On 19 Jul 2007, at 1:36 am, Chris Samuel wrote:
InnoDB: When MySQL/InnoDB is compiled on Mac OS X 10.2 or earlier,
detect the
operating system version at run time and use the fcntl() file flush
method on
Mac OS X versions 10.3 and later. In Mac OS X, fsync() does not
flush the
write cache in the disk drive, but the special fcntl() does;
however, the
flush request is ignored by some external devices. Failure to flush
the
buffers may cause severe database corruption at power outages.
And this is different from Linux how?
Man sync(2) on Linux says:
BUGS
According to the standard specification (e.g., SVID), sync()
schedules
the writes, but may return before the actual writing is
done. However,
since version 1.3.20 Linux does actually wait. (This
still does not
guarantee data integrity: modern disks have large caches.)
Man fsync on Mac OS X 10.4 says:
Note that while fsync() will flush all data from the host to
the drive
(i.e. the "permanent storage device"), the drive itself may not
physi-
cally write the data to the platters for quite some time and it
may be
written in an out-of-order sequence.
Those look like much the same caveats to me. At least later versions
of Mac OS X supply the fcntl() methods to specifically ask the driver
to commit (although how it can possibly guarantee that I don't know -
the device could be a long way away across a SAN); that possibility
does not exist on Linux, as far as I know, although I'm willing to be
corrected.
Any code that relies on sync(), or any other method which just
results in a *request* to flush the data to the physical storage, is
going to have small windows where the data is at risk. Enter UPS,
stage right. :-)
Mind you, MySQL is perfectly capable of corrupting its own data
without relying on rare hardware and power failures to blame for it.
Tim
--
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