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Clarify max_local_block_devices config option usage

The previous wording for the max_local_block_devices config option made
it sound like if you set it to 0 that you could restrict to only booting
instances from volumes, which is not the case - but is apparently
something that some operators want (this came up in the related bug and
in IRC).

This updates the help text with a description that ndipanov put in the
bug report which makes the 0 case more clear.

Co-Authored-By: Nikola Dipanov <>

Related-Bug: #1436314

Change-Id: I01a9e2f193b50285966690efd771da432a7ad40c
Matt Riedemann 5 years ago
1 changed files with 9 additions and 2 deletions
  1. +9

+ 9
- 2
nova/compute/ View File

@@ -114,8 +114,15 @@ compute_opts = [
help='Maximum number of devices that will result '
'in a local image being created on the hypervisor node. '
'Setting this to 0 means nova will allow only '
'boot from volume. A negative number means unlimited.'),
'A negative number means unlimited. Setting '
'max_local_block_devices to 0 means that any request that '
'attempts to create a local disk will fail. This option '
'is meant to limit the number of local discs (so root '
'local disc that is the result of --image being used, and '
'any other ephemeral and swap disks). 0 does not mean '
'that images will be automatically converted to volumes '
'and boot instances from volumes - it just means that all '
'requests that attempt to create a local disk will fail.'),

ephemeral_storage_encryption_group = cfg.OptGroup(