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Taikun OCP Guide

Table of Contents

Database Management

Updating and Migrating the
Database

The default metadata driver for Glance uses SQLAlchemy, which implies there
exists a backend database which must be managed. The
glance-manage binary provides a set of commands for making
this easier.

The commands should be executed as a subcommand of ‘db’:

glance-manage db <cmd> <args>

Note

In the Ocata release (14.0.0), the database migration engine was
changed from SQLAlchemy Migrate to Alembic. This
necessitated some changes in the glance-manage tool. While
the user interface has been kept as similar as possible, the
glance-manage tool included with the Ocata and more recent
releases is incompatible with the “legacy” tool. If you are consulting
these documents for information about the glance-manage
tool in the Newton or earlier releases, please see the legacy-database-management
page.

Migration Scripts

The migration scripts are stored in the directory:
glance/db/sqlalchemy/alembic_migrations/versions

As mentioned above, these scripts utilize the Alembic migration
engine, which was first introduced in the Ocata release. All database
migrations up through the Liberty release are consolidated into one
Alembic migration script named liberty_initial. Mitaka
migrations are retained, but have been rewritten for Alembic and named
using the new naming convention.

A fresh Glance installation will apply the following migrations:

  • liberty-initial
  • mitaka01
  • mitaka02
  • ocata01

Note

The “old-style” migration scripts have been retained in their current
directory
in the Ocata release so that interested operators can
correlate them with the new migrations. This directory will be removed
in future releases.

In particular, the “old-style” script for the Ocata migration, 045_add_visibility.py
is retained for operators who are conversant in SQLAlchemy Migrate and
are interested in comparing it with a “new-style” Alembic migration
script. The Alembic script, which is the one actually used to do the
upgrade to Ocata, is ocata01_add_visibility_remove_is_public.py.

Sync the Database

glance-manage db sync [VERSION]

Place an existing database under migration control and upgrade it to
the specified VERSION or to the latest migration level if VERSION is not
specified.

Note

Prior to Ocata release the database version was a numeric value. For
example: for the Newton release, the latest migration level was
44. Starting with Ocata, database version is a revision
name corresponding to the latest migration included in the release. For
the Ocata release, there is only one database migration and it is
identified by revision ocata01. So, the database version
for Ocata release is ocata01.

This naming convention will change slightly with the introduction of
zero-downtime upgrades, which is EXPERIMENTAL in Ocata, but is projected
to be the official upgrade method beginning with the Pike release. See
zero-downtime for more
information.

Determining the Database
Version

glance-manage db version

This will print the current migration level of a Glance database.

Upgrading an Existing
Database

glance-manage db upgrade [VERSION]

This will take an existing database and upgrade it to the specified
VERSION.

Downgrading an Existing Database

Downgrading an existing database is NOT
SUPPORTED
.

Upgrades involve complex operations and can fail. Before attempting
any upgrade, you should make a full database backup of your production
data. As of the OpenStack Kilo release (April 2013), database downgrades
are not supported, and the only method available to get back to a prior
database version is to restore from backup.

Database Maintenance

Like most OpenStack systems, Glance performs soft deletions
when it deletes records from its database. Depending on usage patterns
in your cloud, you may occasionally want to actually remove such soft
deleted table rows. This operation is called purging the
database, and you can use the glance-manage tool to do
this.

High-Level Database
Architecture

Roughly, what we’ve got in the glance database is an
images table that stores the image id
and some other core image properties. All the other information about
the image (for example: where the image data is stored in the backend,
what projects an image has been shared with, image tags, custom image
properties) is stored in other tables in which the image
id
is a foreign key.

Because the images table keeps track of what image
identifiers have been issued, it must be treated differently from the
other tables with respect to purging the database.

Note

Before the Rocky release (17.0.0), the images table
was not treated differently, which made Glance vulnerable to OSSN-0075,
“Deleted Glance image IDs may be reassigned”. Please read through that
OpenStack Security Note to understand the nature of the problem.

Additionally, the Glance spec Mitigate
OSSN-0075
contains a discussion of the issue and explains the
changes made to the glance-manage tool for the Rocky
release. The Gerrit
review of the spec
contains an extensive discussion of several
alternative approaches and will give you an idea of why the Glance team
provided a “mitigation” instead of a fix.

Purging the Database

You can use the glance-manage tool to purge the
soft-deleted rows from all tables except the images table:

glance-manage db purge

This command takes two optional parameters:

–age_in_days NUM Only purge rows that have been deleted for
longer

than NUM days. The default is 30 days.

–max_rows NUM Purge a maximum of NUM rows from each
table.

The default is 100. All deleted rows are purged if equals -1

Purging the Images Table

Remember that image identifiers are used by other OpenStack services
that require access to images. These services expect that when an image
is requested by ID, they will receive the same data every time. When the
images table is purged of its soft-deleted rows, Glance
loses its memory that those image IDs were ever mapped to some
particular payload. Thus, care must be taken in purging the
images table. We recommend that it be done much less
frequently than the “regular” purge operation.

Use the following command to purge the images table:

glance-manage db purge_images_table

Be sure you have read and understood the implications of OSSN-0075
before you use this command, which purges the soft-deleted rows from the
images table.

It takes two optional parameters:

–age_in_days NUM Only purge rows that have been deleted for
longer

than NUM days. The default is 180 days.

–max_rows NUM Purge a maximum of NUM rows from the
images table.

The default is 100. All deleted rows are purged if equals -1

It is possible for this command to fail with an IntegrityError saying
something like “Cannot delete or update a parent row: a foreign key
constraint fails”. This can happen when you try to purge records from
the images table when related records have not yet been
purged from other tables. The purge_images_table command
should only be issued after all related information has been purged
using the “regular” purge command.

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