Date: Thu, 22 Jan 2009 15:39:54 +0100 From: Attila Nagy <bra@fsn.hu> To: freebsd-current@FreeBSD.org Subject: Creating swap based ramdisks from rc.initdiskless by default Message-ID: <4978853A.2000107@fsn.hu>
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Hello,
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
to
/sbin/mdmfs -S -i 4096 -s $1 md $2
?
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