Swap on RAM disk in CentOS 6.5 x86_64

RAM ディスク上にスワップを設定できるかの調査。

環境

$ uname -r
2.6.32-431.5.1.el6.x86_64
$ grep CONFIG_SWAP /usr/src/kernels/`uname -r`/.config
CONFIG_SWAP=y
$ grep CONFIG_BLK_DEV_RAM /usr/src/kernels/`uname -r`/.config
CONFIG_BLK_DEV_RAM=y
CONFIG_BLK_DEV_RAM_COUNT=16
CONFIG_BLK_DEV_RAM_SIZE=16384
$ cat /proc/swaps
Filename    Type  Size Used Priority
/dev/dm-1                               partition 835576 0 -1

swap file on ext4 @/tmp -> PASS

$ mount
(近頃(?) /tmp は RAM 扱いではない模様)
$ dd if=/dev/zero of=/tmp/swapfile bs=1M count=1
$ mkswap -f /tmp/swapfile
Setting up swapspace version 1, size = 1020 KiB
no label, UUID=fd6633bd-5d5c-4e6a-99fc-f8ee9741eacc
$ file /tmp/swapfile
/tmp/swapfile: Linux/i386 swap file (new style) 1 (4K pages) size 255 pages
$ chmod 600 /tmp/swapfile
# swapon -v /tmp/swapfile
swapon on /tmp/swapfile
swapon: /tmp/swapfile: found swap signature: version 1, page-size 4, same byte order
swapon: /tmp/swapfile: pagesize=4096, swapsize=1048576, devsize=1048576
# swapon -s
Filename    Type  Size Used Priority
/dev/dm-1                               partition 835576 0 -1
/tmp/swapfile                           file  1016 0 -2
# swapoff /tmp/swapfile

ちなみに、swapon -v オプションを有効にしているとスワップファイルの permission が 600 でないと警告されたが、 600 であっても root 以外がオーナになら警告すべきだが警告されなかった。

$ swapon -V
swapon: (util-linux-ng 2.17.2)

stable/v2.17 の swapon.c は util-linux/util-linux.git [kernel.org] で uid を確認していないが、最新 stable/v2.24 の swapon.c util-linux/util-linux.git [kernel.org] では、uid もチェックしている模様。
ちなみに、util-linux [wikipedia.org] によると、util-linux-ng は util-linux の後継として開発されたが、また util-linux という名前に戻ったらしい。

swap file on ramfs @/dev/ram0 -> FAIL

# mkdir /mnt/ram0
# mount -t ramfs /dev/ram0 /mnt/ram0
# dd if=/dev/zero of=/mnt/ram0/swapfile bs=1M count=1
# mkswap -f /mnt/ram0/swapfile
Setting up swapspace version 1, size = 1020 KiB
no label, UUID=7a23a073-c7a0-4e13-abfb-a0746abe3afc
mkswap: unable to relabel /mnt/ram0/swapfile to system_u:object_r:swapfile_t:s0: Operation not supported
# file /mnt/ram0/swapfile
/mnt/ram0/swapfile: Linux/i386 swap file (new style) 1 (4K pages) size 255 pages
# chmod 600 /mnt/ram0/swapfile
# swapon -v swapfile
swapon on swapfile
swapon: /mnt/ram0/swapfile: found swap signature: version 1, page-size 4, same byte order
swapon: /mnt/ram0/swapfile: pagesize=4096, swapsize=1048576, devsize=1048576
swapon: swapfile: swapon failed: Invalid argument
# dmesg | tail -1
swapon: swapfile has holes
# cp /tmp/swapfile /mnt/ram0/
# chmod 600 /mnt/ram0/swapfile
# swapon -v /mnt/ram0/swapfile
swapon on /mnt/ram0/swapfile
swapon: /mnt/ram0/swapfile: found swap signature: version 1, page-size 4, same byte order
swapon: /mnt/ram0/swapfile: pagesize=4096, swapsize=1048576, devsize=1048576
swapon: /mnt/ram0/swapfile: swapon failed: Invalid argument
# dmesg | tail -2
swapon: swapfile has holes
swapon: swapfile has holes

swap file on ext2 @/dev/ram0 -> PASS

# mkfs /dev/ram0
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
Stride=0 blocks, Stripe width=0 blocks
4096 inodes, 16384 blocks
819 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=16777216
2 block groups
8192 blocks per group, 8192 fragments per group
2048 inodes per group
Superblock backups stored on blocks:
 8193

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

This filesystem will be automatically checked every 23 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.
# mount -t ext2 /dev/ram0 /mnt/ram0
# dd if=/dev/zero of=/mnt/ram0/swapfile bs=1M count=1
# mkswap -f /mnt/ram0/swapfile
Setting up swapspace version 1, size = 1020 KiB
no label, UUID=acaff689-a570-43a7-8c97-2c8c80045bbc
# chmod 600 /mnt/ram0/swapfile
# swapon -v /mnt/ram0/swapfile
swapon on /mnt/ram0/swapfile
swapon: /mnt/ram0/swapfile: found swap signature: version 1, page-size 4, same byte order
swapon: /mnt/ram0/swapfile: pagesize=4096, swapsize=1048576, devsize=1048576
# swapon -s
Filename    Type  Size Used Priority
/dev/dm-1                               partition 835576 0 -1
/mnt/ram0/swapfile                      file  1004 0 -2
# dmesg | tail -1
Adding 1004k swap on /mnt/ram0/swapfile.  Priority:-2 extents:5 across:1020k
# swapoff /mnt/ram0/swapfile

swap partition @/dev/ram0 -> PASS

# mkswap /dev/ram0
Setting up swapspace version 1, size = 16380 KiB
no label, UUID=efac3cf3-9d79-4f68-935c-f4da81845dbb
# swapon -v /dev/ram0
swapon on /dev/ram0
swapon: /dev/ram0: found swap signature: version 1, page-size 4, same byte order
swapon: /dev/ram0: pagesize=4096, swapsize=16777216, devsize=16777216
# swapon -s
Filename    Type  Size Used Priority
/dev/dm-1                               partition 835576 0 -1
/dev/ram0                               partition 16376 0 -2
# dmesg | tail -1
Adding 16376k swap on /dev/ram0.  Priority:-2 extents:1 across:16376k
# swapoff

swap file on tmpfs -> FAIL

# mkdir /mnt/tmp
# mount -t tmpfs tmpfs /mnt/tmp
# dd if=/dev/zero of=/mnt/tmp/tmpfsswap bs=1M count=1
# mkswap -f /mnt/tmp/tmpfsswap
Setting up swapspace version 1, size = 1020 KiB
no label, UUID=ed44d266-47e6-40a2-8ac7-7f925d88fa6e
# chmod 600 /mnt/tmp/tmpfsswap
# swapon -v /mnt/tmp/tmpfsswap
swapon on /mnt/tmp/tmpfsswap
swapon: /mnt/tmp/tmpfsswap: found swap signature: version 1, page-size 4, same byte order
swapon: /mnt/tmp/tmpfsswap: pagesize=4096, swapsize=1048576, devsize=1048576
swapon: /mnt/tmp/tmpfsswap: swapon failed: Invalid argument
# dmesg | tail
(no message invoked by this swapon)

tmpfs 自体 swap を使うのだから swap を tmpfs 上に作れるとは思えなかったが、念のためやってみた。