Sindbad~EG File Manager
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<h2 class="title" style="clear: both"><a id="transapp_archival"></a>Database and log file
archival</h2>
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<p>
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.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Berkeley DB backups (archives) can be recovered using
machines of differing byte order. That is, a backup taken
on a big-endian machine can be used to restore a database
residing on a little-endian machine.
</p>
</div>
<p>
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 <span class="emphasis"><em>hot</em></span> 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.
</p>
<p>
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.
</p>
<p>
When incremental backups are implemented using this
procedure, it is important to know that a database copy taken
prior to a bulk loading event (that is, a transaction started
with the <a href="../api_reference/C/txnbegin.html#txnbegin_DB_TXN_BULK" class="olink">DB_TXN_BULK</a> flag) can no longer be used as the
target of an incremental backup. This is true because bulk
loading omits logging of some record insertions, so these
insertions cannot be rolled forward by recovery. It is
recommended that a full backup be scheduled following a bulk
loading event.
</p>
<p>
To create a standard backup of your database that can be
used to recover from catastrophic failure, take the following
steps:
</p>
<div class="orderedlist">
<ol type="1">
<li>
Commit or abort all ongoing transactions.
</li>
<li>
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 <a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a> and <a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a>
methods may not be called).
</li>
<li>
Force an environment checkpoint (see the
<a href="../api_reference/C/db_checkpoint.html" class="olink">db_checkpoint</a> utility for more information).
</li>
<li>
<p>
Run the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility with option <span class="bold"><strong>-s</strong></span> to identify all the
database data files, and copy them to a backup device
such as CD-ROM, alternate disk, or tape.
</p>
<p>
If the database files are stored in a separate
directory from the other Berkeley DB files, it may be
simpler to archive the directory itself instead of the
individual files (see <a href="../api_reference/C/envadd_data_dir.html" class="olink">DB_ENV->add_data_dir()</a> for additional
information on storing database files in separate
directories).
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
If any of the database files did not have an
open <a href="../api_reference/C/db.html" class="olink">DB</a> handle during the lifetime of the
current log files, the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility 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 the
<a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility.
</p>
</div>
</li>
<li>
Run the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility with option <span class="bold"><strong>-l</strong></span> 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.
</li>
</ol>
</div>
<p>
To create a <span class="emphasis"><em>hot</em></span> backup of your
database that can be used to recover from catastrophic
failure, take the following steps:
</p>
<div class="orderedlist">
<ol type="1">
<li>
Set the <a href="../api_reference/C/envset_flags.html#set_flags_DB_HOTBACKUP_IN_PROGRESS" class="olink">DB_HOTBACKUP_IN_PROGRESS</a> flag in the
environment. This affects the behavior of transactions
started with the <a href="../api_reference/C/txnbegin.html#txnbegin_DB_TXN_BULK" class="olink">DB_TXN_BULK</a> flag.
</li>
<li>
<p>
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, it is critical that database pages be read
atomically as described by <a class="xref" href="transapp_reclimit.html" title="Berkeley DB recoverability">Berkeley DB recoverability</a>. </p>
<p>
Note that only UNIX based systems are known to
support the atomicity of reads. These systems include:
Solaris, Mac OSX, HPUX and various BSD based systems.
Linux and Windows based systems do not support atomic
filesystem reads directly. The XFS file system
supports atomic reads despite the lack of it in Linux.
On systems that do not support atomic file system
reads, the <a href="../api_reference/C/db_hotbackup.html" class="olink">db_hotbackup</a> utility should be used or a tool can
be constructed using the <a href="../api_reference/C/envbackup.html" class="olink">DB_ENV->backup()</a> method.
Alternatively, you can construct a tool using the the
<a href="../api_reference/C/db_copy.html" class="olink">db_copy()</a> method. You can also perform a hot backup of
just a single database in your environment using the
<a href="../api_reference/C/envdbbackup.html" class="olink">DB_ENV->dbbackup()</a> method.
</p>
</li>
<li>
<p>
Archive <span class="bold"><strong>all</strong></span> of the
log files. The order of these two operations is
required, and the database files must be archived
<span class="bold"><strong>before</strong></span> 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.
</p>
<p>
To archive your log files, run the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility
using the <span class="bold"><strong>-l</strong></span> 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
<a href="../api_reference/C/envset_lg_dir.html" class="olink">DB_ENV->set_lg_dir()</a> method for more information).
</p>
</li>
<li> Reset the <a href="../api_reference/C/envset_flags.html#set_flags_DB_HOTBACKUP_IN_PROGRESS" class="olink">DB_HOTBACKUP_IN_PROGRESS</a> flag.
</li>
</ol>
</div>
<p>
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.
</p>
<p>
After completing one of these two sets of steps, the
database environment can be recovered from catastrophic
failure (see <a class="xref" href="transapp_recovery.html" title="Recovery procedures">Recovery procedures</a> for more information).
</p>
<p>
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
<span class="bold"><strong>all</strong></span> 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.
</p>
<p>
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.
</p>
<p>
<span class="bold"><strong>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!</strong></span>
</p>
<p>
The functionality provided by the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility is also
available directly from the Berkeley DB library. The following
code fragment prints out a list of log and database files that
need to be archived:
</p>
<pre class="programlisting">void
log_archlist(DB_ENV *dbenv)
{
int ret;
char **begin, **list;
/* Get the list of database files. */
if ((ret = dbenv->log_archive(dbenv,
&list, DB_ARCH_ABS | DB_ARCH_DATA)) != 0) {
dbenv->err(dbenv, ret, "DB_ENV->log_archive: DB_ARCH_DATA");
exit (1);
}
if (list != NULL) {
for (begin = list; *list != NULL; ++list)
printf("database file: %s\n", *list);
free (begin);
}
/* Get the list of log files. */
if ((ret = dbenv->log_archive(dbenv,
&list, DB_ARCH_ABS | DB_ARCH_LOG)) != 0) {
dbenv->err(dbenv, ret, "DB_ENV->log_archive: DB_ARCH_LOG");
exit (1);
}
if (list != NULL) {
for (begin = list; *list != NULL; ++list)
printf("log file: %s\n", *list);
free (begin);
}
}</pre>
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Sindbad File Manager Version 1.0, Coded By Sindbad EG ~ The Terrorists