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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="RUNTIME-CONFIG-RESOURCE"
>18.4. Resource Consumption</A
></H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-MEMORY"
>18.4.1. Memory</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-SHARED-BUFFERS"
></A
><TT
CLASS="VARNAME"
>shared_buffers</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Sets the amount of memory the database server uses for shared
memory buffers. The default is typically 32 megabytes
(<TT
CLASS="LITERAL"
>32MB</TT
>), but might be less if your kernel settings will
not support it (as determined during <SPAN
CLASS="APPLICATION"
>initdb</SPAN
>).
This setting must be at least 128 kilobytes. (Non-default
values of <TT
CLASS="SYMBOL"
>BLCKSZ</TT
> change the minimum.) However,
settings significantly higher than the minimum are usually needed
for good performance. This parameter can only be set at server start.
</P
><P
> If you have a dedicated database server with 1GB or more of RAM, a
reasonable starting value for <TT
CLASS="VARNAME"
>shared_buffers</TT
> is 25%
of the memory in your system. There are some workloads where even
large settings for <TT
CLASS="VARNAME"
>shared_buffers</TT
> are effective, but
because <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> also relies on the
operating system cache, it is unlikely that an allocation of more than
40% of RAM to <TT
CLASS="VARNAME"
>shared_buffers</TT
> will work better than a
smaller amount. Larger settings for <TT
CLASS="VARNAME"
>shared_buffers</TT
>
usually require a corresponding increase in
<TT
CLASS="VARNAME"
>checkpoint_segments</TT
>, in order to spread out the
process of writing large quantities of new or changed data over a
longer period of time.
</P
><P
> On systems with less than 1GB of RAM, a smaller percentage of RAM is
appropriate, so as to leave adequate space for the operating system.
Also, on Windows, large values for <TT
CLASS="VARNAME"
>shared_buffers</TT
>
aren't as effective. You may find better results keeping the setting
relatively low and using the operating system cache more instead. The
useful range for <TT
CLASS="VARNAME"
>shared_buffers</TT
> on Windows systems
is generally from 64MB to 512MB.
</P
><P
> Increasing this parameter might cause <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
to request more <SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
> shared
memory than your operating system's default configuration
allows. See <A
HREF="kernel-resources.html#SYSVIPC"
>Section 17.4.1</A
> for information on how to
adjust those parameters, if necessary.
</P
></DD
><DT
><A
NAME="GUC-TEMP-BUFFERS"
></A
><TT
CLASS="VARNAME"
>temp_buffers</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Sets the maximum number of temporary buffers used by each database
session. These are session-local buffers used only for access to
temporary tables. The default is eight megabytes
(<TT
CLASS="LITERAL"
>8MB</TT
>). The setting can be changed within individual
sessions, but only before the first use of temporary tables
within the session; subsequent attempts to change the value will
have no effect on that session.
</P
><P
> A session will allocate temporary buffers as needed up to the limit
given by <TT
CLASS="VARNAME"
>temp_buffers</TT
>. The cost of setting a large
value in sessions that do not actually need many temporary
buffers is only a buffer descriptor, or about 64 bytes, per
increment in <TT
CLASS="VARNAME"
>temp_buffers</TT
>. However if a buffer is
actually used an additional 8192 bytes will be consumed for it
(or in general, <TT
CLASS="SYMBOL"
>BLCKSZ</TT
> bytes).
</P
></DD
><DT
><A
NAME="GUC-MAX-PREPARED-TRANSACTIONS"
></A
><TT
CLASS="VARNAME"
>max_prepared_transactions</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Sets the maximum number of transactions that can be in the
<SPAN
CLASS="QUOTE"
>"prepared"</SPAN
> state simultaneously (see <A
HREF="sql-prepare-transaction.html"
>PREPARE TRANSACTION</A
>).
Setting this parameter to zero (which is the default)
disables the prepared-transaction feature.
This parameter can only be set at server start.
