linux-kernel - Re: [RFC v2] UBI: UBIVIS (aka checkpointing) support

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Message-ID: <4FAB7D70.1040704@nod.at>
Date:	Thu, 10 May 2012 10:33:52 +0200
From:	Richard Weinberger <richard@....at>
To:	dedekind1@...il.com
CC:	linux-mtd@...ts.infradead.org, tim.bird@...sony.com,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	Heinz.Egger@...utronix.de
Subject: Re: [RFC v2] UBI: UBIVIS (aka checkpointing) support

Hi Artem!

Am 10.05.2012 06:26, schrieb Artem Bityutskiy:
> Hi Richard, I would like to complain about the names again. I though I
> better give this feed back as soon as possible...

No problem. :)

> Firs of all, thanks for doing this, I will look closer, and I am very
> keen of merging this stuff once we are sure its design is good, allows
> for future extensions and is backward-compatible.
> 

Yeah.
First of all, yes it's fully backward-compatible. It uses two new internal volume IDs
with compat = UBI_COMPAT_DELETE.
Old UBI implementations will delete the checkpoint and continue with scanning...

Regarding design, ubi_wl_get_peb() currently offers three types of data types.
UBI_LONGTERM, UBI_SHORTTERM and UBI_UNKNOWN. Do we really need them?
Checkpointing has a pool of unknown PEBs. This PEBs have to be scanned while attaching.
For now I had to create three pools (for UBI_LONGTERM, UBI_SHORTTERM and UBI_UNKNOWN).
This makes the whole thing complexer than needed.
It introduces also some nasty corner cases.

To make the review easier for you:
The most critical code path is scan_pool() -> process_pool_seb() -> update_vol().
It searches within a pool for PEBs which are no longer empty and scans them.
After that it updates the corresponding volume.

ubi_update_checkpoint() is also very important because it has to find
unused PEBs at the beginning of the MTD to place the super block.

> So may be just naming your stuff UBI2, having terms like "UBI2 format",
> would be the easiest? Then someone could make this to be UBI3. A
> documentation section could describe what UBI2 is and how it is
> different from UBI1 or just UBI.

Okay, got your point.
I think "fastmap" is a good name because I can also use it within the code.
So, while reviewing the code please keep s/checkpoint/fastmap/g and s/cp/fm/g in mind. ;-)

I like the UBI2 idea. UBI2 = UBI + fastmap.
After the UBI2/fastmap design is stable I will happily write a detailed design paper
for http://www.linux-mtd.infradead.org/doc/ubi.html.

Thanks,
//richard

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