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Re: bad fsync? (A.M.)
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Re: bad fsync? (A.M.)



MySQL makes the following claim at:
http://dev.mysql.com/doc/mysql/en/news-4-1-9.html

"InnoDB: Use the fcntl() file flush method on Mac OS X versions 10.3
and up. Apple had disabled fsync() in Mac OS X for internal disk
drives, which caused corruption at power outages."

First of all, is this accurate? A pointer to some docs or a tech note
on this would be helpful.

The comments about fsync() are wrong...

On MacOS X, fsync() always has and always will flush all file data
from host memory to the drive on which the file resides.  The behavior
of fsync() on MacOS X is the same as it is on every other version of
Unix since the dawn of time (well, since the introduction of fsync
anyway :-).

I believe that what the above comment refers to is the fact that
fsync() is not sufficient to guarantee that your data is on stable
storage and on MacOS X we provide a fcntl(), called F_FULLFSYNC,
to ask the drive to flush all buffered data to stable storage.

Let me explain in more detail.  With fsync() even though the OS
writes the data through to the disk and the disk says "yes I wrote
the data", the data is not actually on permanent storage.  Unless
you explicitly disable it, all disks have a write buffer which holds
data you've written.  The disk buffers the data you wrote until it
decides to flush it to the platters (and the writes may not be in
the order you wrote them).  If you lose power or the system crashes
before the data is written, you can wind up in a situation where only
some of your data is actually on disk.  What is worse is that even if
you write blocks A, B and C, call fsync() and then write block D you
may find after rebooting that blocks A and D are on disk but B and C
are not (in fact any ordering of A, B, C, and D is possible).

While this may seem like a rare case it is not.  In fact if you sit
down and pull the plug on a system you can make it happen in one or
two plug pulls.  I have even gone so far as to watch this behavior
with a logic analyzer on the ATA bus: I saw the data for two writes
come across the ATA cable, the drive replied and said the writes were
successful and then when we rebooted the data from the second write
was correct on disk but the data from the first write was not.

To deal with this we introduced the F_FULLFSYNC fcntl which will ask
the drive to flush all of its buffered data to disk.  When an app
needs to guarantee that data is on disk it should use F_FULLFSYNC.
In most cases you do not need such a heavy handed operation and
fsync() is good enough.  But in an app like a database, it is
essential if you want transactional integrity.

Now, a little bit more detail: on ATA drives we implement F_FULLFSYNC
with the FLUSH_TRACK_CACHE command.  All drives sold by Apple will
honor this command.  Unfortunately quite a few firewire drive vendors
disable this command and do not pass it to the drive.  This means that
most external firewire drives are not reliable if you lose power or
the system crashes.  We can't work-around that unless we ask the drive
to disable the write cache completely (which hurts performance quite
badly -- and even that may not be enough as some drives will ignore
that request too).

So in summary, I believe that the comments in the MySQL news posting
are slightly confused.  On MacOS X fsync() behaves the same as it does
on all Unices.  That's not good enough if you really care about data
integrity and so we also provide the F_FULLFSYNC fcntl.  As far as I
know, MacOS X is the only OS to provide this feature for apps that
need to truly guarantee their data is on disk.

Hope this clears things up.


--dominic


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