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July 21, 2008

Apple said that a software glitch in Mac OS X can cause severe loss of data in VM-Ware Fusion virtual machines. Fusion is VM-Ware's server virtualisation product for Mac OS X.

According to VM-Ware, the underlying problem is in the way Mac OS X handles unbuffered I/O requests. The issue is variously referred to as either a flaw or "a disagreement between Fusion and OS X about what sequences of disk-write calls are legal to make."

VM-Ware says the good news is that with its default settings, Fusion does not use unbuffered I/O.

Overall, unbuffered Input/Output works by sending data directly to a file, as opposed to buffered I/O which stages the data to be written into a block of memory (called a buffer) and then writes the contents of the buffer out to that file.

But VM-Ware's server virtualisation software does give its users the choice of optimising for virtual machine disk performance or for the performance of other Mac OS applications.

This is achieved by enabling unbuffered I/O, which allows Fusion to use less memory and it also allows the server to run a bit faster at the same time.

But VM-Ware now believes that version 10.5.3 failed to provide a proper fix for the problem, and said that it is "aggressively pushing" Apple for a permanent fix.

The feature had been disabled in versions 1.1.1 and 1.1.2 of Fusion. It was restored in version 1.1.3 providing the software was running under Mac OS X 10.5.3 or later, as an unbuffered I/O problem had supposedly been fixed by Apple.

Source: VMware.

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