On Sunday 12 February 2006 20:19, Alan Stern wrote: > On Sun, 12 Feb 2006, Phillip Susi wrote: > > Alan Stern wrote: > > > Both of you are missing an important difference between Suspend-to-RAM > > > and Suspend-to-Disk. > > > > > > Suspend-to-RAM is a true suspend operation, in that the hardware's > > > state is maintained _in the hardware_. External buses like USB will > > > retain suspend power, for instance (assuming the motherboard supports > > > it; some don't). > > > > > > Suspend-to-Disk, by contrast, is _not_ a true suspend. It can more > > > accurately be described as checkpoint-and-turn-off. Hardware state is > > > not maintained. (Some systems may support a special ACPI state that > > > does maintain suspend power to external buses during shutdown, I forget > > > what it's called. And I down't know whether swsusp uses this state.) > > > > I would disagree. The only difference between the two is WHERE the > > state is maintained - ram vs. disk. I won't really argue it though, > > because it's just semantics -- call it whatever you want. > > It's not just semantics. There's a real difference between maintaining > state in the hardware and maintaining it somewhere else. The biggest > difference is that if the hardware retains suspend power, it is able to > detect disconnections. When the system resumes, it _knows_ whether a > device was attached the entire time, as opposed to being unplugged and > replugged (or possibly a different device plugged in!) while the system > was asleep. If the hardware is down completely, there is no way of > telling for certain whether a device attached to some port is the same one > that was there when the system got suspended. > > Another difference is the possibility of remote wakeup. Clearly it can't > happen when there's no power available. Well... sort of. I know that my notebook can be brought up over LAN when it is off - suspended to disk or otherwise. > > > > So for example, let's say you have a filesystem mounted on a USB flash > > > or disk drive. With Suspend-to-RAM, there's a very good chance that > > > the connection and filesystem will still be intact when you resume. > > > With Suspend-to-Disk, the USB connection will terminate when the > > > computer shuts down. When you resume, the device will be gone and your > > > filesystem will be screwed. > > > > This is not true. The USB bus is shut down either way, and provided > > that you have not unplugged the disk, nothing will be screwed when you > > resume from disk or ram. > > Have you actually tried it? I have. > > In any case, it is undeniably true that if the bus is shut down then all > the USB connections are lost. When you resume it will be the same as if > you had unplugged all the USB devices and then replugged them. Not a good > thing to do when they contain mounted filesystems; all the memory mappings > are invalidated. It all depends on the machine I guess. Mine keeps even the CD drive running when it's off! > > (Bear in mind that whether a USB bus gets shut down depends on the > motherboard; some supply suspend power and some don't. It depends on the > USB controller too; some support low-power states other than "completely > off" and others don't.) > > Alan Stern > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- --hackmiester If you can read this, you don't need glasses.
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