RAM size on Target Server

[algreco72]

Hi , I m trying REAR and if I have the Target Server where I want to have recovery of an ISO file b uild by REAR the system reboot because the physical RAM is lower than the one on the original/Source Server. Any workaround to make the ISO image to work on a server with lower RAM? The problem is I have 2 servers , on Primary site the RAM is higher than on the Server on the secondary site, any workaround to fix the issue? I 'd like to build an ISO file to work on a Server with lower RAM than the primary one.

Thanks everyone for the help,

[jsmeix]

How much RAM does each one of your servers exactly have?

The whole ReaR recovery system runs in main memory via a ramdisk.

The main needed space in the ReaR recovery system is nowadays the
firmware files for various hardware (e.g. network interface cards or graphics cards)
that are by default all included in the recovery system to let it work on all hardware,
see FIRMWARE_FILES in usr/share/rear/conf/default.conf

Another major needed space in the ReaR recovery system are the kernel modules
for various hardware and various functionality (e.g. access of various filesystems)
that are by default all included in the recovery system to let it work in general,
see MODULES in usr/share/rear/conf/default.conf

On my openSUSE Leap 15.2 system I get this ReaR recovery system sizes:
By default:

# du -hs /tmp/rear.EHYoS4gnFtyPRE4/rootfs/
953M    /tmp/rear.EHYoS4gnFtyPRE4/rootfs/

With FIRMWARE_FILES=( 'no' ):

# du -hs /tmp/rear.GuUugJjsXSfC3G2/rootfs/
454M    /tmp/rear.GuUugJjsXSfC3G2/rootfs/

With FIRMWARE_FILES=( 'no' ) and MODULES=( 'loaded_modules' ):

# du -hs /tmp/rear.OF6uW70Cp1l2yv1/rootfs/
201M    /tmp/rear.OF6uW70Cp1l2yv1/rootfs/

So I get about 200M for the bare ReaR recovery system
plus about 250M for all kernel modules
plus about 500M for the firmware files.

My 953M default ReaR recovery system won't work with only 1G RAM.

Furthermore when you use special third-party backup software
that could also much increase the ReaR recovery system size
because to do the backup restore during "rear recover" the restore tool
and all what it needs to run (libraries, config files, whatever else)
must be included in the ReaR recovery system.
Depending on what third-party backup software you use
its restore tool and all what it needs to run could be rather big.
See "Relax-and-Recover versus backup and restore" in
https://en.opensuse.org/SDB:Disaster_Recovery

To inspect the ReaR recovery system contents after "rear mkrescue/mkbackup"
use KEEP_BUILD_DIR="yes" or run ReaR in debug mode to keep the build area,
see KEEP_BUILD_DIR in usr/share/rear/conf/default.conf

See also
#2386 (comment)

[algreco72]

Hello we tested using Primary server With 128gb and recovery server 32 gb we receive a panic ram error message and the recovery doesnt work. It seems to have the Recovery server with the same or higher ram of Primary server is mandatory requirement to have rear working . Any additional suggestions? Thanks in advance

[jsmeix]

I run ReaR on my QEMU/KVM test VMs with 1G RAM or 2G RAM.
32G RAM is much more than what ReaR normally ever needs.
So it seems the root cause of your particular issue here is something special
but I have no idea what that "something special" actually is in this case.
To have a chance to imagine what the root cause could be in this case see
https://raw.githubusercontent.com/rear/rear/master/.github/ISSUE_TEMPLATE.md
what information we usually need to be able to analyze a ReaR issue.
This list is automatically shown when you use the [New issue] button on
https://github.com/rear/rear/issues

To me it looks as if it is "something special" with your specific servers hardware.
Perhaps something with special kernel things like special kernel modules
or special kernel command line options or even special kernels for those servers?
The ReaR recovery system uses the kernel from the system where "rear mkrescue"
was run so this kernel must also work on the replacement hardware,
cf. the section "Fully compatible replacement hardware is needed" in
https://en.opensuse.org/SDB:Disaster_Recovery
So perhaps your replacement server is not sufficiently compatible
or you may need adapted kernel command line options for it
cf. KERNEL_CMDLINE and COPY_KERNEL_PARAMETERS
in usr/share/rear/conf/default.conf

[gdha]

Have you tested out if you could boot the ISO on the primary server? And, check the rear.log file for errors.