</P
><P
> If you are not planning to use prepared transactions, this parameter
should be set to zero to prevent accidental creation of prepared
transactions. If you are using prepared transactions, you will
probably want <TT
CLASS="VARNAME"
>max_prepared_transactions</TT
> to be at
least as large as <A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
>max_connections</A
>, so that every
session can have a prepared transaction pending.
</P
><P
> Increasing this parameter might cause <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
to request more <SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
> shared
memory than your operating system's default configuration
allows. See <A
HREF="kernel-resources.html#SYSVIPC"
>Section 17.4.1</A
> for information on how to
adjust those parameters, if necessary.
</P
><P
> When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</P
></DD
><DT
><A
NAME="GUC-WORK-MEM"
></A
><TT
CLASS="VARNAME"
>work_mem</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Specifies the amount of memory to be used by internal sort operations
and hash tables before writing to temporary disk files. The value
defaults to one megabyte (<TT
CLASS="LITERAL"
>1MB</TT
>).
Note that for a complex query, several sort or hash operations might be
running in parallel; each operation will be allowed to use as much memory
as this value specifies before it starts to write data into temporary
files. Also, several running sessions could be doing such operations
concurrently. Therefore, the total memory used could be many
times the value of <TT
CLASS="VARNAME"
>work_mem</TT
>; it is necessary to
keep this fact in mind when choosing the value. Sort operations are
used for <TT
CLASS="LITERAL"
>ORDER BY</TT
>, <TT
CLASS="LITERAL"
>DISTINCT</TT
>, and
merge joins.
Hash tables are used in hash joins, hash-based aggregation, and
hash-based processing of <TT
CLASS="LITERAL"
>IN</TT
> subqueries.
</P
></DD
><DT
><A
NAME="GUC-MAINTENANCE-WORK-MEM"
></A
><TT
CLASS="VARNAME"
>maintenance_work_mem</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Specifies the maximum amount of memory to be used by maintenance
operations, such as <TT
CLASS="COMMAND"
>VACUUM</TT
>, <TT
CLASS="COMMAND"
>CREATE
INDEX</TT
>, and <TT
CLASS="COMMAND"
>ALTER TABLE ADD FOREIGN KEY</TT
>. It defaults
to 16 megabytes (<TT
CLASS="LITERAL"
>16MB</TT
>). Since only one of these
operations can be executed at a time by a database session, and
an installation normally doesn't have many of them running
concurrently, it's safe to set this value significantly larger
than <TT
CLASS="VARNAME"
>work_mem</TT
>. Larger settings might improve
performance for vacuuming and for restoring database dumps.
</P
><P
> Note that when autovacuum runs, up to
<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
> times this memory may be
allocated, so be careful not to set the default value too high.
</P
></DD
><DT
><A
NAME="GUC-MAX-STACK-DEPTH"
></A
><TT
CLASS="VARNAME"
>max_stack_depth</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Specifies the maximum safe depth of the server's execution stack.
The ideal setting for this parameter is the actual stack size limit
enforced by the kernel (as set by <TT
CLASS="LITERAL"
>ulimit -s</TT
> or local
equivalent), less a safety margin of a megabyte or so. The safety
margin is needed because the stack depth is not checked in every
routine in the server, but only in key potentially-recursive routines
such as expression evaluation. The default setting is two
megabytes (<TT
CLASS="LITERAL"
>2MB</TT
>), which is conservatively small and
unlikely to risk crashes. However, it might be too small to allow
execution of complex functions. Only superusers can change this
setting.
</P
><P
> Setting <TT
CLASS="VARNAME"
>max_stack_depth</TT
> higher than
the actual kernel limit will mean that a runaway recursive function
can crash an individual backend process. On platforms where
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> can determine the kernel limit,
the server will not allow this variable to be set to an unsafe
value. However, not all platforms provide the information,
so caution is recommended in selecting a value.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-DISK"
>18.4.2. Disk</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-TEMP-FILE-LIMIT"
></A
><TT
CLASS="VARNAME"
>temp_file_limit</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Specifies the maximum amount of disk space that a session can use
for temporary files, such as sort and hash temporary files, or the
storage file for a held cursor. A transaction attempting to exceed
this limit will be cancelled.
