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Date: Wed, 22 Jul 2009 10:38:49 +0300
From: Adrian Hunter <adrian.hunter@nokia.com>
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To: Corentin Chary <corentin.chary@gmail.com>
Subject: Re: UBI FS mounting time
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Cc: "linux-mtd@lists.infradead.org" <linux-mtd@lists.infradead.org>,
	Bosi Daniele <Daniele.Bosi@mta.it>, Brijesh Singh <brij.singh@samsung.com>
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Corentin Chary wrote:
> On Wed, Jul 22, 2009 at 8:44 AM, Adrian Hunter<adrian.hunter@nokia.com> wrote:
>> Bosi Daniele wrote:
>>> We have a UBI+UBIFS 1GB partition on a NAND chip, which takes around 6s to
>>> mount.
>>>
>>> We'd like to have the data available at least read-only earlier than that.
>>>
>>> We know UBI on principle needs to read the map of all blocks in order to
>>> rebuild the logical view of the memory, but maybe there's another way around
>>> it to make it available earlier...
>>>
>>> Someone know how?
>> Some options are:
>>
>>
>> a) Use another smaller partition that can mount first and provide some
>> functionality while the larger partition mounts.
>> b) Reduce the number of eraseblocks by combining them into larger logical
>> blocks.
>> c) Change UBI to write its mapping table when it is unloaded, so it can be
>> read quickly when loading (still have to scan if UBI was not unloaded
>> cleanly).
> 
> Do you know if someone is trying to implement that ?

Not that particular approach.  Some Samsung people have looked at
creating a scalable version of UBI.

> I did some test, and using lzo/zlib it should be possible to store
> such a mapping
> store in only one PEB.
> Then we can choose this PEB to be near the beginning of the
> flash to speed up scanning (using ec and pnum to calculate a "score").
> This is also possible to use an "anchor".

Yes, it is better to use anchor blocks.  Probably just one is enough if
it is only updated twice per mount.