Linux – Ramdisk

Step 0: 簡介
Step 1: 系統預設的 RAM DISK
Step 2: 修改 ramdisk 大小
Step 3: Format the ramdisk
Step 4: mount RAM Disk
Step 5: 使用 ramdisk

Step 0: 簡介

RAM Disk 就是將系統上一塊 RAM 把它當成是 Disk 來存取.相較於一般的 Disk 他的資料在下次開機後就不能保存了.

Step 1: 系統預設的 RAM DISK

系統在一開始就已經存在 16 個 RAM DISK, 從 ram0 – ram15  大小為 16M,一開始他並不占用任何系統上 RAM 的空間空間,直到我們掛載他為止.

[root @benjr ~]# ll /dev/ram*
lrwxrwxrwx    1 root     root            4 Jun 12 00:31 /dev/ram -> ram1
brw-rw—-    1 root     disk       1,   0 Jan 30  2003 /dev/ram0
brw-rw—-    1 root     disk       1,   1 Jan 30  2003 /dev/ram1
brw-rw—-    1 root     disk       1,  10 Jan 30  2003 /dev/ram10
brw-rw—-    1 root     disk       1,  11 Jan 30  2003 /dev/ram11
brw-rw—-    1 root     disk       1,  12 Jan 30  2003 /dev/ram12
brw-rw—-    1 root     disk       1,  13 Jan 30  2003 /dev/ram13
brw-rw—-    1 root     disk       1,  14 Jan 30  2003 /dev/ram14
brw-rw—-    1 root     disk       1,  15 Jan 30  2003 /dev/ram15
brw-rw—-    1 root     disk       1,  16 Jan 30  2003 /dev/ram16
brw-rw—-    1 root     disk       1,  17 Jan 30  2003 /dev/ram17
brw-rw—-    1 root     disk       1,  18 Jan 30  2003 /dev/ram18
brw-rw—-    1 root     disk       1,  19 Jan 30  2003 /dev/ram19
brw-rw—-    1 root     disk       1,   2 Jan 30  2003 /dev/ram2
brw-rw—-    1 root     disk       1,   3 Jan 30  2003 /dev/ram3
brw-rw—-    1 root     disk       1,   4 Jan 30  2003 /dev/ram4
brw-rw—-    1 root     disk       1,   5 Jan 30  2003 /dev/ram5
brw-rw—-    1 root     disk       1,   6 Jan 30  2003 /dev/ram6
brw-rw—-    1 root     disk       1,   7 Jan 30  2003 /dev/ram7
brw-rw—-    1 root     disk       1,   8 Jan 30  2003 /dev/ram8
brw-rw—-    1 root     disk       1,   9 Jan 30  2003 /dev/ram9
lrwxrwxrwx    1 root     root            4 Jun 12 00:31 /dev/ramdisk -> ram0
[root@benjr ~]#

dmesg | grep -i ramdisk
RAMDISK driver initialized: 16 RAM disks of 16000K size 1024 blocksize

如果我們需要更大的空間此時才要修改一下大小的參數.

Step 2: 修改 ramdisk 大小

要修改 ramdisk 大小的方法很簡單,只要傳送我們想要的大小參數給 kernel(ramdisk_size=xxxxx) 就可以了,我們也可以直接修改 grub.conf 在每次開機都可以生效.

[root @benjr ~]# vi /etc/grub.conf
# grub.conf generated by anaconda
# 
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/hda5
#          initrd /initrd-version.img
#boot=/dev/hda
default=0
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
title Red Hat Linux (2.4.20-20.9)
root (hd0,0)
kernel /vmlinuz-2.4.20-20.9 ro root=LABEL=/ hdc=ide-scsiramdisk_size=32000   
initrd /initrd-2.4.20-20.9.img

重新開機就會生效了.

[root@benjr ~]# dmesg | grep -i ramdisk 
RAMDISK driver initialized: 16 RAM disks of 32000K size 1024 blocksize RAMDISK:

Step 3: Format the ramdisk

因為是 RAM Disk 的關係所以不需要 Format 成 ext3(Journaling file system)我們只需 ext2 的格式就好.選一個你需要的 ram0 ~ ram15,我這裡使用的是 /dev/ram0

[root@benjr ~]# mkfs.ext2 /dev/ram0
mke2fs 1.32 (09-Nov-2002)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
4000 inodes,
…….略……………..
Writing inode tables: done
Writing superblocks and filesystem accounting information: done 
This filesystem will be automatically checked every 22 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

Step 4: mount RAM Disk

[root@benjr ~]# mkdir /mnt/ramdisk
[root@benjr ~]# mount /dev/ram0 /mnt/ramdisk
[root@benjr ~]# df -h | grep ram0
/dev/ram0              31M   386K   29M   2% /mnt/ramdisk

我們一樣可以用 tune2fs 來看一些詳細的資料.

[root@benjr ~]# tune2fs -l /dev/ram0
tune2fs 1.32 (09-Nov-2002)
Filesystem volume name:   none
Last mounted on:          not available
Filesystem UUID:          fbb80e9a-8e7c-4bd4-b3d9-37c29813a5f5 Filesystem magic
…….略……………..

Step 5: 使用 ramdisk

接下來我們就可以像是用硬碟一樣使用這個空間.不需要時直接 umount 就可以了.

[root@benjr ~]# umount -v /mnt/rd /dev/ram0

RAM Disk 的資料會一直保留起來,直到你下次開機前系統都不會動到這一塊的空間.參考資料
http://www.linuxfocus.org/English/November1999/article124.htmlhttp://www.vanemery.com/Linux/Ramdisk/ramdisk.html