@ -27,7 +27,7 @@
</para>
<para>
While forcing data periodically to the disk platters might seem like
While forcing data to the disk platters periodically might seem like
a simple operation, it is not. Because disk drives are dramatically
slower than main memory and CPUs, several layers of caching exist
between the computer's main memory and the disk platters.
@ -48,7 +48,7 @@
some later time. Such caches can be a reliability hazard because the
memory in the disk controller cache is volatile, and will lose its
contents in a power failure. Better controller cards have
<firstterm>battery-backed unit</> (<acronym>BBU</>) caches , meaning
<firstterm>battery-backup units</> (<acronym>BBU</>s) , meaning
the card has a battery that
maintains power to the cache in case of system power loss. After power
is restored the data will be written to the disk drives.
@ -57,15 +57,10 @@
<para>
And finally, most disk drives have caches. Some are write-through
while some are write-back, and the same concerns about data loss
exist for write-back drive caches as exist for disk controller
exist for write-back drive caches as for disk controller
caches. Consumer-grade IDE and SATA drives are particularly likely
to have write-back caches that will not survive a power failure,
though <acronym>ATAPI-6</> introduced a drive cache flush command
(<command>FLUSH CACHE EXT</>) that some file systems use, e.g.
<acronym>ZFS</>, <acronym>ext4</>. (The SCSI command
<command>SYNCHRONIZE CACHE</> has long been available.) Many
solid-state drives (SSD) also have volatile write-back caches, and
many do not honor cache flush commands by default.
to have write-back caches that will not survive a power failure. Many
solid-state drives (SSD) also have volatile write-back caches.
</para>
<para>
@ -81,7 +76,7 @@
a <literal>*</> next to <literal>Write cache</>. <command>hdparm -W</>
can be used to turn off write caching. SCSI drives can be queried
using <ulink url="http://sg.danny.cz/sg/sdparm.html"><application>sdparm</></ulink>.
for SCSI drives. Use <command>sdparm --get=WCE</command> to check
Use <command>sdparm --get=WCE</command> to check
whether the write cache is enabled and <command>sdparm --clear=WCE</>
to disable it.
</para>
@ -107,35 +102,40 @@
<listitem>
<para>
On <productname>Windows</>, if <varname>wal_sync_method</> is
<literal>open_datasync</> (the default), write caching is disabled
by unchecking <literal>My Computer\Open\{select disk drive}\Properties\Hardware\Properties\Policies\Enable write caching on the disk</>.
Alternatively, set <varname>wal_sync_method</varname> to <literal>fsync</> or <literal>fsync_writethrough</>, which never do write caching.
<literal>open_datasync</> (the default), write caching can be disabled
by unchecking <literal>My Computer\Open\<replaceable>disk drive</>\Properties\Hardware\Properties\Policies\Enable write caching on the disk</>.
Alternatively, set <varname>wal_sync_method</varname> to
<literal>fsync</> or <literal>fsync_writethrough</>, which prevent
write caching.
</para>
</listitem>
<listitem>
<para>
On <productname>MacOS X</productname>, write caching can be disabl ed by
On <productname>Mac OS X</productname>, write caching can be prevent ed by
setting <varname>wal_sync_method</> to <literal>fsync_writethrough</>.
</para>
</listitem>
</itemizedlist>
<para>
Many file systems that use write barriers (e.g. <acronym>ZFS</>,
<acronym>ext4</>) internally use <command>FLUSH CACHE EXT</> or
<command>SYNCHRONIZE CACHE</> commands to flush data to the platters on
write-back-enabled drives. Unfortunately, such write barrier file
systems behave suboptimally when combined with battery-backed unit
Recent SATA drives (those following <acronym>ATAPI-6</> or later)
offer a drive cache flush command (<command>FLUSH CACHE EXT</>),
while SCSI drives have long supported a similar command
<command>SYNCHRONIZE CACHE</>. These commands are not directly
accessible to <productname>PostgreSQL</>, but some file systems
(e.g., <acronym>ZFS</>, <acronym>ext4</>) can use them to flush
data to the platters on write-back-enabled drives. Unfortunately, such
file systems behave suboptimally when combined with battery-backup unit
(<acronym>BBU</>) disk controllers. In such setups, the synchronize
command forces all data from the BBU to the disks, eliminating much
of the benefit of the BBU. You can run the utility
command forces all data from the controller cache to the disks,
eliminating much of the benefit of the BBU. You can run the utility
<filename>src/tools/fsync</> in the PostgreSQL source tree to see
if you are affected. If you are affected, the performance benefits
of the BBU cache ca n be regained by turning off write barriers in
of the BBU can be regained by turning off write barriers in
the file system or reconfiguring the disk controller, if that is
an option. If write barriers are turned off, make sure the battery
remains active ; a faulty battery can potentially lead to data loss.
remains functional ; a faulty battery can potentially lead to data loss.
Hopefully file system and disk controller designers will eventually
address this suboptimal behavior.
</para>
@ -148,6 +148,8 @@
ensure data integrity. Avoid disk controllers that have non-battery-backed
write caches. At the drive level, disable write-back caching if the
drive cannot guarantee the data will be written before shutdown.
If you use SSDs, be aware that many of these do not honor cache flush
commands by default.
You can test for reliable I/O subsystem behavior using <ulink
url="http://brad.livejournal.com/2116715.html"><filename>diskchecker.pl</filename></ulink>.
</para>
@ -157,16 +159,17 @@
operations themselves. Disk platters are divided into sectors,
commonly 512 bytes each. Every physical read or write operation
processes a whole sector.
When a write request arrives at the drive, it might be for 512 bytes,
1024 bytes, or 8192 bytes, and the process of writing could fail due
When a write request arrives at the drive, it might be for some multiple
of 512 bytes (<productname>PostgreSQL</> typically writes 8192 bytes, or
16 sectors, at a time), and the process of writing could fail due
to power loss at any time, meaning some of the 512-byte sectors were
written, and others were not. To guard against such failures,
written while others were not. To guard against such failures,
<productname>PostgreSQL</> periodically writes full page images to
permanent WAL storage <emphasis>before</> modifying the actual page on
disk. By doing this, during crash recovery <productname>PostgreSQL</> can
restore partially-written pages. If you have a battery-backed disk
restore partially-written pages from WAL . If you have a battery-backed disk
controller or file-system software that prevents partial page writes
(e.g., ZFS), you can turn off this page imaging by turning off the
(e.g., ZFS), you can safely turn off this page imaging by turning off the
<xref linkend="guc-full-page-writes"> parameter.
</para>
</sect1>
Tom Lane
15 years ago