Vacuum I/O throttling

Below is a patch for the lazy vacuum. It implements a simple I/O throttle so
boxen arnt killed for hours a day when VACUUM runs. Patch includes a
paragraph for the manual. The new setting is VACUUM_THROTTLE. It can be SET
from a client connection, too.

The usleep() could be replaced with a select() call with a timeout an no
fd_set's to aid portability..

The intention is I can simply startup a daemon like shellscript that spends
its whole life doing VACUUM, like:

$ while :; do echo "SET vacuum_throttle = 20; VACUUM ANALYZE VERBOSE"; done

It is against version 7.3.2 with a bunch of Debian specific patches applied,
hopefully it will apply fine you people. [Debian's promiscuous patching of
everything always irks me]

Now, some things I noticed while writing this patch:

- Is it correct that the database backend appears to have absolutely no idea
what tables have free space until VACUUM runs for the first time?

- If a table is VACUUM'd multiple times simultaneously, what happens? [Can
this even happen? I didn't look for per-vacuum locks]. From what I can
see, this is a dangerous thing to do...

- Would this patch be more useful as a general I/O throttle for reading
pages in from disk? [Down in ReadBuffer() somewhere I guess, but I didnt
look to close (yet;)] I didn't do it this way, because I didnt want sleeps
forced upon processes that could have important locks held.

Note that I'm fairly noob to these database thingys, and comments are
appreciated.

- Guy

diff -bBur postgresql-7.3.2/doc/src/sgml/runtime.sgml postgresql-7.3.2-guy/doc/src/sgml/runtime.sgml
--- postgresql-7.3.2/doc/src/sgml/runtime.sgml 2003-01-11 05:04:26.000000000 +0000
+++ postgresql-7.3.2-guy/doc/src/sgml/runtime.sgml 2003-08-28 04:16:28.000000000 +0000
@@ -2045,6 +2045,23 @@
</varlistentry>

<varlistentry>
+ <term><varname>VACUUM_THROTTLE</varname> (<type>integer</type>)</term>
+ <listitem>
+ <para>
+ Optionally throttle the rate at which the lazy
+ <command>VACUUM</command> will scan database pages. The value
+ specified is either 0 to disable the throttle (the default) or the
+ number of pages/second <command>VACUUM</command> is permitted to
+ look at. If you are having problems with <command>VACUUM</command>
+ nuking your I/O subsystem, try tuning this parameter. Values larger
+ than your OS scheduling frequency will probably not be useful. This
+ does not affect <command>VACUUM FULL</command> or
+ <command>ANALYZE</command>.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
<term><varname>VIRTUAL_HOST</varname> (<type>string</type>)</term>
<listitem>
<para>
diff -bBur postgresql-7.3.2/src/backend/commands/vacuumlazy.c postgresql-7.3.2-guy/src/backend/commands/vacuumlazy.c
--- postgresql-7.3.2/src/backend/commands/vacuumlazy.c 2002-09-20 19:56:01.000000000 +0000
+++ postgresql-7.3.2-guy/src/backend/commands/vacuumlazy.c 2003-08-28 03:34:27.000000000 +0000
@@ -204,6 +205,7 @@
bool did_vacuum_index = false;
int i;
VacRUsage ru0;
+ int page_delay = 0;

vac_init_rusage(&ru0);

@@ -221,6 +223,9 @@

lazy_space_alloc(vacrelstats, nblocks);

+ if (VacuumThrottle > 0)
+ page_delay = 1000000 / VacuumThrottle;
+
for (blkno = 0; blkno < nblocks; blkno++)
{
Buffer buf;
@@ -232,6 +237,9 @@
hastup;
int prev_dead_count;

+ if(page_delay > 0)
+ usleep(page_delay);
+
CHECK_FOR_INTERRUPTS();

/*
diff -bBur postgresql-7.3.2/src/backend/utils/init/globals.c postgresql-7.3.2-guy/src/backend/utils/init/globals.c
--- postgresql-7.3.2/src/backend/utils/init/globals.c 2002-10-03 17:07:53.000000000 +0000
+++ postgresql-7.3.2-guy/src/backend/utils/init/globals.c 2003-08-28 02:55:16.000000000 +0000
@@ -70,4 +70,5 @@
bool allowSystemTableMods = false;
int SortMem = 1024;
int VacuumMem = 8192;
+int VacuumThrottle = 0;
int NBuffers = DEF_NBUFFERS;
diff -bBur postgresql-7.3.2/src/backend/utils/misc/guc.c postgresql-7.3.2-guy/src/backend/utils/misc/guc.c
--- postgresql-7.3.2/src/backend/utils/misc/guc.c 2003-01-28 18:04:13.000000000 +0000
+++ postgresql-7.3.2-guy/src/backend/utils/misc/guc.c 2003-08-28 03:07:27.000000000 +0000
@@ -602,6 +602,11 @@
},

{
+ {"vacuum_throttle", PGC_USERSET}, &VacuumThrottle,
+ 0, 0, INT_MAX, NULL, NULL
+ },
+
+ {
{"max_files_per_process", PGC_BACKEND}, &max_files_per_process,
1000, 25, INT_MAX, NULL, NULL
},
diff -bBur postgresql-7.3.2/src/include/c.h postgresql-7.3.2-guy/src/include/c.h
--- postgresql-7.3.2/src/include/c.h 2002-10-24 03:11:05.000000000 +0000
+++ postgresql-7.3.2-guy/src/include/c.h 2003-08-28 03:17:03.000000000 +0000
@@ -58,6 +58,7 @@
#include <string.h>
#include <stddef.h>
#include <stdarg.h>
+#include <unistd.h>
#ifdef HAVE_STRINGS_H
#include <strings.h>
#endif
diff -bBur postgresql-7.3.2/src/include/miscadmin.h postgresql-7.3.2-guy/src/include/miscadmin.h
--- postgresql-7.3.2/src/include/miscadmin.h 2002-10-03 17:07:53.000000000 +0000
+++ postgresql-7.3.2-guy/src/include/miscadmin.h 2003-08-28 03:12:25.000000000 +0000
@@ -165,6 +165,7 @@
extern bool allowSystemTableMods;
extern DLLIMPORT int SortMem;
extern int VacuumMem;
+extern int VacuumThrottle;

/*
* A few postmaster startup options are exported here so the