Monday, October 23, 2006

step-by-step: how to set up a large ramdisk on CentOS 4.4/i386

On our new database server running CentOS 4.4 on a Dell PE 6850, a 2G ramdisk is needed. The option to use 'tmpfs' was considered and forfeited due to its potential of using disk swap.
  • To make the ramdisk 2G apiece, a kernel parameter (ramdisk_size) needs to be passed ( grub.conf or lilo.conf) and a quick reboot is required to make it effective. The ramdisk by default is 16M in size.
Here is my grub.conf entry:
kernel /vmlinuz-2.6.9-42.ELsmp ro root=LABEL=/ console=ttyS0,57600n8 console=tty1 ramdisk_size=2097152
  • To make the ramdisk accessible to the database after reboot, I cooked a chkconfig-compliant init script
# $Id: rc.ramdisk4syb,v 1.00 2006/10/23 13:57:14 jer Exp $
# rc file for set up ramdisk fs for sybase to use for tempdb
# For Redhat-ish systems
# chkconfig: 2345 30 70
# processname: ramdisk4syb
# config: none
# description: set up ramdisk for sybase to use for tempdb
case $1 in
test -d /mnt/ram1 || mkdir /mnt/ram1
mke2fs -m 1 -b 1024 /dev/ram1 >/tmp/ramdisk.mke2fs.log &&
mount -t ext2 /dev/ram1 /mnt/ram1 &&
chmod 770 /mnt/ram1 && chown sybase:dba /mnt/ram1
umount /mnt/ram1
echo "Usage: $0 start|stop"
  • to pin the init script to run levels
chkconfig --add ramdisk4syb
  • to test the init script before reboot
/etc/rc3.d/S30ramdisk4syb start && df -k /mnt/ram1 && ls -ld /man/ram1
  • to make all this effective by reboot.
init 6
  • to verify the resultant ramdisk file system
df -k /mnt/ram1 && ls -ld /man/ram1

Much to my surprise, the ramdisk is not mounted after the reboot! The mke2fs log showed the mke2fs was successful. When I attempted to mount it manually, I got
# mount /dev/ram1 /mnt/ram1 mount: wrong fs type, bad option, bad superblock on /dev/ram1, or too many mounted file systems

In the log, I did notice the block size was 4K was instead of the 1K block size I observed with a 512M ramdisk.
Block size=4096 (log=2) Fragment size=4096 (log=2) 262144 inodes, 524288 blocks

Since the Sybase ASE uses 2K page size, I tried mke2fs using a 2K block. No joy! Mounting attempts were met with the same errors.

1K block size is known to have worked for 512M ramdisk, so I gave it a try. Bingo!
#mke2fs -b 1024 /dev/ram1 Block size=1024 (log=0) Fragment size=1024 (log=0) 262144 inodes, 2097152 blocks
Now we have a working ramdisk as below:
/dev/ram1 2063763 3086 1955820 1% /mnt/ram1
I don't see much of a point to waste space on 5% reserved for superuser. So, I tuned it down to 1%
# tune2fs -m 1 /dev/ram1
/dev/ram1 2063763 3086 2039706 1% /mnt/ram1

From the 'free' output, I am a bit surprised to see the 2G is not counted as 'used'.
total used free shared buffers cached Mem: 16627288 231236 16396052 0 48300 89240 -/+ buffers/cache: 93696 16533592 Swap: 16779884 0 16779884

bootparam man[7] page actually says " These days ram disks use the buffer cache, and grow dynamically." If so, ramdisk truly behaves similar to tmpfs at this point, except
  • ramdisk's maximum size (the reserved?) needs a reboot to adjust, while tmpfs can be adjusted on the fly.
  • ramdisk won't have swap in the picture at all. [good thing]
However, by growing dynamically, FWIW, it may not able to grow to its full reserved size when it needs it [bad bad thing], if physical RAM is used up by some real processes already. Questions yet to be answered:
  • Does the 2.6 kernel or other series know or set ramdisk has higher priority on its claim to thus reserved memory?!
  • why ext2 file system made with block sizes other than 1K failed to be mounted ? ramdisk is the only case here?

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