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<div class="sect1" lang="en" xml:lang="en">
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<h2 class="title" style="clear: both"><a id="dbcompact"></a>Db::compact()</h2>
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<pre class="programlisting">#include <db_cxx.h>
int
Db::compact(DbTxn *txnid,
Dbt *start, Dbt *stop, DB_COMPACT *c_data, u_int32_t flags, Dbt *end);</pre>
<p>
The <code class="methodname">Db::compact()</code> method compacts Btree and Recno access method
databases, and optionally returns unused Btree, Hash or Recno database
pages to the underlying filesystem.
</p>
<p>
The <code class="methodname">Db::compact()</code> <span>
<span>
method either returns a non-zero error value or throws an
exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
</span>
</span>
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1632000"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632332"></a>txnid</h4>
</div>
</div>
</div>
<p>
If the operation is part of an application-specified transaction, the
<span class="bold"><strong>txnid</strong></span> parameter is a transaction
handle returned from <a class="xref" href="txnbegin.html" title="DbEnv::txn_begin()">DbEnv::txn_begin()</a>;
if the operation is part of a Berkeley DB Concurrent Data Store group, the
<span class="bold"><strong>txnid</strong></span> parameter is a handle returned
from <a class="xref" href="envcdsgroup_begin.html" title="DbEnv::cdsgroup_begin()">DbEnv::cdsgroup_begin()</a>;
otherwise NULL.
</p>
<p>
If a transaction handle is supplied to this method, then the operation is
performed using that transaction. In this event, large sections of the
tree may be locked during the course of the transaction.
</p>
<p>
If no transaction handle is specified, but the operation occurs
in a transactional database, the operation will be implicitly
transaction protected using multiple transactions. These transactions
will be periodically committed to avoid locking large sections of the
tree. Any deadlocks encountered cause the compaction operation to
be retried from the point of the last transaction commit.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632516"></a>start</h4>
</div>
</div>
</div>
<p>
If non-NULL, the <span class="bold"><strong>start</strong></span> parameter is
the starting point for compaction in a Btree or Recno database.
Compaction will start at the smallest key greater than or equal to the
specified key. If NULL, compaction will start at the beginning of the
database.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632206"></a>stop</h4>
</div>
</div>
</div>
<p>
If non-NULL, the <span class="bold"><strong>stop</strong></span> parameter is
the stopping point for compaction in a Btree or Recno database.
Compaction will stop at the page with the smallest key greater than
the specified key. If NULL, compaction will stop at the end of the
database.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632623"></a>c_data</h4>
</div>
</div>
</div>
<p>
</p>
<p>
If non-NULL, the <span class="bold"><strong>c_data</strong></span> parameter
contains additional compaction configuration parameters, and returns
compaction operation statistics, in a structure of type DB_COMPACT.
</p>
<p>
The following input configuration fields are available from the
DB_COMPACT structure:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">int compact_fillpercent;</code>
</p>
<p>
If non-zero, this provides the goal for filling pages, specified as a percentage
between 1 and 100. Any page in a Btree or Recno databases not at or
above this percentage full will be considered for compaction. The
default behavior is to consider every page for compaction, regardless
of its page fill percentage.
</p>
</li>
<li>
<p>
<code class="literal">int compact_pages;</code>
</p>
<p>
If non-zero, the call will return after the
specified number of pages have been freed, or
no more pages can be freed.
</p>
</li>
<li>
<p>
<code class="literal">db_timeout_t compact_timeout;</code>
</p>
<p>
If non-zero, and no <span class="bold"><strong>txnid</strong></span> parameter
was specified, this parameter identifies the lock timeout used for implicit transactions, in
microseconds.
</p>
</li>
</ul>
</div>
<p>
The following output statistics fields are available from the
<code class="literal">DB_COMPACT</code> structure:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">u_int32_t compact_deadlock;</code>
</p>
<p>
An output statistics parameter: if no <span class="bold"><strong>txnid</strong></span> parameter was specified, the number of
deadlocks which occurred.
</p>
</li>
<li>
<p>
<code class="literal">u_int32_t compact_pages_examine;</code>
</p>
<p>
An output statistics parameter: the number of database pages reviewed
during the compaction phase.
</p>
</li>
<li>
<p>
<code class="literal">u_int32_t compact_pages_free;</code>
</p>
<p>
An output statistics parameter: the number of database pages freed
during the compaction phase.
