On Sun, 2008-04-20 at 17:04 +0100, Jamie Lokier wrote:
Josh Boyer wrote:
Is it even a good idea? The UBI (version 1 :-) initial scan is not fast for large flash, and if the bootloader does it too, that's twice as much time to boot.
It's a good idea, yes. Particularly if you want to boot from NAND flash.
Can you define "large device"? It only scans the first 1 or 2 pages for each eraseblock to build up the volume tables. That really isn't that slow...
I was thinking this:
Hamish Moffatt wrote (Message-ID: 20080407073227.GA6317@cloud.net.au):
Sorry I should've said 512MiB perhaps: 512 megabytes. UBI attach time appears to be about 6 seconds.
If 6 seconds is as fast as it can be done, annoying but fair enough.
You should read that thread a bit more carefully. The scan time is highly dependent upon the NAND driver beneath UBI. For example, a UBI scan/attach on a 1GiB device on OLPC was 2 seconds.
Adding _another_ 6 seconds to the boot time seems a lot to me.
You mean adding another "X amount of time depending on factors outside of UBI's control."
The only ways I see to improve the speed in general are:
Partition the NAND into multiple independent UBIs, so the boot loader doesn't have the scan the whole chip, but that is obviously not so good for wear levelling. (It's probably alright, though, if it's like the /boot partition on a standard Linux distro - not updated very often.)
Change UBI's data structure so that the number of pages it needs to read is a sub-linear function of the number of erase blocks. I think this is what's meant by 'UBI 2'.
To remove the double scan:
However, if there was a protocol for bootloader to pass the scan results to the booted kernel, that would be very nice.
Maybe. I was never fully convinced of that.
I can understand the hesitation, but I think 6 seconds just to find the kernel - especially when doing a 'disk resume' - is quite a lot.
You should really stop quoting this 6 second number.
Anyway, passing the scan results from a bootloader becomes very involved. You have to store it somewhere, probably preserving a section of DRAM, which isn't always easy.
Note that I haven't tried UBI myself yet. I'm going on what has been written to the list so far, as quoted above.
Maybe you should try it :).
josh