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><H1
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><A
NAME="WAL-RELIABILITY"
>29.1. Reliability</A
></H1
><P
> Reliability is an important property of any serious database
system, and <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does everything possible to
guarantee reliable operation. One aspect of reliable operation is
that all data recorded by a committed transaction should be stored
in a nonvolatile area that is safe from power loss, operating
system failure, and hardware failure (except failure of the
nonvolatile area itself, of course). Successfully writing the data
to the computer's permanent storage (disk drive or equivalent)
ordinarily meets this requirement. In fact, even if a computer is
fatally damaged, if the disk drives survive they can be moved to
another computer with similar hardware and all committed
transactions will remain intact.
</P
><P
> 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.
First, there is the operating system's buffer cache, which caches
frequently requested disk blocks and combines disk writes. Fortunately,
all operating systems give applications a way to force writes from
the buffer cache to disk, and <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> uses those
features. (See the <A
HREF="runtime-config-wal.html#GUC-WAL-SYNC-METHOD"
>wal_sync_method</A
> parameter
to adjust how this is done.)
</P
><P
> Next, there might be a cache in the disk drive controller; this is
particularly common on <ACRONYM
CLASS="ACRONYM"
>RAID</ACRONYM
> controller cards. Some of
these caches are <I
CLASS="FIRSTTERM"
>write-through</I
>, meaning writes are sent
to the drive as soon as they arrive. Others are
<I
CLASS="FIRSTTERM"
>write-back</I
>, meaning data is sent to the drive at
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
<I
CLASS="FIRSTTERM"
>battery-backup units</I
> (<ACRONYM
CLASS="ACRONYM"
>BBU</ACRONYM
>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.
</P
><P
> 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 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. Many
solid-state drives (SSD) also have volatile write-back caches.
</P
><P
> These caches can typically be disabled; however, the method for doing
this varies by operating system and drive type:
</P
><P
></P
><UL
><LI
><P
> On <SPAN
CLASS="PRODUCTNAME"
>Linux</SPAN
>, IDE drives can be queried using
<TT
CLASS="COMMAND"
>hdparm -I</TT
>; write caching is enabled if there is
a <TT
CLASS="LITERAL"
>*</TT
> next to <TT
CLASS="LITERAL"
>Write cache</TT
>. <TT
CLASS="COMMAND"
>hdparm -W 0</TT
>
can be used to turn off write caching. SCSI drives can be queried
using <A
HREF="http://sg.danny.cz/sg/sdparm.html"
TARGET="_top"
><SPAN
CLASS="APPLICATION"
>sdparm</SPAN
></A
>.
Use <TT
CLASS="COMMAND"
>sdparm --get=WCE</TT
> to check
whether the write cache is enabled and <TT
CLASS="COMMAND"
>sdparm --clear=WCE</TT
>
to disable it.
</P
></LI
><LI
><P
> On <SPAN
CLASS="PRODUCTNAME"
>FreeBSD</SPAN
>, IDE drives can be queried using
<TT
CLASS="COMMAND"
>atacontrol</TT
> and write caching turned off using
<TT
CLASS="LITERAL"
>hw.ata.wc=0</TT
> in <TT
CLASS="FILENAME"
>/boot/loader.conf</TT
>;
SCSI drives can be queried using <TT
CLASS="COMMAND"
>camcontrol identify</TT
>,
and the write cache both queried and changed using
<TT
CLASS="COMMAND"
>sdparm</TT
> when available.
</P
></LI
><LI
><P
> On <SPAN
CLASS="PRODUCTNAME"
>Solaris</SPAN
>, the disk write cache is controlled by
<A
HREF="http://www.sun.com/bigadmin/content/submitted/format_utility.jsp"
TARGET="_top"
><TT
CLASS="LITERAL"
>format -e</TT
></A
>.
(The Solaris <ACRONYM
CLASS="ACRONYM"
>ZFS</ACRONYM
> file system is safe with disk write-cache
enabled because it issues its own disk cache flush commands.)
</P
></LI
><LI
><P
> On <SPAN
CLASS="PRODUCTNAME"
>Windows</SPAN
>, if <TT
CLASS="VARNAME"
>wal_sync_method</TT
> is
<TT
CLASS="LITERAL"
>open_datasync</TT
> (the default), write caching can be disabled
by unchecking <TT
CLASS="LITERAL"
>My Computer\Open\<TT
CLASS="REPLACEABLE"
><I
>disk drive</I
></TT
>\Properties\Hardware\Properties\Policies\Enable write caching on the disk</TT
>.
Alternatively, set <TT
CLASS="VARNAME"
>wal_sync_method</TT
> to
<TT
CLASS="LITERAL"
>fsync</TT
> or <TT
CLASS="LITERAL"
>fsync_writethrough</TT
>, which prevent
write caching.
</P
></LI
><LI
><P
> On <SPAN
CLASS="PRODUCTNAME"
>Mac OS X</SPAN
>, write caching can be prevented by
setting <TT
CLASS="VARNAME"
>wal_sync_method</TT
> to <TT
CLASS="LITERAL"
>fsync_writethrough</TT
>.
</P
></LI
></UL
><P
> Recent SATA drives (those following <ACRONYM
CLASS="ACRONYM"
>ATAPI-6</ACRONYM
> or later)
offer a drive cache flush command (<TT
CLASS="COMMAND"
>FLUSH CACHE EXT</TT
>),
while SCSI drives have long supported a similar command
<TT
CLASS="COMMAND"
>SYNCHRONIZE CACHE</TT
>. These commands are not directly
accessible to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, but some file systems
(e.g., <ACRONYM
CLASS="ACRONYM"
>ZFS</ACRONYM
>, <ACRONYM
CLASS="ACRONYM"
>ext4</ACRONYM
>) 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
CLASS="ACRONYM"
>BBU</ACRONYM
>) disk controllers. In such setups, the synchronize
command forces all data from the controller cache to the disks,
eliminating much of the benefit of the BBU. You can run the
<A
HREF="pgtestfsync.html"
><SPAN
CLASS="APPLICATION"
>pg_test_fsync</SPAN
></A
> module to see
if you are affected. If you are affected, the performance benefits
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 functional; a faulty battery can potentially lead to data loss.
Hopefully file system and disk controller designers will eventually
address this suboptimal behavior.
</P
><P
> When the operating system sends a write request to the storage hardware,
there is little it can do to make sure the data has arrived at a truly
non-volatile storage area. Rather, it is the
administrator's responsibility to make certain that all storage components
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 <A
HREF="http://brad.livejournal.com/2116715.html"
TARGET="_top"
><TT
CLASS="FILENAME"
>diskchecker.pl</TT
></A
>.
</P
><P
> Another risk of data loss is posed by the disk platter write
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 some multiple
of 512 bytes (<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 while others were not. To guard against such failures,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> periodically writes full page images to
permanent WAL storage <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>before</I
></SPAN
> modifying the actual page on
disk. By doing this, during crash recovery <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> can
restore partially-written pages from WAL. If you have file-system software
that prevents partial page writes (e.g., ZFS), you can turn off
this page imaging by turning off the <A
HREF="runtime-config-wal.html#GUC-FULL-PAGE-WRITES"
>full_page_writes</A
> parameter. Battery-Backed Unit
(BBU) disk controllers do not prevent partial page writes unless
they guarantee that data is written to the BBU as full (8kB) pages.
</P
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