand what "UBI liability" is.  Can you please
> > clarify? What breaks if the PEBs get consumed?
> 
> let me try. Let's forget about bad blocks and assume we are talking
> about NOR flash. For simplicity.
> 
> Let's also first forget about fastmap so far and talk about the
> current design.
> 
> Suppose you have 100 PEBs on your flash. Suppose UBI reserves 10 for
> its internal needs (volume table, etc). 90 PEBs are available to the
> user.
> 
> User now can create one or many volumes, but the overall size of the
> volumes cannot be larger than 90 LEBs.
> 
> This means that UBI guarantees that you can always fill all volumes
> with data there will always be enough PEBs to map to. This is UBI
> liability.
> 
> UBI will not allow you to create a volume of 100 LEBs because in this
> case it will not be able to guarantee that all LEBs will be writable.
> 
> I have invented this "liability" term on the spot actually. It
> basically means what UBI already "promised", what it reserved an put
> aside.
> 
> Now let's add fastmap support to the picture.
> 
> Suppose fastmap took another 10 PEBs and now we have 80 PEBs for the
> user.
> 
> The user can create a volume of 80 LEBs in size. And UBI has to
> guarantee that the user can at any point of time fill all of them with
> data.
> 
> This means that fastmap in no circumstances can grab any more than 10
> PEBs, because they are all reserved, UBI liability is 80 PEBs.
> 
> On other words, fastmap has to know how much PEBs it needs at the UBI
> initialization time, and reserve them. The _maximum_ value.
> 
> The same way other UBI sub-systems do. E.g., the volume table code
> reserves 2 PEBs, because this is the maximum it needs at any point of
> time. The WL subsystem reserves 1 PEB.
> 
> Of course this is not about reserving any specific PEB, this is just
> accounting - we have a couple of variable for reserved PEBs count.
> 
> So let's return to the error messages I spotted. They say that fastmap
> needs a PEB but cannot find one. The flash is nandsim and has no
> badblocks. Why fastmap did not find one? Because it did not reserve
> enough. And UBI tests create volumes of maximum possible size and fill
> it with data, so all available PEBs are mapped and thus, used.
> 
> What this means that the following situation is possible: the UBI
> volume is not fully filled yet and not all LEBs are mapped, so there
> are available PEBs, and fastmap successfully grows and reduces the
> amount of available PEBs. And when the user writes more data, he gets
> -ENOSPC.
> 
> And this is basically the problem I indicated.
> 
> To make my description complete, let's add NAND to the picture. We
> simply reserve 2% (by default, it is configurable) of PEBs for bad
> blocks handling. This is because vendors typically say that this is
> the maximum amount for the flash life-time.
> 
> If NAND wears-out a lot, and we run out of reserved PEBs, we switch to
> R/O mode, because we cannot anymore keep our "promise", the liability.
> 
> You can look at it this way. If you create an UBI volume, and mount it
> with UBIFS, you usually expect that all the free file-system space is
> available to you.
> 
> You probably will be disappointed if you write your file and get
> -ENOSPC because fastmap does grew and consumed a PEB which which was
> promised to your volume.
> 

Okay, then let's explicitly reserve a few PEBs for fastmap.
This should be very easy task.
How much PEB should be reserved? 2 x sizeof(fastmap)?

Thanks,
//richard
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