Creating swap based ramdisks from rc.initdiskless by default

Attila Nagy bra at
Thu Jan 22 06:40:00 PST 2009


In /etc/rc.initdiskless there is a function, which creates memory disks 
in diskless environments:
# Create a generic memory disk
mount_md() {
    /sbin/mdmfs -S -i 4096 -s $1 -M md $2

I have a lot of remote booted diskless and "with disks" machines, which 
rely on this kind of storage. The problem is that the above command 
specifies "-M", so it will create MD_MALLOC disks, which can't be 
swapped out, so it constantly takes away RAM, even if there is only a 
lightly used dataset on the storage, which could be in swap too in 
cases, when there is a memory pressure on the system.

So the question is: what is the rationale behind creating malloc backed 
disks by default, instead of swap-backed ones?
I can only think of two:
- MD_SWAP disks cannot be created, if NO_SWAPPING is enabled in the 
kernel (I haven't checked, if the swap code is enabled (default) and 
there is no swap, I can create swap based disks, like malloc based ones)
- under memory pressure, the swap based disks will be slow, so maybe 
it's not a goot idea to put /etc (in netbooted environment, this is by 
default on memory disks) onto it. BTW, I don't see the difference here 
between a netbooted machine, having /etc on a swap backed memory disk, 
which also holds swap and a locally booted machine, having /etc on a 
disk, which also holds swap. (of course there is a difference, if the 
swap is on another disk(s)

So, are there any objections on changing
    /sbin/mdmfs -S -i 4096 -s $1 -M md $2
    /sbin/mdmfs -S -i 4096 -s $1 md $2


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