fsync and hardware write cache
Something to think about:
if you run PostgreSQL with fsync on, but you use the hardware write cache
on your disk drives, how likely are you to lose data? Obviously, this is a
fairly limited problem, as it only applies to power down (which you can
control) or power loss where the risks may be reduced but not eliminated
with a UPS.
Does it make sense to add a platform specific call that will flush a write
cache when fsync is enable?
pgsql@mohawksoft.com wrote:
Something to think about:
if you run PostgreSQL with fsync on, but you use the hardware write cache
on your disk drives, how likely are you to lose data? Obviously, this is a
fairly limited problem, as it only applies to power down (which you can
control) or power loss where the risks may be reduced but not eliminated
with a UPS.Does it make sense to add a platform specific call that will flush a write
cache when fsync is enable?
Pete Zaitsev from mysql wrote that there is a special call on Mac OS:
Quoting him:
Mac OS X also has this "optimization", but at least it provides an
alternative flush method for Database Servers:fcntl(fd, F_FULLFSYNC, NULL)
can be used instead of fsync() to get true fsync() behavior.
I couldn't confirm this with a quick google search - perhaps someone
with MacOS docs (or mysql sources) should check it.
What might be useful is a test tool that benchmarks fsync: if it's
faster than the rotational speed of a 15k rpm disk then probably someone
caches the write calls.
--
Manfred
On Mon, Aug 23, 2004 at 10:19:20PM +0200, Manfred Spraul wrote:
Does it make sense to add a platform specific call that will flush a write
cache when fsync is enable?Pete Zaitsev from mysql wrote that there is a special call on Mac OS:
Quoting him:Mac OS X also has this "optimization", but at least it provides an
alternative flush method for Database Servers:fcntl(fd, F_FULLFSYNC, NULL)
can be used instead of fsync() to get true fsync() behavior.
I couldn't confirm this with a quick google search - perhaps someone
with MacOS docs (or mysql sources) should check it.
I can confirm it exists.
#define F_FULLFSYNC 51 /* fsync + ask the drive to flush to the media */
What might be useful is a test tool that benchmarks fsync: if it's
faster than the rotational speed of a 15k rpm disk then probably someone
caches the write calls.
I played with doing that - and can't find any system where a naive
looped write, fsync, write, fsync took more that about 600us, so I
guess I'm missing something somewhere.
Cheers,
Steve