The value is specified in kilobytes, and <TT
CLASS="LITERAL"
>-1</TT
> (the
default) means no limit.
Only superusers can change this setting.
</P
><P
> This setting constrains the total space used at any instant by all
temporary files used by a given <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> session.
It should be noted that disk space used for explicit temporary
tables, as opposed to temporary files used behind-the-scenes in query
execution, does <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> count against this limit.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-KERNEL"
>18.4.3. Kernel Resource Usage</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-MAX-FILES-PER-PROCESS"
></A
><TT
CLASS="VARNAME"
>max_files_per_process</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Sets the maximum number of simultaneously open files allowed to each
server subprocess. The default is one thousand files. If the kernel is enforcing
a safe per-process limit, you don't need to worry about this setting.
But on some platforms (notably, most BSD systems), the kernel will
allow individual processes to open many more files than the system
can actually support if many processes all try to open
that many files. If you find yourself seeing <SPAN
CLASS="QUOTE"
>"Too many open
files"</SPAN
> failures, try reducing this setting.
This parameter can only be set at server start.
</P
></DD
><DT
><A
NAME="GUC-SHARED-PRELOAD-LIBRARIES"
></A
><TT
CLASS="VARNAME"
>shared_preload_libraries</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> This variable specifies one or more shared libraries
to be preloaded at server start. For example,
<TT
CLASS="LITERAL"
>'$libdir/mylib'</TT
> would cause
<TT
CLASS="LITERAL"
>mylib.so</TT
> (or on some platforms,
<TT
CLASS="LITERAL"
>mylib.sl</TT
>) to be preloaded from the installation's
standard library directory.
All library names are converted to lower case unless double-quoted.
If more than one library is to be loaded, separate their names
with commas. This parameter can only be set at server start.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> procedural language
libraries can be preloaded in this way, typically by using the
syntax <TT
CLASS="LITERAL"
>'$libdir/plXXX'</TT
> where
<TT
CLASS="LITERAL"
>XXX</TT
> is <TT
CLASS="LITERAL"
>pgsql</TT
>, <TT
CLASS="LITERAL"
>perl</TT
>,
<TT
CLASS="LITERAL"
>tcl</TT
>, or <TT
CLASS="LITERAL"
>python</TT
>.
</P
><P
> By preloading a shared library, the library startup time is avoided
when the library is first used. However, the time to start each new
server process might increase slightly, even if that process never
uses the library. So this parameter is recommended only for
libraries that will be used in most sessions.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> On Windows hosts, preloading a library at server start will not reduce
the time required to start each new server process; each server process
will re-load all preload libraries. However, <TT
CLASS="VARNAME"
>shared_preload_libraries
</TT
> is still useful on Windows hosts because some shared libraries may
need to perform certain operations that only take place at postmaster start
(for example, a shared library may need to reserve lightweight locks
or shared memory and you can't do that after the postmaster has started).
</P
></BLOCKQUOTE
></DIV
><P
> If a specified library is not found,
the server will fail to start.
</P
><P
> Every PostgreSQL-supported library has a <SPAN
CLASS="QUOTE"
>"magic
block"</SPAN
> that is checked to guarantee compatibility.
For this reason, non-PostgreSQL libraries cannot be
loaded in this way.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-VACUUM-COST"
>18.4.4. Cost-based Vacuum Delay</A
></H2
><P
> During the execution of <A
HREF="sql-vacuum.html"
>VACUUM</A
>
and <A
HREF="sql-analyze.html"
>ANALYZE</A
>
commands, the system maintains an
internal counter that keeps track of the estimated cost of the
various I/O operations that are performed. When the accumulated
cost reaches a limit (specified by
<TT
CLASS="VARNAME"
>vacuum_cost_limit</TT
>), the process performing
the operation will sleep for a short period of time, as specified by
<TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
>. Then it will reset the
counter and continue execution.
