Random Musings of an Insane Mind

A few days ago a client came to me and asked how they could back up a lot of data on a nightly basis that only had a few changes. Many solutions with their current hosting providers were discussed, Amazon’s S3 and Glacier, but, none gave him the flexibility he was after. So, the logical conclusion here was Amazon EC2 + Elastic Block store (EBS).

We created a micro instance of Debian, logged in and added rsync, created a volume large enough to hold his backup with some room for growth and with a little command line magic he was able to mirror his data to EBS. Of course, the instance didn’t need to be running all the time so he would need to start it, run the rsync, shut it down.

After some thought I decided it would be easy enough to write a quick Python script using boto to start the instance, rsync the volume and stop the instance. If he needed access to the instance he could start it manually and log in when needed. Now, his backups could be run via cron on a regular basis.

I put the code on GitHub: https://github.com/cd34/spawncamping-octo-ninja

Most of the instance startup situations are handled and so far it seems robust.

I created an EC2 instance a while back to test a theory and had some time this evening to take a look at it again. I went to start the instance and:

    Xen Minimal OS!
  start_info: 0xac4000(VA)
    nr_pages: 0x26700
  shared_inf: 0x7de16000(MA)
     pt_base: 0xac7000(VA)
nr_pt_frames: 0x9
    mfn_list: 0x990000(VA)
   mod_start: 0x0(VA)
     mod_len: 0
       flags: 0x0
    cmd_line: root=/dev/sda1 ro 4
  stack:      0x94f860-0x96f860
MM: Init
      _text: 0x0(VA)
     _etext: 0x5ffbd(VA)
   _erodata: 0x78000(VA)
     _edata: 0x80ae0(VA)
stack start: 0x94f860(VA)
       _end: 0x98fe68(VA)
  start_pfn: ad3
    max_pfn: 26700
Mapping memory range 0xc00000 - 0x26700000
setting 0x0-0x78000 readonly
skipped 0x1000
MM: Initialise page allocator for c01000(c01000)-26700000(26700000)
MM: done
Demand map pfns at 26701000-2026701000.
Heap resides at 2026702000-4026702000.
Initialising timer interface
Initialising console ... done.
gnttab_table mapped at 0x26701000.
Initialising scheduler
Thread "Idle": pointer: 0x2026702010, stack: 0x26640000
Initialising xenbus
Thread "xenstore": pointer: 0x20267027c0, stack: 0x26650000
Dummy main: start_info=0x96f960
Thread "main": pointer: 0x2026702f70, stack: 0x26660000
"main" "root=/dev/sda1" "ro" "4" 
vbd 2049 is hd0
******************* BLKFRONT for device/vbd/2049 **********


backend at /local/domain/0/backend/vbd/3617/2049
Failed to read /local/domain/0/backend/vbd/3617/2049/feature-barrier.
Failed to read /local/domain/0/backend/vbd/3617/2049/feature-flush-cache.
16777216 sectors of 512 bytes
**************************

    [H
    [J  Booting '3.9-1-amd64'



root (hd0)

 Filesystem type is ext2fs, using whole disk

kernel /boot/vmlinuz-3.9-1-amd64 root=/dev/xvda1 ro 

initrd /boot/initrd.img-3.9-1-amd64



ERROR Invalid kernel: xc_dom_probe_bzimage_kernel: unknown compression format

xc_dom_bzimageloader.c:394: panic: xc_dom_probe_bzimage_kernel: unknown compression format
ERROR Invalid kernel: xc_dom_find_loader: no loader found

xc_dom_core.c:536: panic: xc_dom_find_loader: no loader found
xc_dom_parse_image returned -1



Error 9: Unknown boot failure



Press any key to continue...

This happens when you use a kernel compiled with .xz and the Xen Instance you’re using has the old Xen hypervisor which cannot support .xz.

What you would normally do to fix this is take another instance in the same availability zone, detach the EBS volume from the broken instance, attach the EBS volume to the other instance, make the changes to grub or put a new kernel on, detach the volume from the new instance, attach the volume to the old instance, and restart.

However, if you’re not using your own AMI, you might get the following message:

'vol-xxxxxxxx' with Marketplace codes may not be attached as a secondary device.

in which case I believe you’re stuck.