With a D6 instllation I am seeing 'Killed' returned whenever I try to clear the Drupal cache.
Verbose output gives me this:
Initialized Drupal 6.30 root directory at /var/www [notice] Intialized Drupal site default at sites/default [notice] Undefined variable: may_cache feed_field.module 12 [notice] Killed
This is most likely a system issue. I have had similar issues when using middleware on our db servers. For example, if you are using tungsten as a db replicator, it could be configured to disable certain queries or anything that takes longer than a predefined value. This is a mysql or middle ware configuration that is killing a process that takes longer than expected. Similarly, from another SA question
you might be triggering the Linux out-of-memory (OOM) killer. Check dmesg for messages about it. It says which process was killed when this happens.
You can use dmesg | grep php for further information
The string "Killed" doesn't exist in the Drupal 6.30 codebase.
You might have a custom/contrib module which is responsible, or perhaps a Drush extension.
If that's the case, to find it:
grep -nr "Killed" /path/to/drupal/root
grep -nr "Killed" /path/to/drush/extension/folder
(The extension path will probably be ~/.drush)
trigger_error, which is probably the function that's ultimately responsible. You'll see it in database.mysql.inc, but unless your error message contains a query string, you can be pretty sure that's not where it's coming from
I increased the PHP memory_limit to form 196M to 256M in the php.ini and the command now executes as expected so it was a memory issue as suspected by @awm.
I'm not sure what had changed to cause Drush to consume more memory however and whether it's safe to allow php scripts to consume so much memory.
For for the help all.