linux-kernel - Re: UBI fastmap updates

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Date:	Thu, 2 Aug 2012 10:03:04 -0700
From:	Tim Bird <tim.bird@...sony.com>
To:	"artem.bityutskiy@...ux.intel.com" <artem.bityutskiy@...ux.intel.com>
CC:	Richard Weinberger <richard@....at>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"adrian.hunter@...el.com" <adrian.hunter@...el.com>,
	"Heinz.Egger@...utronix.de" <Heinz.Egger@...utronix.de>,
	"thomas.wucher@...utronix.de" <thomas.wucher@...utronix.de>,
	"shmulik.ladkani@...il.com" <shmulik.ladkani@...il.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Marius.Mazarel@...l.ro" <Marius.Mazarel@...l.ro>,
	"nyoushchenko@...sta.com" <nyoushchenko@...sta.com>
Subject: Re: UBI fastmap updates

On 08/02/2012 09:45 AM, Artem Bityutskiy wrote:
> Richard,
> 
> On Thu, 2012-08-02 at 18:32 +0200, Richard Weinberger wrote:
>>> This should not happen. Fastmap should _reserve_ enough of PEBs for it
>>> to operate. It should always find the PEB to write.
>>
>> What is the benefit?
>> IOW what is wrong with the current approach?
> 
> Several reasons. The main is: fastmap will start consuming PEBs reserved
> for volumes when the amount of available PEBs is just enough to map all
> LEBs. This will break UBI liability.

I'm don't understand what "UBI liability" is.  Can you please clarify?
What breaks if the PEBs get consumed?

> 
>> Why?
>> If everything goes wrong, fastmap makes sure that no fastmap is on
>> flash.
>> In case of a powercut we fall back to scanning mode.
>> R/O mode is overkill IMHO.
> 
> So can I interpret this the following way. Not only fastmap give no
> guarantees that it exists after an unclean reboot, it does not even give
> guarantees that it exists after a clean reboot.
> 
> Unless I am confused, the fastmap design is over-simplified.

Fastmap is an optimization.  Maybe I'm missing something, but
I'm not sure why, if the optimization stopped working, you
would want to reduce the functionality of the file system.

=============================
Tim Bird
Architecture Group Chair, CE Workgroup of the Linux Foundation
Senior Staff Engineer, Sony Network Entertainment
=============================

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