[Yaffs] Re: yaffs2 status

[Wookey]

+++ Sergei Sharonov [05-04-18 14:57 +0000]:
> Hi,
> 
> > For best mount-time splitting your flash into more than one partition is
> > often smart so you can boot from a small partition and give user feedback
> > whilst mounting a larger data partition. Obviously this doesn't suit all
> > applications.
> 
> Can anybody post any reference numbers? For example it takes me 20+
> minutes to mount 256 MByte NAND partion with a 200 MByte file on it
> when using jffs2.

That does sound remarkably slow. But obviously it does depend a lot of the
hardware speed - what is the platform?

> Any idea what I may gain by moving to yaffs2?

The main technical difference is that YAFFS doesn't need to read the body of
each page (due to not having compression), only the OOB data, during initial
boot scan. And the generally simpler code should be faster.

Some ballpark figures reckon YAFFS(1) is 4 times as fast to boot per MB of
flash, or twice as fast per meg of data (assuming the JFFS2 FS is
compressed at 2:1, which is farily typical). (from
http://www.aleph1.co.uk/talks/yaffs/mgp00025.html which came from collated
posts to this list).

YAFFS2 is up to twice as fast on read (with 2K pages and 16bit bus) - on 512byte
pages and 8 bit bus it is same speed as YAFFS1.

ecc checking can really slow things down if it is done in software not
hardware - although that's primarily on writing, so not important for
initial scan time. 

The best way to get some reliable numbers would of course be to try it - it
should definately be at least twice as fast. We'd be interested to hear what
you find.

Ulitimately the solution to the problem that all log-structured filesystems have
to scan the whole media on boot, and this takes time is checkpointing
(storing intermediate, or shutdown, status), and smarter partitioning so you
can start before scanning the whole media. The latter is available today -
the former needs sponsoring.

Wookey
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