5 sec boot

Mitch Bradley wmb at laptop.org
Sat Oct 4 22:32:27 EDT 2008

Deepak Saxena wrote:
> On Oct 04 2008, at 15:49, Mitch Bradley was caught saying:
>> c) Raw FLASH read time maxes out at 20 MB/sec.  But you don't get that  
>> speed from the filesystem; JFFS2 is good for between 5 and 10 MB/sec.
>> Considering all the intricacies of JFFS2, my best guess is that it's  
>> going to be close to a wash whether the kernel + initrd is stored in  
>> compressed or uncompressed form.
>> OTOH, if the kernel + initrd were in a separate partition in e.g. romfs  
>> format, where OFW could just blast them into memory without doing JFFS2  
>> node processing, we could probably get close to the 20 MB/sec speed.
> We can probably just get away with making all of /boot into a
> romfs; however, do we even need to bother with a filesystem 
> representation of the images?  We could have four partions 
> (kernel0, kernel1, initrd0, initrd1) that contain the binary 
> data for current and alternate images and some sort of way
> to tell which one is current and which one is alternate.
> ~Deepak
I have considered something like that off and on.  It's sort of nice to 
have a definite length for the images.  There are ways around that, but 
they are a bit ugly at some level.  It's sort of a tossup at some level.

One difficultly with having a lot of partitions is that it makes it more 
likely that you will encounter the resizing issue.

On a related topic, I would like to see us start bundling the initrd 
into the kernel image.  It's certainly possible to do that with existing 
kernel mechanisms.  The two pieces are interdependent enough that they 
really have to be updated together, at which point it's really better to 
have them in the same image.

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