m4_comment([$Id: archival.so,v 10.56 2005/02/10 20:02:41 bostic Exp $])

m4_ref_title(m4_tam Applications,
    Database and log file archival,
    [archival in @transaction protected applications, catastrophic recovery],
    transapp/checkpoint, transapp/logfile)

m4_p([dnl
The third component of the administrative infrastructure, archival for
catastrophic recovery, concerns the recoverability of the database in
the face of catastrophic failure.  Recovery after catastrophic failure
is intended to minimize data loss when physical hardware has been
destroyed -- for example, loss of a disk that contains databases or log
files.  Although the application may still experience data loss in this
case, it is possible to minimize it.])

m4_p([dnl
First, you may want to periodically create snapshots (that is, backups)
of your databases to make it possible to recover from catastrophic
failure.  These snapshots are either a standard backup, which creates a
consistent picture of the databases as of a single instant in time; or
an on-line backup (also known as a m4_italic(hot) backup), which creates
a consistent picture of the databases as of an unspecified instant
during the period of time when the snapshot was made.  The advantage of
a hot backup is that applications may continue to read and write the
databases while the snapshot is being taken.  The disadvantage of a hot
backup is that more information must be archived, and recovery based on
a hot backup is to an unspecified time between the start of the backup
and when the backup is completed.])

m4_p([dnl
Second, after taking a snapshot, you should periodically archive the
log files being created in the environment.  It is often helpful to
think of database archival in terms of full and incremental filesystem
backups.  A snapshot is a full backup, whereas the periodic archival of
the current log files is an incremental backup.  For example, it might
be reasonable to take a full snapshot of a database environment weekly
or monthly, and archive additional log files daily.  Using both the
snapshot and the log files, a catastrophic crash at any time can be
recovered to the time of the most recent log archival; a time long after
the original snapshot.])

m4_p([dnl
To create a standard backup of your database that can be used to recover
from catastrophic failure, take the following steps:])

m4_nlistbegin
m4_nlist([Commit or abort all ongoing transactions.])
m4_nlist([dnl
Stop writing your databases until the backup has completed.  Read-only
operations are permitted, but no write operations and no filesystem
operations may be performed (for example, the m4_ref(dbenv_remove) and
m4_refT(dbh_open)s may not be called).])
m4_nlist([dnl
Force an environment checkpoint (see m4_ref(db_checkpoint) for more
information).])
m4_nlist([dnl
Run m4_ref(db_archive) m4_option(s) to identify all the database data
files, and copy them to a backup device such as CD-ROM, alternate disk,
or tape.])
m4_p([dnl
If the database files are stored in a separate directory from the other
m4_db files, it may be simpler to archive the directory itself instead
of the individual files (see m4_ref(dbenv_set_data_dir) for additional
information).  m4_bold([Note: if any of the database files did not have
an open m4_ref(Db) handle during the lifetime of the current log files,
m4_ref(db_archive) will not list them in its output!])  This is another
reason it may be simpler to use a separate database file directory and
archive the entire directory instead of archiving only the files listed
by m4_ref(db_archive).])
m4_nlist([dnl
Run m4_ref(db_archive) m4_option(l) to identify all the log files,
and copy the last one (that is, the one with the highest number) to a
backup device such as CD-ROM, alternate disk, or tape.])
m4_nlistend

m4_idefz(hot @backup)
m4_p([dnl
To create a m4_italic(hot) backup of your database that can be used to
recover from catastrophic failure, take the following steps:])

m4_nlistbegin
m4_nlist([Archive your databases, as described in the previous step [#4].
You do not have to halt ongoing transactions or force a checkpoint.  As
this is a hot backup, and the databases may be modified during the copy,
the utility you use to copy the databases must read database pages
atomically (as described by m4_link(M4RELDIR/ref/transapp/reclimit,
[m4_db recoverability])).])
m4_nlist([dnl
Archive m4_bold(all) of the log files.  The order of these two operations
is required, and the database files must be archived m4_bold(before) the
log files.  This means that if the database files and log files are in
the same directory, you cannot simply archive the directory; you must
make sure that the correct order of archival is maintained.
m4_p([dnl
To archive your log files, run the m4_ref(db_archive) utility using
the m4_option(l) option to identify all the database log files, and
copy them to your backup media.  If the database log files are stored
in a separate directory from the other database files, it may be simpler
to archive the directory itself instead of the individual files (see
the m4_refT(dbenv_set_lg_dir) for more information).])])
m4_nlistend

m4_p([dnl
To minimize the archival space needed for log files when doing a hot
backup, run db_archive to identify those log files which are not in use.
Log files which are not in use do not need to be included when creating
a hot backup, and you can discard them or move them aside for use with
previous backups (whichever is appropriate), before beginning the hot
backup.])

m4_p([dnl
After completing one of these two sets of steps, the database
environment can be recovered from catastrophic failure (see
m4_link(recovery, Recovery procedures) for more information).])

m4_p([dnl
For an example of a hot backup implementation in the m4_db distribution,
see the source code for the m4_ref(db_hotbackup) utility.])

m4_p([dnl
To update either a hot or cold backup so that recovery from catastrophic
failure is possible to a new point in time, repeat step [#2] under the
hot backup instructions and archive m4_bold(all) of the log files in the
database environment.  Each time both the database and log files are
copied to backup media, you may discard all previous database snapshots
and saved log files.  Archiving additional log files does not allow you
to discard either previous database snapshots or log files.  Generally,
updating a backup must be integrated with the application's log file
removal procedures.])

m4_p([dnl
The time to restore from catastrophic failure is a function of the
number of log records that have been written since the snapshot was
originally created.  Perhaps more importantly, the more separate pieces
of backup media you use, the more likely it is that you will have a
problem reading from one of them.  For these reasons, it is often best
to make snapshots on a regular basis.])

m4_p([m4_bold([dnl
Obviously, the reliability of your archive media will affect the safety
of your data.  For archival safety, ensure that you have multiple copies
of your database backups, verify that your archival media is error-free
and readable, and that copies of your backups are stored offsite!])])

m4_p([dnl
The functionality provided by the m4_ref(db_archive) utility is also
available directly from the m4_db library.  The following code fragment
prints out a list of log and database files that need to be archived:])

include(ref/transapp/archival.cs)

m4_page_footer