Chapter 2
Supporting Perforce:
Backup and Recovery

• Versioned files are files submitted by Perforce users. Versioned files are stored in directory trees called depots. There is one subdirectory under the server's root directory for each depot in your Perforce installation. The versioned files for a given depot are stored in a tree of directories beneath this subdirectory.

• Database files store metadata, including changelists, opened files, client specs, branch specs, and other data concerning the history and present state of the versioned files. Database files appear as db.* files in the top level of the server root directory. Each db.* file contains a single, binary-encoded database table.

Backup and Recovery Concepts

As mentioned earlier, versioned files are stored in subdirectories beneath your Perforce server root, and can be restored directly from backups without any loss of integrity.

The files making up the Perforce database, on the other hand, may not have been in a state of transactional integrity at the moment they were copied to the system backups. Restoring the db.* files from system backups may result in an inconsistent database. The only way to guarantee the integrity of the database after it's been damaged is to reconstruct the db.* files from Perforce checkpoint and journal files.

• A checkpoint is a snapshot or copy of the database at a particular moment in time.

• A journal is a log that records updates made to the database since the last snapshot was taken.

Both the checkpoint and journal are text files, and have the same format. A checkpoint and, if available, its subsequent journal, can restore the Perforce database.

Because the information stored in the Perforce database is as irreplaceable as your versioned files, checkpointing and journaling are an integral part of administering a Perforce server, and should be performed regularly.

Checkpoint files

Versioned files are backed up separately from checkpoints. This means that a checkpoint does not contain the contents of versioned files, and as such, you cannot restore any versioned files from a checkpoint. You can, however, restore all changelists, labels, jobs, and so on, from a checkpoint.

To guarantee database integrity upon restoration, the checkpoint must be as old as, or older than, the versioned files in the depot. This means that the database should be checkpointed, and the checkpoint generation must be complete, before the backup of the versioned files starts.

Regular checkpointing is important to keep the journal from getting too long. Making a checkpoint immediately before backing up your system is good practice.

Creating a checkpoint

p4d -r root -jc

To make the checkpoint, p4d locks the database and then dumps its contents to a file named checkpoint.n in the P4ROOT directory, where n is a sequence number. Before unlocking the database, p4d also copies the journal file to a file named journal.n-1 in the P4ROOT directory (regardless of the directory in which the current journal is stored), and then truncates the current journal. This guarantees that the last checkpoint (checkpoint.n) combined with the current journal (journal) always reflects the full contents of the database at the time the checkpoint was created.

The sequence numbers reflect the roll-forward nature of the journal; to restore databases to older checkpoints, match the sequence numbers. That is, the database reflected by checkpoint.6 can be restored by restoring the database stored in checkpoint.5 and rolling forward the changes recorded in journal.5. In most cases, you're only interested in restoring the current database, which is reflected by the highest-numbered checkpoint.n rolled forward with the changes in the current journal.

To specify a prefix or directory location for the checkpoint and journal, use the -jc option. For instance, if you create a checkpoint with:

p4d -jc prefix

To create a checkpoint without being logged in to the machine running the Perforce server, use the command:

p4 admin checkpoint [-z] [prefix]

You can set up an automated program to create your checkpoints on a regular schedule. Be sure to always check the program's output to ensure that checkpoint creation was started. After successful creation, a checkpoint file may be compressed, archived, or moved onto another disk. At that time or shortly thereafter, back up the versioned files stored in the depot subdirectories.

To restore from a backup, the checkpoint must be at least as old as the files in the depots. (that is, the versioned files can be newer than the checkpoint, but not the other way around.) As you might expect, the shorter this time gap, the better.

If the checkpoint command itself fails, contact Perforce technical support immediately. Checkpoint failure is usually a symptom of a resource problem (disk space, permissions, etc.) that can put your database at risk if not handled correctly.

Journal files

If you have Monday's checkpoint and the journal that was collected from then until Wednesday, those two files (Monday's checkpoint plus the accumulated journal) contain the same information as a checkpoint made Wednesday. If a disk crash were to cause corruption in your Perforce database on Wednesday at noon, for instance, you could still restore the database even though Wednesday's checkpoint hadn't yet been made.

To restore your database, you only need to keep the most recent journal file accessible, but it doesn't hurt to archive old journals with old checkpoints, should you ever need to restore to an older checkpoint.

Enabling journaling on Windows

If you installed Perforce without the installer (for an example of when you might do this, see "Multiple Perforce services under Windows" on page 129), you do not have to create an empty file named journal in order to enable journaling under a manual installation on Windows.

Enabling journaling on UNIX

If P4JOURNAL is left unset (and no location is specified on the command line), the default location for the journal is $P4ROOT/journal.

After enabling journaling

Every checkpoint after your first checkpoint starts a new journal file and renames the old one. The old journal is renamed to journal.n, where n is a sequence number, and a new journal file is created.

