OS Suspend to disk (Google Summer of Code)

[Andrew Pilloud]

John Baldwin wrote:
> Hmm, the idea of reading the saved image in during early boot is a bit 
> different.  One thing to note is that / does have to be mounted for rc 
> scripts to run (so at least one filesystem will already be mounted).  One 
> thing that might make it easier though, is to just require a separate disk 
> partition (like swap, but a dedicated one, not reusing swap) to dump the RAM 
> image into.  Later on you could extend it to use a pre-allocated file on a 
> filesystem perhaps, but using a dedicated partition for now might reduce the 
> work needed.  I think if you can dump all of RAM to a suspend partition then 
> it actually is not too hard to make the loader just read that into RAM with 
> some sort of header to give an entry point (and to validate it).
>
> Some things that would be really nice for S4 suspend though would be to have 
> the system try to quiesce things more during suspend than it does now.  For 
> example, it would be nice if suspend would flush pending writes to disk and 
> possibly mark filesystems as clean before actually going to sleep to minimize 
> data loss (this would be useful even for S1 or S3 if the battery runs out 
> while suspended).

Thanks for the advice. I don't think / being mounted read only will be a 
problem. As long as the disk is not modified everything should be fine. 
A dedicated partition would make things easier, especially with resuming 
in the boot loader.

There seem to be different advantages to both ways of resuming the 
system. Microsoft products resume in the boot loader, and Linux in a 
script just after the kernel loads. This seems like something that could 
turn into a bikeshed debate rather quickly. I think the best way to 
determine which is better is to implement them both.

As for the file systems, I don't really know what more you can do to 
quiesce the system then is already being done. When you suspend, the 
script rc.suspend is suppose to run. That script runs sync, which should 
take care of flushing pending disk writes. I don't know if marking a 
file system clean is wise or even possible without completely 
un-mounting it.

Andrew Pilloud