SwappingOverNFS < DULG

%SECTION0{name=SwappingOverNFS}% Swapping over NFS

In case that the available memory is not sufficient, i.e. for compiling
the X.org server, and no hard-drive can be attached to the system it _is_
possible to swap over NFS, although it is not quite straightforward.

Usually one would create a blank file, _mkswap_ it and simply do a _swapon swapfile_.
Doing this on a filesystem mounted over NFS, i.e. the ELDK root filesystem, fails however.

With one level of indirection we can trick the kernel into doing it anyway.  First we
create a filesystem image (ext2 will do) on the NFS filesystem and mount it with the aid of the
loopback device.  Then we create a blank swapfile _inside_ of this filesystem and turn
on swapping:

<pre>
bash-2.05b# mount
/dev/nfs on / type nfs (rw)
none on /proc type proc (rw)
bash-2.05b# cd /tmp
bash-2.05b# dd if=/dev/zero of=ext2.img bs=1M count=66
66+0 records in
66+0 records out
bash-2.05b# mkfs.ext2 ext2.img
mke2fs 1.27 (8-Mar-2002)
ext2.img is not a block special device.
Proceed anyway? (y,n) y
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
16920 inodes, 67584 blocks
3379 blocks (5.00%) reserved for the super user
First data block=1
9 block groups
8192 blocks per group, 8192 fragments per group
1880 inodes per group
Superblock backups stored on blocks:
        8193, 24577, 40961, 57345

Writing inode tables: done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 26 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.
bash-2.05b# for i in `seq 0 9` ; do mknod /dev/loop$i b 7 $i ; done
bash-2.05b# mkdir /mnt2
bash-2.05b# mount -o loop ext2.img /mnt2
bash-2.05b# cd /mnt2
bash-2.05b# dd if=/dev/zero of=swapfile bs=1M count=62
62+0 records in
62+0 records out
bash-2.05b# mkswap swapfile
Setting up swapspace version 1, size = 65007 kB
bash-2.05b# free
             total       used       free     shared    buffers     cached
Mem:         14556      14260        296          0        772       9116
-/+ buffers/cache:       4372      10184
Swap:            0          0          0
bash-2.05b# swapon swapfile
bash-2.05b# free
             total       used       free     shared    buffers     cached
Mem:         14556      14172        384          0        784       9020
-/+ buffers/cache:       4368      10188
Swap:        63480          0      63480
bash-2.05b# 
</pre>

Because the ELDK right now has no device nodes for the loopback driver we create them
along the way.  It goes without saying that the _loop_ driver has to be included in
the kernel configuration.  You can check this by looking for a driver for major number
7 (block devices) in _/proc/devices_.
14.3.29. The =appWeb= server hangs OR /dev/random hangs		1. Abstract		14.3.31. Using NFSv3 for NFS Root Filesystem
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