How To Make Your Xen-PAE Kernel Work With More Than 4GB RAM (Debian Etch With GRUB)
Version 1.0
Author: Falko Timme
If you have a server with more than 4GB RAM and want to install a 32bit Debian Etch on it (following this tutorial: Debian Etch And Xen From The Debian Repository), you'd expect the Xen-PAE kernel to see all your RAM because the Xen-PAE kernel supports up to 64GB RAM. In fact, it recognizes only about 3.3GB RAM due to a bug in the GRUB bootloader. This article explains how you can fix GRUB so that all your RAM gets recognized.
I do not issue any guarantee that this will work for you!
1 Preliminary Note
This bug exists on 64bit Debian systems as well - the default Debian Xen kernel also recognizes only 3.3GB of RAM. It's possible that the following procedure works for 64bit Debian Etch systems as well, although I have tested it only on a 32bit Debian Etch system.
I assume that you have already set up Xen on your system according to this tutorial: Debian Etch And Xen From The Debian Repository
I have tested this on a system with 6GB RAM.
Don't follow this tutorial if you're using another bootloader than GRUB (e.g. lilo)!
2 Check Your Memory
You can run
cat /proc/meminfo
to see how much memory your system recognizes:
Debian-40-etch-32-minimal:~# cat /proc/meminfo
MemTotal: 3468288 kB
MemFree: 3237948 kB
Buffers: 55884 kB
Cached: 46124 kB
SwapCached: 0 kB
Active: 68820 kB
Inactive: 41412 kB
HighTotal: 2731012 kB
HighFree: 2667548 kB
LowTotal: 737276 kB
LowFree: 570400 kB
SwapTotal: 2104440 kB
SwapFree: 2104440 kB
Dirty: 3524 kB
Writeback: 0 kB
AnonPages: 8196 kB
Mapped: 3932 kB
Slab: 10140 kB
PageTables: 304 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
CommitLimit: 3838584 kB
Committed_AS: 50368 kB
VmallocTotal: 116728 kB
VmallocUsed: 3776 kB
VmallocChunk: 112612 kB
Debian-40-etch-32-minimal:~#
As you see, only about 3.3GB are recognized by the system...
3 Rebuild GRUB
Now let's rebuild the GRUB bootloader. We need the package dpkg-dev for it:
apt-get install dpkg-dev
Then we download the GRUB sources...
apt-get source grub
... and install all requirements to rebuild GRUB:
apt-get build-dep grub
We must modify the stage2/common.c file in the GRUB source directory. First we make a copy of that file:
cd grub-0.97/
cp -a stage2/common.c stage2/common.c.original
Then we open it:
vi stage2/common.c
Around line 143, you should find this section:
[...] #ifndef STAGE1_5 unsigned long cont, memtmp, addr; int drive; #endif [...] |
Modify it as follows:
[...] #ifndef STAGE1_5 unsigned long memtmp, addr; volatile unsigned long cont; int drive; #endif [...] |
Save the file, then build the new GRUB package:
debian/rules binary
Afterwards we can install the new GRUB package as follows:
dpkg -i ../grub_0.97-27_i386.deb
Now we must install GRUB on our hard drives. I'm using two hard drives, /dev/sda and /dev/sdb (for software RAID1), so I run
grub-install /dev/sda
grub-install /dev/sdb
If you only got one hard drive, then run grub-install only once. Make sure that you use the correct device name for your hard drive (for example, instead of /dev/sda, yours might be /dev/hda - you can find out about the hard drive names by running
fdisk -l
).
Finally, it's time to reboot:
reboot
4 Check Your Memory Again
After the reboot, it's time to check the memory again. If everything went well, the system should now recognize the full amount of RAM:
cat /proc/meminfo
Debian-40-etch-32-minimal:~# cat /proc/meminfo
MemTotal: 6067200 kB
MemFree: 5902700 kB
Buffers: 1596 kB
Cached: 13964 kB
SwapCached: 0 kB
Active: 19316 kB
Inactive: 4408 kB
HighTotal: 5329732 kB
HighFree: 5299600 kB
LowTotal: 737468 kB
LowFree: 603100 kB
SwapTotal: 2104440 kB
SwapFree: 2104440 kB
Dirty: 784 kB
Writeback: 0 kB
AnonPages: 8160 kB
Mapped: 3932 kB
Slab: 8616 kB
PageTables: 324 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
CommitLimit: 5138040 kB
Committed_AS: 50388 kB
VmallocTotal: 116728 kB
VmallocUsed: 3776 kB
VmallocChunk: 112612 kB
Debian-40-etch-32-minimal:~#
As you see, my system is now recognizing my full 6GB of RAM.
5 Links
- Debian: http://www.debian.org