</P
><P
> The intent of this feature is to allow administrators to reduce
the I/O impact of these commands on concurrent database
activity. There are many situations where it is not
important that maintenance commands like
<TT
CLASS="COMMAND"
>VACUUM</TT
> and <TT
CLASS="COMMAND"
>ANALYZE</TT
> finish
quickly; however, it is usually very important that these
commands do not significantly interfere with the ability of the
system to perform other database operations. Cost-based vacuum
delay provides a way for administrators to achieve this.
</P
><P
> This feature is disabled by default for manually issued
<TT
CLASS="COMMAND"
>VACUUM</TT
> commands. To enable it, set the
<TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> variable to a nonzero
value.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-VACUUM-COST-DELAY"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> The length of time, in milliseconds, that the process will sleep
when the cost limit has been exceeded.
The default value is zero, which disables the cost-based vacuum
delay feature. Positive values enable cost-based vacuuming.
Note that on many systems, the effective resolution
of sleep delays is 10 milliseconds; setting
<TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> to a value that is
not a multiple of 10 might have the same results as setting it
to the next higher multiple of 10.
</P
><P
> When using cost-based vacuuming, appropriate values for
<TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> are usually quite small, perhaps
10 or 20 milliseconds. Adjusting vacuum's resource consumption
is best done by changing the other vacuum cost parameters.
</P
></DD
><DT
><A
NAME="GUC-VACUUM-COST-PAGE-HIT"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_page_hit</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> The estimated cost for vacuuming a buffer found in the shared buffer
cache. It represents the cost to lock the buffer pool, lookup
the shared hash table and scan the content of the page. The
default value is one.
</P
></DD
><DT
><A
NAME="GUC-VACUUM-COST-PAGE-MISS"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_page_miss</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> The estimated cost for vacuuming a buffer that has to be read from
disk. This represents the effort to lock the buffer pool,
lookup the shared hash table, read the desired block in from
the disk and scan its content. The default value is 10.
</P
></DD
><DT
><A
NAME="GUC-VACUUM-COST-PAGE-DIRTY"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_page_dirty</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> The estimated cost charged when vacuum modifies a block that was
previously clean. It represents the extra I/O required to
flush the dirty block out to disk again. The default value is
20.
</P
></DD
><DT
><A
NAME="GUC-VACUUM-COST-LIMIT"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_limit</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> The accumulated cost that will cause the vacuuming process to sleep.
The default value is 200.
</P
></DD
></DL
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> There are certain operations that hold critical locks and should
therefore complete as quickly as possible. Cost-based vacuum
delays do not occur during such operations. Therefore it is
possible that the cost accumulates far higher than the specified
limit. To avoid uselessly long delays in such cases, the actual
delay is calculated as <TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> *
<TT
CLASS="VARNAME"
>accumulated_balance</TT
> /
<TT
CLASS="VARNAME"
>vacuum_cost_limit</TT
> with a maximum of
<TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> * 4.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER"
>18.4.5. Background Writer</A
></H2
><P
> There is a separate server
process called the <I
CLASS="FIRSTTERM"
>background writer</I
>, whose function
is to issue writes of <SPAN
CLASS="QUOTE"
>"dirty"</SPAN
> (new or modified) shared
buffers. It writes shared buffers so server processes handling
user queries seldom or never need to wait for a write to occur.
