moonfire-nvr/guide/schema.md

Moonfire NVR Schema Guide

This document has notes about the Moonfire NVR storage schema. As described in README.md, this consists of two kinds of state:

• a SQLite database, typically <1 GiB. It should be stored on flash if available.
• the "sample file directory", which holds the actual samples/frames of H.264 video. This should be quite large and typically is stored on a hard drive.

Upgrading

The database schema includes a version number to quickly identify if a the database is compatible with a particular version of the software. Some software upgrades will require you to upgrade the database.

Note that in general upgrades are one-way and backward-incompatible. That is, you can't downgrade the database to the old version, and you can't run the old software on the new database. To minimize the corresponding risk, you should save a backup of the old SQLite database and verify the new software works in read-only mode prior to deleting the old database.

Procedure

First ensure there is sufficient space available for four copies of the SQLite database:

• copy 1: the copy to upgrade

• copy 2: a backup you manually create so that you can restore if you discover a problem while running the new software against the upgraded database in read-only mode. If disk space is tight, you can save this to a different filesystem than the primary copy.

• copies 3 and 4: internal copies made and destroyed by Moonfire NVR and SQLite during the upgrade:

  • during earlier steps, possibly duplicate copies of tables, which may occupy space both in the main database and the journal
  • during the final vacuum step, a complete database copy

  If disk space is tight, and you are very careful, you can skip these copies with the --preset-journal=off --no-vacuum arguments to the updater. If you aren't confident in your ability to do this, don't do it. If you are confident, take additional safety precautions anyway:

  • double-check you have the full backup described above. Without the journal any problems during the upgrade will corrupt your database and you will need to restore.
  • ensure you re-enable journalling via pragma journal_mode = wal; before using the upgraded database, or any problems after the upgrade will corrupt your database. The upgrade procedure should do this automatically, but you will want to verify by hand that you are no longer in the dangerous mode.

Next ensure Moonfire NVR is not running and does not automatically restart if the system is rebooted during the upgrade. If you are using systemd with the service name moonfire-nvr, you can do this as follows:

$ sudo systemctl stop moonfire-nvr
$ sudo systemctl disable moonfire-nvr

The service takes a moment to shut down; wait until the following command reports that it is not running:

$ sudo systemctl status moonfire-nvr

Then back up your SQLite database. If you are using the default path, you can do so as follows:

$ sudo -u moonfire-nvr cp /var/lib/moonfire-nvr/db/db{,.pre-upgrade}

By default, the upgrade command will reset the SQLite journal_mode to delete prior to the upgrade. This works around a problem with journal_mode = wal in older SQLite versions, as documented in the SQLite manual for write-ahead logging:

WAL works best with smaller transactions. WAL does not work well for very large transactions. For transactions larger than about 100 megabytes, traditional rollback journal modes will likely be faster. For transactions in excess of a gigabyte, WAL mode may fail with an I/O or disk-full error. It is recommended that one of the rollback journal modes be used for transactions larger than a few dozen megabytes. Beginning with version 3.11.0 (2016-02-15), WAL mode works as efficiently with large transactions as does rollback mode.

Run the upgrade procedure using the new software binary (here referred to as new-moonfire-nvr; if you are installing from source, you may find it as target/release/moonfire-nvr).

$ sudo -u moonfire-nvr new-moonfire-nvr upgrade

Then run the system in read-only mode to verify correct operation:

$ sudo -u moonfire-nvr new-moonfire-nvr run --read-only

Go to the web interface and ensure the system is operating correctly. If you detect a problem now, you can copy the old database back over the new one. If you detect a problem after enabling read-write operation, a restore will be more complicated.

Then install the new software to the path expected by your systemd configuration and start it up:

$ sudo install -m 755 new-moonfire-nvr /usr/local/bin/moonfire-nvr
$ sudo systemctl enable moonfire-nvr
$ sudo systemctl start moonfire-nvr

Hopefully your system is functioning correctly. If not, there are two options for restore; neither are easy:

• go back to your old database. There will be two classes of problems:
  • If the new system deleted any recordings, the old system will incorrectly believe they are still present. You could wait until all existing files are rotated away, or you could try to delete them manually from the database.
  • if the new system created any recordings, the old system will not know about them and will not delete them. Your disk may become full. You should find some way to discover these files and manually delete them.
• undo the changes by hand. There's no documentation on this; you'll need to read the code and come up with a reverse transformation.

Once you're confident of correct operation, delete the unneeded backup:

$ sudo systemctl rm /var/lib/moonfire-nvr/db/db.pre-upgrade

Unversioned to version 0

Early versions of Moonfire NVR (prior to 2016-12-20) did not include the version information in the schema. You can manually add this information to your schema using the sqlite3 commandline. This process is backward compatible, meaning that software versions that accept an unversioned database will also accept a version 0 database.

Version 0 makes two changes:

• it adds schema versioning, as described above.
• it adds a column (video_sync_samples) to a database index to speed up certain operations.

There's a special procedure for this upgrade. The good news is that a backup is unnecessary; there's no risk with this procedure.

First ensure Moonfire NVR is not running as described in the general procedure above.

Then use sqlite3 to manually edit the database. The default path is /var/lib/moonfire-nvr/db/db; if you've specified a different --db_dir, use that directory with a suffix of /db.

$ sudo -u moonfire-nvr sqlite3 /var/lib/moonfire-nvr/db/db
sqlite3>

At the prompt, run the following commands:

begin transaction;

create table version (
  id integer primary key,
  unix_time integer not null,
  notes text
);

insert into version values (0, cast(strftime('%s', 'now') as int),
                            'manual upgrade to version 0');

drop index recording_cover;

create index recording_cover on recording (
  camera_id,
  start_time_90k,
  duration_90k,
  video_samples,
  video_sample_entry_id,
  sample_file_bytes
);

commit transaction;

When you are done, you can restart the service via systemctl and continue using it with your existing or new version of Moonfire NVR.

Version 0 to version 1

Version 1 makes several changes to the recording tables and indices. These changes allow overlapping recordings to be unambiguously listed and viewed. They also reduce the amount of I/O; in one test of retrieving playback indexes, the number of (mostly 1024-byte) read syscalls on the database dropped from 605 to 39.

The general upgrade procedure applies to this upgrade.

Version 1 to version 2 to version 3

This upgrade affects the sample file directory as well as the database. Thus, the restore procedure written above of simply copying back the databae is insufficient. To do a full restore, you would need to back up and restore the sample file directory as well. This directory is considerably larger, so consider an alternate procedure of crossing your fingers, and being prepared to start over from scratch if there's a problem.

Version 2 represents a half-finished upgrade from version 1 to version 3; it is never used.

Version 3 adds over version 1:

• recording of sub streams (splits a new stream table out of camera)
• a per-stream knob flush_if_sec meant to reduce database commits (and thus SSD write cycles). This improves practicality of many streams.
• support for multiple sample file directories, to take advantage of multiple hard drives (or multiple RAID volumes).
• an interlock between database and sample file directories to avoid various mixups that could cause data integrity problems.
• recording the RFC-6381 codec associated with a video sample entry, so that logic for determining this is no longer needed as part of the database layer.
• a simpler sample file directory layout in which files are represented by the same sequentially increasing id as in the database, rather than a separate uuid which has to be reserved in advance.