Re: [RFC][PATCH 0/7] UBI checkpointing support

[Richard Weinberger <rw@linutronix.de>]

Am 07.03.2012 17:33, schrieb Artem Bityutskiy:
> Just basic questions to make sure I understand things correctly.
>
> Do you have plans to also change the user-space tools?

Maybe ubiattach to make the attach method selectable.
Attaching by scanning or checkpointing...

> On Tue, 2012-02-14 at 21:06 +0100, Richard Weinberger wrote:
>> 1) A primary checkpoint block, which contains merily a pointer to the
>>     erase block(s) which hold the real checkpointing data.
>>
>>     This primary block is guaranteed to be held within the first N
>>     eraseblocks of a device. N is momentarily set to 16, but it might
>>     be necessary to make this configurable in some way.
>
> Does it mean that you reserve the first 16 PEBs for the primary block?

The current implementation selects out one of the first 64 blocks.
I know I wrote 16 in the initial RFC, but it's 64.
But it does not reserve them.

While writing a new checkpoint it tries to select an other early block.
If no new early block is available it reuses the old one.

> I guess we need to carefully look an this number and make the default to
> be good enough for the general case.

Yep. The current number was chosen randomly. :D

>> 2) The secondary checkpoint blocks, which contain the real
>>     checkpointing data (physical to logical eraseblock relations,
>>     erase counts, sequence numbers ...)
>
>>     Aside of that the checkpointing data contains a list of blocks
>>     which belong to the active working pool. The active working pool is
>>     a fixed number of blocks for shortterm, longterm and unknown
>>     storage time, which can be modified before the next checkpoint set
>>     is written to FLASH. These blocks need to be scanned in the
>>     conventional UBI scan mode.
>
> BTW, WRT short-term etc - how about just killing these concepts? I am
> really not sure they make much sense anyway and give any improvements.

Good idea!

> I guess this would simplify things for you as well. I'd vote for
> removing them.
>
>>     The reason for these pool blocks is to reduce the checkpoint
>>     updates to the necessary minimum to avoid accelerated device
>>     wearout in scenarios where data changes rapidly. The checkpoint
>>     data is updated whenever a working pool runs out of blocks.
>>
>>     The number of pool blocks can be defined with a config option at
>>     the moment, but this could also be done at runtime via sysfs. In
>>     case of a change the checkpointing data would be reconstructed.
>
> Id suggest to introduce as few configuration knob as possible. My
> experience show that they usually only hurt. I'd stick to this rule for
> most cases: no user, no knob.

Yeah.

>> So the checkpoint scan consists of the following steps:
>>
>>     1) Find the primary checkpoint block by scanning the start of the
>>        device.
>>
>>     2) Read the real checkpoint data and construct the UBI device info
>>        structures.
>>
>>     3) Scan the pool blocks.
>>
>> All these operations scan a limited number of erase blocks which makes
>> the UBI init O(1) and independent of the device size.
>
> Well, is it really true? The larger is the flash the more you read and
> process anyway, and it is still linear, but the multiplier becomes very
> small, so this is a huge improvement.

Yes. :)

Using checkpointing UBI only has to scan a fixed (independent of the 
flash size!) number of blocks.

>> The checkpoint functionality is fully compatible with existing UBI
>> deployments. If no checkpoint blocks can be found then the device is
>> scanned and the checkpoint blocks are created from the scanned
>> information.
>>
>> Aside of review and testing it needs to be decided, whether the number
>> of pool blocks should be deduced from the device size (number of
>> physical eraseblocks) or made configurable at compile or runtime.
>
> I would go for automatic decisions. Manual configuration can always be
> added later if needed.
>
>> Thanks to the folks at CELF who sponsored this work!
>
> Indeed thanks! And thank you Richard!

Thanks,
//richard