Transferring the root filesystem to a ramdisk?

[Cordula's Web]

> in an attempt to save power on FreeBSD servers, I'm
> trying to replace the hdd with a ramdisk. The idea is to:

Thanks to all who helped, on and off-list. To sum up:

* Spin down and up of ATA devices: /usr/ports/sysutils/ataidle
                  or SCSI devices: camcontrol {start | stop} device

* Changing the root device of an already running kernel
  is very hard, and should be avoided at all costs.

  (A suggestion was to modify loader(8), and have
   the kernel use a chunk of pre-loaded memory as initial
   root device a.k.a. md(4). However, this would involve
   a _lot_ of work.)

* /sbin/init could chroot() to the new root device.

  (This is possible, but requires easy access to the underlying
  root fs where init was originally located. => disk must
  spin up on request, so it can't be completely turned off.)

* diskless(8) is possible, though only when connectivity
  is available.

Since our intended use is to embed FreeBSD in moving robots
that are connected with each other through an ad hoc mobile
network, where the radio links are both intermittent and
low-bandwidth; and since the systems are not always well
connected or reachable, diskless(8) is not a solution.

We've finally decided to use flash ram for the root device, and
avoid the complete issue of moving root.

Thanks again for all the help and insights.

-cpghost.

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