linux-kernel - Re: [RFC v4] UBI: Fastmap support (aka checkpointing)

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Message-ID: <4FB38683.7030306@nod.at>
Date:	Wed, 16 May 2012 12:50:43 +0200
From:	Richard Weinberger <richard@....at>
To:	dedekind1@...il.com
CC:	linux-mtd@...ts.infradead.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, Heinz.Egger@...utronix.de,
	tim.bird@...sony.com
Subject: Re: [RFC v4] UBI: Fastmap support (aka checkpointing)

On 16.05.2012 11:38, Artem Bityutskiy wrote:
  >> This case can happen if the complete fastmap fits into one PEB, the 
fastmap
>> super block is the first PEB on the MTD partition and the fastmap pool is empty.
>> On the other side, in the worst case fastmap has to scan UBI_FM_MAX_START +
>> UBI_FM_MAX_BLOCKS + UBI_FM_MAX_POOL_SIZE PEBs.
>
> When N ->  inf, UBI_FM_MAX_BLOCKS ->  inf as well. Each PEB requires
> little space in the fastmap table.

No, UBI_FM_MAX_BLOCKS does *not* depend on the MTD partition size.
When N -> inf, UBI_FM_MAX_BLOCKS is still a constant.
--> O(1)

> O(N) would be: N ->  inf, UBI_FM_MAX_BLOCKS ->  C, where C is a constant.
>
> Or did I completely forgot math basics?
>
>> With the current default settings this would be 192 PEBs.
>> So, attaching via fastmap has a complexity of O(1).
>
> No :-) Again, for each PEB you have a little data structure in a fastmap
> which you have to (a) store, (b) read, and (c) process when attaching
> the device. The more PEBs you have, the more you do.

The maximum size of a fastmap is limited to UBI_FM_MAX_BLOCKS.
As I said, in worst case we'd have to scan 192 PEBs, which is a constant.

Thanks,
//richard
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