By default, the journal is written to the file journal in the server root directory (P4ROOT). Because there is no sure protection against disk crashes, the journal file and the Perforce server root should be located on different filesystems, ideally on different physical drives. The name and location of the journal can be changed by specifying the name of the journal file in the environment variable P4JOURNAL, or by providing the -J filename flag to p4d.

Whether you use P4JOURNAL or the -J journalfile option to p4d, the journal file name can be provided either as an absolute path, or as a path relative to the server root.

Example:  Specifying journal files

Starting the server with:

$ p4d -r $P4ROOT -p 1666 -J /usr/local/perforce/journalfile Perforce Server starting...

requires that you either checkpoint with:

$ p4d -r $P4ROOT -J /usr/local/perforce/journalfile -jc Checkpointing to checkpoint.19... Saving journal to journal.18... Truncating /usr/local/perforce/journalfile...

or set P4JOURNAL to /usr/local/perforce/journal and use

$ p4d -r $P4ROOT -jc Checkpointing to checkpoint.19... Saving journal to journal.18... Truncating /usr/local/perforce/journalfile...

If your P4JOURNAL environment variable (or command-line specification) doesn't match the setting used when you started the Perforce server, the checkpoint is still created, but the journal is neither saved nor truncated. This is highly undesirable!

Disabling journaling

Versioned files

Versioned file formats

Perforce also supports the AppleSingle file format for Macintosh. These files are stored on the server in full and compressed, just like other binary files. They are stored in the Mac's AppleSingle file format; if need be, the files can be copied directly from the server root, uncompressed, and used as-is on a Macintosh.

Because Perforce uses compression in the depot files, do not assume compressibility of the data when sizing backup media. Both text and binary files are either compressed by the Perforce server (denoted by the .gz suffix) before storage, or are stored uncompressed. At most installations, if any binary files in the depot subdirectories are being stored uncompressed, they were probably incompressible to begin with. (For example, many image, music, and video file formats are incompressible.)

Back up after checkpointing

While your versioned files can be newer than the data stored in your checkpoint, it is in your best interest to keep this difference to a minimum; in general, you'll want your backup script to back up your versioned files immediately after successfully completing a checkpoint.

Backup Procedures

1. Verify the integrity of your server and add MD5 digests and file length metadata to any new files:
   p4 verify //...

   You may wish to pass the -q (quiet) option to p4 verify. If called with the -q option, p4 verify produces output only when errors are detected.

   The p4 verify command recomputes the MD5 signatures of all of your archived files and compares them with those stored when the files were first stored, and that all files known to Perforce exist in the depot subdirectories.

   By running p4 verify before the backup, you ensure that you create and store checksums and file length metadata for any files new to the depot since your last backup, and that this information is stored as part of the backup you're about to make.

   Regular use of p4 verify is good practice not only because it enables you to spot any server corruption before a backup is made, but because it also gives you the ability, following a crash, to determine whether or not the files restored from your backups are in good condition.

   Note	If your site is very large, p4 verify may take some time to run, and you may wish to perform this step on a weekly basis rather than on a daily basis. For more about the p4 verify command, see "File verification by signature" on page 47.

2. Make a checkpoint by invoking p4d with the -jc (journal-create) flag, or by using the p4 admin command. Use one of:
   p4d -jc

   or:

   p4 admin checkpoint

   Because p4d locks the entire database when making the checkpoint, you do not generally have to stop your Perforce server during any part of the backup procedure.

   Note

   If your site is very large (say, several gigabytes off db.* files), creating a checkpoint may take a considerable length of time. Under such circumstances, you may wish to defer checkpoint creation and journal truncation until times of low system activity. You might, for instance, archive only the journal file in your nightly backup, and only create checkpoints and roll the journal file on a weekly basis.

3. Ensure that the checkpoint has been created successfully before backing up any files. (After a disk crash, the last thing you want to discover is that the checkpoints you've been backing up for the past three weeks were incomplete!)

   You can tell that the checkpoint command has completed successfully by examining the error code returned from p4d -jc, or by observing the truncation of the current journal file.

4. Once the checkpoint has been created successfully, back up the checkpoint file, the old journal file, and your versioned files.

   (If you don't require an audit trail, you don't actually need to back up the journal. It is, however, usually good practice to do so.)

   Note

   There are rare instances (for instance, users obliterating files during backup, or submitting files on Windows during the file backup portion of the process) in which your depot files may change during the interval between the time the checkpoint was taken and the time at which the depot files get backed up by the backup utility. Most sites are affected by these issues; having the Perforce server available on a 24/7 basis is generally a benefit worth this minor risk, especially if backups are being performed at times of low system activity. If, however, the reliability of every backup is of paramount importance, consider stopping the Perforce server before checkpointing, and restarting it after the backup process has completed. Doing so will eliminate all risk of the system state changing during the backup process.

   You never need to back up the db.* files. Your latest checkpoint and journal contain all the information necessary to re-create them. More significantly, a database restored from db.* files is not guaranteed to be in a state of transactional integrity. A database restored from a checkpoint is.

