- If a client renews before 'dhcp-cache-threshold' percent of its lease

has elapsed (default 25%), the server will reuse the allocated lease
  (provide a lease within the currently allocated lease-time) rather
  than extend or renew the lease.  This absolves the server of needing
  to perform an fsync() operation on the lease database before reply,
  which improves performance. [ISC-Bugs #22228]

RELNOTES

@@ -141,6 +141,13 @@ work on other platforms. Please report any problems and suggested fixes to
 
 - Compilation problem with gcc4.5 and omshell.c resolved. [ISC-Bugs #23831]
 
+- If a client renews before 'dhcp-cache-threshold' percent of its lease
+  has elapsed (default 25%), the server will reuse the allocated lease
+  (provide a lease within the currently allocated lease-time) rather
+  than extend or renew the lease.  This absolves the server of needing
+  to perform an fsync() operation on the lease database before reply,
+  which improves performance. [ISC-Bugs #22228]
+
 			Changes since 4.2.0
 
 - Documentation cleanup covering multiple tickets

includes/dhcpd.h

@@ -691,27 +691,6 @@ struct lease_state {
 #define SV_LIMIT_PREFS_PER_IA		57
 #define SV_DELAYED_ACK			58
 #define SV_MAX_ACK_DELAY		59
-
-#if !defined (DEFAULT_PING_TIMEOUT)
-# define DEFAULT_PING_TIMEOUT 1
-#endif
-
-#if !defined (DEFAULT_DELAYED_ACK)
-# define DEFAULT_DELAYED_ACK 28  /* default SO_SNDBUF size / 576 bytes */
-#endif
-
-#if !defined (DEFAULT_ACK_DELAY_SECS)
-# define DEFAULT_ACK_DELAY_SECS 0
-#endif
-
-#if !defined (DEFAULT_ACK_DELAY_USECS)
-# define DEFAULT_ACK_DELAY_USECS 250000 /* 1/4 of a second */
-#endif
-
-#if !defined (DEFAULT_MIN_ACK_DELAY_USECS)
-# define DEFAULT_MIN_ACK_DELAY_USECS 10000 /* 1/100 second */
-#endif
-
 #if defined(LDAP_CONFIGURATION)
 # define SV_LDAP_SERVER                 60
 # define SV_LDAP_PORT                   61
@@ -734,6 +713,31 @@ struct lease_state {
 # define SV_LDAP_TLS_RANDFILE           77
 #endif
 #endif
+#define SV_CACHE_THRESHOLD		78
+
+#if !defined (DEFAULT_PING_TIMEOUT)
+# define DEFAULT_PING_TIMEOUT 1
+#endif
+
+#if !defined (DEFAULT_DELAYED_ACK)
+# define DEFAULT_DELAYED_ACK 28  /* default SO_SNDBUF size / 576 bytes */
+#endif
+
+#if !defined (DEFAULT_ACK_DELAY_SECS)
+# define DEFAULT_ACK_DELAY_SECS 0
+#endif
+
+#if !defined (DEFAULT_ACK_DELAY_USECS)
+# define DEFAULT_ACK_DELAY_USECS 250000 /* 1/4 of a second */
+#endif
+
+#if !defined (DEFAULT_MIN_ACK_DELAY_USECS)
+# define DEFAULT_MIN_ACK_DELAY_USECS 10000 /* 1/100 second */
+#endif
+
+#if !defined (DEFAULT_CACHE_THRESHOLD)
+# define DEFAULT_CACHE_THRESHOLD 25
+#endif
 
 #if !defined (DEFAULT_DEFAULT_LEASE_TIME)
 # define DEFAULT_DEFAULT_LEASE_TIME 43200

server/dhcp.c

@@ -1502,6 +1502,10 @@ void ack_lease (packet, lease, offer, when, msg, ms_nulltp, hp)
 	struct in_addr from;
 	TIME remaining_time;
 	struct iaddr cip;
+#if defined(DELAYED_ACK)
+	isc_boolean_t enqueue = ISC_TRUE;
+#endif
+	int use_old_lease = 0;
 