However, the background writer does cause a net overall
increase in I/O load, because while a repeatedly-dirtied page might
otherwise be written only once per checkpoint interval, the
background writer might write it several times as it is dirtied
in the same interval. The parameters discussed in this subsection
can be used to tune the behavior for local needs.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-BGWRITER-DELAY"
></A
><TT
CLASS="VARNAME"
>bgwriter_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Specifies the delay between activity rounds for the
background writer. In each round the writer issues writes
for some number of dirty buffers (controllable by the
following parameters). It then sleeps for <TT
CLASS="VARNAME"
>bgwriter_delay</TT
>
milliseconds, and repeats. When there are no dirty buffers in the
buffer pool, though, it goes into a longer sleep regardless of
<TT
CLASS="VARNAME"
>bgwriter_delay</TT
>. The default value is 200
milliseconds (<TT
CLASS="LITERAL"
>200ms</TT
>). Note that on many systems, the
effective resolution of sleep delays is 10 milliseconds; setting
<TT
CLASS="VARNAME"
>bgwriter_delay</TT
> to a value that is not a multiple of 10
might have the same results as setting it to the next higher multiple
of 10. This parameter can only be set in the
<TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-BGWRITER-LRU-MAXPAGES"
></A
><TT
CLASS="VARNAME"
>bgwriter_lru_maxpages</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> In each round, no more than this many buffers will be written
by the background writer. Setting this to zero disables
background writing. (Note that checkpoints, which are managed by
a separate, dedicated auxiliary process, are unaffected.)
The default value is 100 buffers.
This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-BGWRITER-LRU-MULTIPLIER"
></A
><TT
CLASS="VARNAME"
>bgwriter_lru_multiplier</TT
> (<TT
CLASS="TYPE"
>floating point</TT
>)</DT
><DD
><P
> The number of dirty buffers written in each round is based on the
number of new buffers that have been needed by server processes
during recent rounds. The average recent need is multiplied by
<TT
CLASS="VARNAME"
>bgwriter_lru_multiplier</TT
> to arrive at an estimate of the
number of buffers that will be needed during the next round. Dirty
buffers are written until there are that many clean, reusable buffers
available. (However, no more than <TT
CLASS="VARNAME"
>bgwriter_lru_maxpages</TT
>
buffers will be written per round.)
Thus, a setting of 1.0 represents a <SPAN
CLASS="QUOTE"
>"just in time"</SPAN
> policy
of writing exactly the number of buffers predicted to be needed.
Larger values provide some cushion against spikes in demand,
while smaller values intentionally leave writes to be done by
server processes.
The default is 2.0.
This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
></DD
></DL
></DIV
><P
> Smaller values of <TT
CLASS="VARNAME"
>bgwriter_lru_maxpages</TT
> and
<TT
CLASS="VARNAME"
>bgwriter_lru_multiplier</TT
> reduce the extra I/O load
caused by the background writer, but make it more likely that server
processes will have to issue writes for themselves, delaying interactive
queries.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-ASYNC-BEHAVIOR"
>18.4.6. Asynchronous Behavior</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-EFFECTIVE-IO-CONCURRENCY"
></A
><TT
CLASS="VARNAME"
>effective_io_concurrency</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Sets the number of concurrent disk I/O operations that
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> expects can be executed
simultaneously. Raising this value will increase the number of I/O
operations that any individual <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> session
attempts to initiate in parallel. The allowed range is 1 to 1000,
or zero to disable issuance of asynchronous I/O requests. Currently,
this setting only affects bitmap heap scans.
</P
><P
> A good starting point for this setting is the number of separate
drives comprising a RAID 0 stripe or RAID 1 mirror being used for the
database. (For RAID 5 the parity drive should not be counted.)
However, if the database is often busy with multiple queries issued in
concurrent sessions, lower values may be sufficient to keep the disk
array busy. A value higher than needed to keep the disks busy will
only result in extra CPU overhead.
</P
><P
> For more exotic systems, such as memory-based storage or a RAID array
that is limited by bus bandwidth, the correct value might be the
number of I/O paths available. Some experimentation may be needed
to find the best value.
</P
><P
> Asynchronous I/O depends on an effective <CODE
CLASS="FUNCTION"
>posix_fadvise</CODE
>
function, which some operating systems lack. If the function is not
present then setting this parameter to anything but zero will result
in an error. On some operating systems (e.g., Solaris), the function
is present but does not actually do anything.
</P
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