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15.1 Reliability

Reliability is an important property of any serious database system, and PostgreSQL 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.

While forcing data periodically to the disk platters 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 PostgreSQL uses those features. (See the wal_sync_method parameter to adjust how this is done.)

Next, there might be a cache in the disk drive controller; this is particularly common on RAID controller cards. Some of these caches are write-through, meaning writes are sent to the drive as soon as they arrive. Others are write-back, 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 battery-backed unit (BBU) caches, 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.

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 caches. Consumer-grade IDE and SATA drives are particularly likely to have write-back caches that will not survive a power failure, though ATAPI-6 introduced a drive cache flush command (FLUSH CACHE EXT) that some file systems use, e.g. ZFS, ext4. (The SCSI 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 check write caching on Linux use hdparm -I; it is enabled if there is a * next to Write cache; hdparm -W to turn off write caching. On FreeBSD use atacontrol. (For SCSI disks use sdparm to turn off WCE.) On Solaris the disk write cache is controlled by format -e. (The Solaris ZFS file system is safe with disk write-cache enabled because it issues its own disk cache flush commands.) On Windows if wal_sync_method is open_datasync (the default), write caching is disabled by unchecking My Computer\Open\{select disk drive}\Properties\Hardware\Properties\Policies\Enable write caching on the disk. Also on Windows, fsync and fsync_writethrough never do write caching.

Many file systems that use write barriers (e.g. ZFS, ext4) internally use FLUSH CACHE EXT or 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 (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 ‘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 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. Hopefully file system and disk controller designers will eventually address this suboptimal behavior.

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. You can test for reliable I/O subsystem behavior using ‘diskchecker.pl’.

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 512 bytes, 1024 bytes, or 8192 bytes, 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, PostgreSQL periodically writes full page images to permanent WAL storage before modifying the actual page on disk. By doing this, during crash recovery PostgreSQL can restore partially-written pages. 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 full_page_writes parameter.