Holding down the power button when the systems freezes

[Sam Varshavchik]

Paul Smith writes:

> Dear All,
>
> When the systems freezes, is it safe to hold down the power button to
> power off the machine? If not, what alternatives do you suggest?
>
> Thanks in advance,

There are two consequences to a hard poweroff, like that:

1) The hard drive does an emergency park of the R/W head. That does incur  
some cost, in terms of wear and tear.

2) The filesystem state is inconsistent. That does not usually result in any  
damage. The filesystem should get automatically re-fscked on the next  
reboot. Still, after a forced poweroff, it is a good idea to "touch  
/forcefsck" and reboot one more time, to force a full fsck on all  
filesystems (which will take some time to complete).

As far as recovering, there are basically two things that can be tried,  
before giving up and yanking the power.

A) Sometimes only X, or the UI is frozen, but the kernel continues to crawl,  
to some extent, underneath. If you were connected to a network, you can try  
ssh-ing in, and running 'poweroff'. Of course, this assumes that you had ssh  
enabled. If you're able to ssh-in and execute 'poweroff', be patient, it may  
take 5-10 minutes for a crippled machine to figure out how to kill off  
everything, and reboot.

B) Execute:

echo 'kernel.sysrq = 1' >/lib/sysctl.d/99-sysrq.conf
sysctl --system

When the machine freezes, try pressing Alt-SysRq-b to force a reboot, if the  
kernel is still alive, somewhere. This will still require a filesystem  
repair, but at least it'll save wear/tear on the hard drives.

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