</p>
</li>
<li>
<p>
<code class="literal">u_int32_t compact_levels;</code>
</p>
<p>
An output statistics parameter: the number of levels removed from the
Btree or Recno database during the compaction phase.
</p>
</li>
<li>
<p>
<code class="literal">u_int32_t compact_pages_truncated;</code>
</p>
<p>
An output statistics parameter: the number of database pages returned
to the filesystem.
</p>
</li>
</ul>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632340"></a>flags</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>flags</strong></span> parameter must be set to 0
or one of the following values:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="compact_DB_FREELIST_ONLY"></a>
<code class="literal">DB_FREELIST_ONLY</code>
</p>
<p>
Do no page compaction, only returning pages to the filesystem that are
already free and at the end of the file. This flag must be set if the
database is a Hash access method database.
</p>
</li>
<li>
<p><a id="compact_DB_FREE_SPACE"></a>
<code class="literal">DB_FREE_SPACE</code>
</p>
<p>
Return pages to the filesystem when possible. If this flag is not
specified, pages emptied as a result of compaction will be placed on
the free list for re-use, but never returned to the filesystem.
</p>
<p>
Note that only pages at the end of a file can be returned to the
filesystem. Because of the one-pass nature of the compaction
algorithm, any unemptied page near the end of the file inhibits
returning pages to the file system. A repeated call to the
<code class="methodname">Db::compact()</code> method with a low <span class="bold"><strong>compact_fillpercent</strong></span> may be used to return pages
in this case.
</p>
</li>
</ul>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632162"></a>end</h4>
</div>
</div>
</div>
<p>
If non-NULL, the <span class="bold"><strong>end</strong></span> parameter will
be filled in with the database key marking the end of the compaction
operation in a Btree or Recno database. This is generally the first
key of the page where the operation stopped.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1632253"></a>Errors</h3>
</div>
</div>
</div>
<p>
The <code class="methodname">Db::compact()</code> <span>
<span>
method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a>
exception, encapsulating one of the following non-zero errors, or return one
of the following non-zero errors:
</span>
</span>
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632934"></a><span>DbDeadlockException or </span>DB_LOCK_DEADLOCK</h4>
</div>
</div>
</div>
<p>
A transactional database environment operation was selected to resolve
a deadlock.
</p>
<p>
<a class="xref" href="dbdeadlock.html" title="DbDeadlockException">DbDeadlockException</a> is thrown if
your Berkeley DB API is configured to throw exceptions.
Otherwise, <code class="literal">DB_LOCK_DEADLOCK</code> is returned.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632308"></a><span>DbLockNotGrantedException or </span>DB_LOCK_NOTGRANTED</h4>
</div>
</div>
</div>
<p>
A Berkeley DB Concurrent Data Store database environment configured
for lock timeouts was unable to grant a lock in the allowed time.
</p>
<p>
<a class="xref" href="dblocknotgranted.html" title="DbLockNotGrantedException">DbLockNotGrantedException</a> is thrown if
your Berkeley DB API is configured to throw exceptions.
Otherwise, <code class="literal">DB_LOCK_NOTGRANTED</code> is returned.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632564"></a><span>DbRepHandleDeadException or</span> DB_REP_HANDLE_DEAD</h4>
</div>
</div>
</div>
<p>
When a client synchronizes with the master, it is possible for committed
transactions to be rolled back. This invalidates all the database and cursor
handles opened in the replication environment. Once this occurs, an attempt to use
such a handle will
<span>
throw a <a class="xref" href="dbrephandledead.html" title="DbRepHandleDeadException">DbRepHandleDeadException</a> (if
your application is configured to throw exceptions), or
</span>
return <code class="literal">DB_REP_HANDLE_DEAD</code>.
The application will need to discard the handle and open a new one in order to
continue processing.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632705"></a><span>DbDeadlockException or </span>DB_REP_LOCKOUT</h4>
</div>
</div>
</div>
<p>
The operation was blocked by client/master synchronization.
</p>
<p>
<a class="xref" href="dbdeadlock.html" title="DbDeadlockException">DbDeadlockException</a> is thrown if
your Berkeley DB API is configured to throw exceptions.
Otherwise, <code class="literal">DB_REP_LOCKOUT</code> is returned.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632648"></a>EACCES</h4>
</div>
</div>
</div>
<p>
An attempt was made to modify a read-only database.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1632646"></a>EINVAL</h4>
</div>
</div>
</div>
<p>
An invalid flag value or parameter was specified.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1632962"></a>Class</h3>
</div>
</div>
</div>
<p>
<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1632265"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
</p>
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