 	unsigned i, j;
 	int s1;
@@ -2445,6 +2449,48 @@ void ack_lease (packet, lease, offer, when, msg, ms_nulltp, hp)
 			packet -> raw -> chaddr,
 			sizeof packet -> raw -> chaddr); /* XXX */
 	} else {
+		int commit = (!offer || (offer == DHCPACK));
+		int thresh = DEFAULT_CACHE_THRESHOLD;
+
+		/*
+		 * Check if the lease was issued recently, if so replay the 
+		 * current lease and do not require a database sync event.  
+		 * Recently is defined as being issued less than a given 
+		 * percentage of the lease previously. The percentage can be 
+		 * chosen either from a default value or via configuration.
+		 *
+		 */
+		if ((oc = lookup_option(&server_universe, state->options,
+					SV_CACHE_THRESHOLD)) &&
+		    evaluate_option_cache(&d1, packet, lt, NULL,
+					  packet->options, state->options,
+					  &lt->scope, oc, MDL)) {
+			if (d1.len == 1 &&
+			    (d1.data[0] >= 0) && (d1.data[0] < 100))
+				thresh = d1.data[0];
+
+			data_string_forget(&d1, MDL);
+		}
+
+		if ((thresh > 0) && (offer == DHCPACK) &&
+		    (lease->binding_state == FTS_ACTIVE)) {
+			int limit;
+			int prev_lease = lease->ends - lease->starts;
+
+			/* it is better to avoid division by 0 */
+			if (prev_lease <= (INT_MAX / thresh))
+				limit = prev_lease * thresh / 100;
+			else
+				limit = prev_lease / 100 * thresh;
+
+			if ((lt->starts - lease->starts) <= limit) {
+				lt->starts = lease->starts;
+				state->offered_expiry = lt->ends = lease->ends;
+				commit = 0;
+				use_old_lease = 1;
+			}
+		}
+
 #if !defined(DELAYED_ACK)
 		/* Install the new information on 'lt' onto the lease at
 		 * 'lease'.  If this is a DHCPOFFER, it is a 'soft' promise,
@@ -2454,9 +2500,19 @@ void ack_lease (packet, lease, offer, when, msg, ms_nulltp, hp)
 		 * the same lease to another client later, and that would be
 		 * a conflict.
 		 */
-		if (!supersede_lease(lease, lt, !offer || (offer == DHCPACK),
+		if (!use_old_lease && !supersede_lease(lease, lt, commit,
 				     offer == DHCPACK, offer == DHCPACK)) {
 #else /* defined(DELAYED_ACK) */
+		/*
+		 * If there already isn't a need for a lease commit, and we
+		 * can just answer right away, set a flag to indicate this.
+		 */
+		if (commit && !(lease->flags & STATIC_LEASE) &&
+		    (!offer || (offer == DHCPACK)))
+			enqueue = ISC_TRUE;
+		else
+			enqueue = ISC_FALSE;
+
 		/* Install the new information on 'lt' onto the lease at
 		 * 'lease'.  We will not 'commit' this information to disk
 		 * yet (fsync()), we will 'propogate' the information if
@@ -2865,8 +2921,7 @@ void ack_lease (packet, lease, offer, when, msg, ms_nulltp, hp)
 	} else {
   		lease->cltt = cur_time;
 #if defined(DELAYED_ACK)
-		if (!(lease->flags & STATIC_LEASE) &&
+		if (enqueue)
-		    (!offer || (offer == DHCPACK)))
 			delayed_ack_enqueue(lease);
 		else 
 #endif

server/dhcpd.conf.5

@@ -27,7 +27,7 @@
 .\" Support and other services are available for ISC products - see
 .\" https://www.isc.org for more information or to learn more about ISC.
 .\"
-.\" $Id: dhcpd.conf.5,v 1.109 2011/04/25 23:43:16 sar Exp $
+.\" $Id: dhcpd.conf.5,v 1.110 2011/05/12 13:26:55 tomasz Exp $
 .\"
 .TH dhcpd.conf 5
 .SH NAME
@@ -2159,6 +2159,30 @@ with \'./configure --enable-delayed-ack\'.
 .RE
 .PP
 The 
+.I dhcp-cache-threshold 
+statement
+.RS 0.25i
+.PP
+.B dhcp-cache-threshold \fIpercentage\fB;\fR
+.PP
+The \fIdhcp-cache-threshold\fR statement takes one integer parameter
+with allowed values between 0 and 100. The default value is 25 (25% of
+the lease time). This parameter expresses the percentage of the total
+lease time, measured from the beginning, during which a
+client's attempt to renew its lease will result in getting
+the already assigned lease, rather than an extended lease.
+.PP
+Clients that attempt renewal frequently can cause the server to
+update and write the database frequently resulting in a performance
+impact on the server.  The \fIdhcp-cache-threshold\fR
+statement instructs the DHCP server to avoid updating leases too
+frequently thus avoiding this behavior.  Instead the server assigns the
+same lease with no modifications except for CLTT (Client Last
+Transmission Time) which does not require disk operations. This
+feature applies to IPv4 only.
+.RE
+.PP
+The
 .I do-forward-updates
 statement
 .RS 0.25i

server/stables.c

@@ -266,6 +266,7 @@ static struct option server_options[] = {
 	{ "ldap-tls-randfile", "t",		&server_universe,  77, 1 },
 #endif /* LDAP_USE_SSL */
 #endif /* LDAP_CONFIGURATION */
+	{ "dhcp-cache-threshold", "B",		&server_universe,  78, 1 },
 	{ NULL, NULL, NULL, 0, 0 }
 };