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Wednesday, October 04, 2006

Hack the Gibson - Episode #59

Read the reason for these posts. Read Steve Gibson's response.

Finally, I'm getting in synch with the released episodes. This one is relatively error-free, I have only just a few comments to make:

buffer overrun doesn't always mean that the buffer is on the stack, it can be in the heap also. Hardware DEP prevents both kind from executing code.

Leo probably meant to say turn it on for essential Windows programs and services only instead of turn it off ...

This episode is the first in which I hear Steve correcting itself, so I think this is worthy of quoting: Remember that I said last week that one of the major failings of Server was that it lacked both sound and USB support. Well, that was wrong.

They support every flavor of Linux you can imagine – FreeBSD, OS/2 Warp, Sun’s Solaris - OS/2 Warp isn't a flavor of Linux by a long shot, but I give him the benefit of the doubt because probably he was meaning every kind of OS.

The only real problem in this podcast (netcast, sorry) is the discussion about the fixed size versus expandable drives. The state of the matter is the following: when you choose to use disks for which the space is not preallocated it saves in the file only the parts of the disk which were written too (because if the guest OS tries to read from any other area, it can just return zeros). There are two problems with this (lumped together by Steve under the name fragmentation): these disk areas are stored in a non-contiguous mode in the file, so at every access a lookup step is necessary and also there is the fact that as the file grows it itself can be fragmented on the disk. A third problem is that these files are never able to shrink. The explanation for this is the fact that the virtual machines don't know about file systems, only about disk sectors. When a sector has been written too, it is marked as dirty and stored permanently in the file, even if the file occupying that space has been deleted. Given all this things I don't think that Parallels's product which probably only goes through the file system and marks the empty disk sectors is worth its price. It would be a nice extra if it was included in the program, but not as a stand-alone product.


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