VMware blames Apple for Fusion corruption

By Stephen Withers
Monday, 21 July 2008 04:57

Technology

A bug in Mac OS X can cause loss of data in VMware Fusion virtual machines, company officials have warned. Fusion is VMware's virtualisation product for Mac OS X.

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

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

The good news is that with its default settings, Fusion does not use unbuffered I/O.

However, the virtualisation software does give users the choice of optimising for virtual machine disk performance (the default), or for the performance of other Mac OS applications. The latter is achieved by enabling unbuffered I/O, which allows Fusion to use less memory.

This 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.

But VMware now believes that 10.5.3 failed to provide a complete fix for the problem, and is "aggressively pushing" Apple for a fix.

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