paradis@encore.UUCP (Jim Paradis) (04/20/87)
Forget what I said earlier about using "chroot" in init to
change the root filesystem system-wide. I keep forgetting
that one of the distinguishing features of MINIX is that
EVERYTHING IS A PROCESS! Init is the granddaddy of all USER
processes only. Therefore, you can do a chroot in init just
fine, but that won't help when it comes to do an exec and the
MEMORY MANAGER has to find the program you're asking it to
execute. Sigh.
Still, for those of you who want to use your hard disk as a
root filesystem so as to free up some memory, there IS hope.
In fact, the changes to the kernel to do so are pretty easy.
Here's what I did:
(1) Change the parameter ROOT_DEV in h/const.h to be the device
that I wanted to use as the root filesystem (0x31 for /dev/hd1)
(2) Replace load_ram() with a new function: mk_ramdisk(size). This
function is a gutted version of load_ram() which simply sets up
the RAM-disk device but puts nothing in it. One possible application
of this is to declare a small ramdisk device (e.g. 100K) and mount
it on /tmp to make compiles go faster. Of course, you could also
make a trivial RAM-disk instead and use the extra memory to increase
the size of the buffer cache instead...
(3) Make sure I had all the root filesystem stuff on my hard disk.
I'm now running with my hard disk as the root filesystem, with no ill
effects (aside from a lot more HD accesses than before... I hope to
fix that by using a bigger buffer cache).
Following is a listing of mk_ramdisk() for those who want it...
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/*======================================================================*
* mk_ramdisk *
*======================================================================*/
PRIVATE mk_ramdisk(size)
int size;
{
/* This function sets up the RAM-disk device to have the specified size.
* Note that no data is actually put on the RAM-disk -- not even
* filesystem information. Therefore, if it is desired to use the RAM
* disk as a filesystem, one must do a mkfs first. /etc/rc would be an
* ideal place to do this.
*/
phys_clicks rm_clicks, init_org, init_text_clicks, init_data_clicks;
extern phys_clicks data_org[INFO + 2];
/* Get size of iINIT by reading block on diskette where 'build' put it */
init_org = data_org[INFO];
init_text_clicks = data_org[INFO + 1];
init_data_clicks = data_org[INFO + 2];
if (size > MAX_RAM) panic("RAM disk is too big. # blocks = ",size);
ram_clicks = size * (BLOCK_SIZE / CLICK_SIZE);
/* Tell MM the origin and size of INIT, and the amount of memory used for the
* system plus RAM disk combined, so it can remove all of it from the map.
*/
m1.m_type = BRK2;
m1.m1_i1 = init_text_clicks;
m1.m1_i2 = init_data_clicks;
m1.m1_i3 = init_org + init_text_clicks + init_data_clicks + ram_clicks;
m1.m1_p1 = (char *)init_org;
if (sendrec(MM_PROC_NR, &m1) != OK) panic("FS Can't report to MM", NO_NUM);
/* Tell RAM driver where RAM disk is and how big it is */
m1.m_type = DISK_IOCTL;
m1.DEVICE = RAM_DEV;
m1.POSITION = (long) init_org + (long) init_text_clicks + init_data_clicks;
m1.POSITION = m1.POSITION << CLICK_SHIFT;
m1.COUNT = size;
if (sendrec(MEM, &m1) != OK) panic("Can't report size to MEM", NO_NUM);
printf("RAM disk set up.\n");
}
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