   Windows

   On Windows, if you make your system backup while the Perforce server is running, you must ensure that your backup program doesn't attempt to back up the db.* files. If you try to back up the db.* files with a running server, Windows locks them while the backup program backs them up. During this brief period, the Perforce server is unable to access the files; if a user attempts to perform an operation that would update the file, the server may fail. If your backup software doesn't allow you to exclude the db.* files from the backup process, stop the server with p4 admin stop before backing up, and restart the server after the backup process is complete.

Recovery Procedures

There are many ways in which systems can fail; while this guide cannot address all of them, it can at least provide a general guideline for recovery from the two most common situations, specifically:

• corruption of your Perforce database only, without damage to your versioned files, and

• corruption to both your database and versioned files.

If you suspect corruption in either your database or versioned files, contact Perforce technical support.

Database corruption, versioned files unaffected

You will need:

• The last checkpoint file, which should be available from the latest P4ROOT directory backup.

• The current journal file, which should be on a separate filesystem from your P4ROOT directory, and which should therefore have been unaffected by any damage to the filesystem where your P4ROOT directory was held.

• Your backup of your versioned files; if they weren't affected by the crash, they're already up to date.

To recover the database

1. Stop the current instance of p4d:
   p4 admin stop

   (You must be a Perforce superuser to use p4 admin.)

2. Rename (or move) the database ("db.*") files:
   mv your_root_dir/db.* /tmp

   There can be no db.* files in the $P4ROOT directory when you start recovery from a checkpoint. Although the old db.* files are never used during recovery, it's good practice not to delete them until you're certain your restoration was successful.

3. Invoke p4d with the -jr (journal-restore) flag, specifying your most recent checkpoint and current journal. If you explicitly specify the server root ($P4ROOT), the -r $P4ROOT argument must precede the -jr flag:
   p4d -r $P4ROOT -jr checkpoint_file journal_file

   This recovers the database as it existed when the last checkpoint was taken, and then apply the changes recorded in the journal file since the checkpoint was taken.

   Note

   If you're using the -z (compress) option to compress your checkpoints upon creation, you'll have to restore the uncompressed journal file separately from the compressed checkpoint. That is, instead of using: p4d -r $P4ROOT -jr checkpoint_file journal_file you'll use two commands: p4d -r $P4ROOT -z -jr checkpoint_file.gz p4d -r $P4ROOT -jr journal_file You must explicitly specify the .gz extension yourself when using the -z flag, and ensure that the -r $P4ROOT argument precedes the -jr flag.

Check your system

Your system state

After recovery, both your database and versioned files should reflect all changes made up to the time of the crash, and no data should have been lost.

Both database and versioned files lost or damaged

You will need:

• The last checkpoint file, which should be available from the latest P4ROOT directory backup.

• Your versioned files, which should be available from the latest P4ROOT directory backup.

• Your current journal file. The journal contains a record of changes to the metadata and versioned files that occurred between the last backup and the crash; because you'll be restoring a set of versioned files from a backup taken before that crash, the checkpoint alone contains the metadata useful for the recovery, and the information in the journal is of limited or no use.

To recover the database

1. Stop the current instance of p4d:
   p4 admin stop

   (You must be a Perforce superuser to use p4 admin.)

2. Rename (or move) the corrupt database ("db.") files:
   mv your_root_dir/db.* /tmp

   The corrupt db.* files aren't actually used in the restoration process, but it's safe practice not to delete them until you're certain your restoration was successful.

3. Invoke p4d with the -jr (journal-restore) flag, specifying only your most recent checkpoint:
   p4d -r $P4ROOT -jr checkpoint_file

   This recovers the database as it existed when the last checkpoint was taken, but does not apply any of the changes in the journal file. (The -r $P4ROOT argument must precede the -jr flag.)

   The database recovery without the roll-forward of changes in the journal file brings the database up to date as of the time of your last backup. In this scenario, you do not want to apply the changes in the journal file, because the versioned files you restored reflect only the depot as it existed as of the last checkpoint.

To recover your versioned files

1. After recovering the database, you then need to restore the versioned files according to your system's restoration procedures (for instance, the UNIX restore(1) command) to ensure that they are as new as the database.

Check your system

Note that files submitted to the depot between the time of the last system backup and the disk crash will not be present in the restored depot.

Your system state

• In most cases, the latest revisions of such files can be restored from the copies still residing in your users' client workspaces.

• In a case where only your versioned files (but not the database, which may have resided on a separate disk and remained unaffected by the crash) were lost, you may also be able to make a separate copy of your database and apply your journal to it in order to examine recent changelists to track down files submitted between the last backup and the disk crash.

Ensuring system integrity after any restoration

p4 verify -q //...

If any versioned files are reported as MISSING by the p4 verify command, you'll know that there is information in the database concerning files that didn't get restored. The usual cause is that you restored from a checkpoint and journal made after the backup of your versioned files. (that is, that your backup of the versioned files was older than the database.)

If (as recommended) you've been using p4 verify as part of your backup routine, you can run p4 verify on the server after restoration to reassure yourself that your restoration was successful.

If you have any difficulties restoring your system after a crash, contact Perforce Technical Support for assistance.