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<chapter id="wal">

<title>Reliability and the Write-Ahead Log</title>

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 drives are

exist for disk controller caches. Consumer-grade IDEand SATAdrives are

particularly likely to have write-back caches that will not survive a

power failure. To check write caching on <productname>Linux</> use

<command>hdparm -I</>; it is enabled if there is a <literal>*</> next

to <literal>Write cache</>. <command>hdparm -W</>canto turn off

to <literal>Write cache</>. <command>hdparm -W</> to turn off

write caching. On <productname>FreeBSD</> use

<application>atacontrol</>. (For SCSI disks use <ulink

url="http://sg.torque.net/sg/sdparm.html"><application>sdparm</></ulink>