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Both InnoDB and PostgreSQL - as well as many other databases - use a technique called multi-version concurrency control (MVCC) to provide transaction isolation: transactions should not see the work of other, uncommitted transactions. MVCC means that, when a row is updated, the database stores both the old and new versions of the row.

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Wednesday, March 13, 2013

Kernel Panic - Fallback to Old Kernel

Few days back, i was testing a kernel module, unfortunately it didn't work and instantly after reboot, kernel Panic occurs. Well i was clueless what goes wrong, but now I cannot select the older kernel version to boot. But, there is a way out, if we choose these first before reboot (off course after installing new kernel), and this is inbuilt in GRUB. This mechanism is called Grub Fall-back. In this case you can set your older/tested kernel as the Fallback kernel in case new kernel panics. And here is how you do it:All you have to do is to open your /boot/grub/grub.conf and add a few lines in it like this

—————————————————————————————————————
default=saved

timeout=15
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
fallback 0 1

title CentOS LIDS (2.6.18-128)
root (hd0,0)
kernel /vmlinuz ro root=/dev/VolGroup00/LogVol00
initrd /initrd-lids-2.6.18-128.img
savedefault fallback

title CentOS (2.6.18-128.el5xen)
root (hd0,0)
kernel /vmlinuz-2.6.18-128.el5 ro root=/dev/VolGroup00/LogVol00
initrd /initrd-2.6.18-128.el5.img
savedefault

—————————————————————————————————————

If the above grub.conf is booted, then first Grub.conf will try to boot the system with “CentOS Testing” and if and only if the kenrel panics, it will call “CentOS Working” and boot the system

Hope this quick howto helps.

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