Eu acho que você quer dizer algo assim:
Carregue o bloco módulo ramdisk, defina o tamanho desejado em blocos usando o parâmetro rd_size=...
.
# modprobe brd rd_size=123456
... após este passo /dev/ram0
existe.
Você agora pode colocar um sistema de arquivos nele.
# 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
30976 inodes, 123456 blocks
6172 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
16 block groups
8192 blocks per group, 8192 fragments per group
1936 inodes per group
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 38 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
Monte e verifique o espaço usado e livre ...
# mount /dev/ram0 /mnt
# df /mnt
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/ram0 119539 1550 111817 2% /mnt