Commits on May 8, 2019

1. mxgrub: Add --reboot …

   Add a new command `mxgrub --reboot` which reboots to the selected kernel
   via kexec.
   
   Notes:
   
       * Do a sync before triggering the reboot, so that services are still
       available while the bulk of dirty pages is written, which can take
       a relevant time on big machines.
   
       * kexec (without -l, -p, -e, or -f) loads the kernel and than calls
       shutdown, which is handled by systemd. So this is a full shutdown ;
       systemd will stop services, unmount filesystems and sync all disks
       (again). It just recognizes the loaded kernel and calls the
       equivalent of (`kexec -e` instead of `reboot -f`) in the very last
       step. We just avoid the EFI/BIOS initialization and grub and kernel
       loading.
   
       * When an nfs server restarts, clients currently get 90 seconds to
       renew their leases. Some nfs file operations from the clients are
       blocked during this time, so this adds to the time, a workstation
       appears to be frozen to the user. We can consider to reduce this
       lease_time of our servers to a lower value. The disadvantage is,
       that the clients need to send more RENEW request (2/3 of the lease
       time, so every minute currently) and might lose lock state if the
       network is interrupted for a longer time.
   
       * Strictly speaking, kexec() has not much to do with grub. However,
       in our implementation of the grub based boot it is related because
       the record of which kernel to boot next (the selected kernel)
       is kept in the grub environment file. So if we want the kexec based
       reboot to start the same kernel as a reboot sequence over grub
       would do, we need to read the grub environment file. This should be
       enough excude to implement the command in mxgrub.
   
       * Loading of the new kernel might fail on machines where not enough
       continuous physical memory is availble because of fragmentation.
   
   Timings:
   
       * reboot theinernet (to lightdm login prompt) :  47 ->  22 seconds
       * reboot claptrap (to console login prompt)   : 160 ->  59 seconds
       * reboot claptrap (to NFS server available)   : 165 ->  43 seconds
       * reboot claptrap (to locks available)        : 262 -> 141 seconds

   

   donald committed May 8, 2019
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