Test box at home - Ubuntu ext3 fs, I think about.. 2 years old.

5gb - 32k	2m24.716s	2m25.723s	2m24.012s
5gb - 64k	2m23.235s	2m25.939s
5gb - 128k	2m18.724s

11gb - 32k	5m48.613s	5m50.557s	5m55.207s
11gb - 128k	5m38.264s	5m29.513s	5m38.922s

15.5gb 128k

9m31.711s

7m45.154s

9m32.641s

Beefy server with SAN storage

14gb - 64k	2m16.941s	2m40.087s	2m30.454s
14gb - 128k	2m14.720s	2m22.201s	2m26.875s

We roughly judge the penalty for failure at about 40 minutes and discard the home server results. The script penalty is about 2 and a half minutes of time and suppose the payoff is a failure rate of 0%. So I interpret loosely this to mean that, if the edit in place script fails more than once out of every sixteen runs, it is not worth running. If it fails less than once out of every sixteen runs, it is worth the risk of damaging the file, and having to redo everything.

14gb is a good estimate for how large these files will be, but when they become smaller it looks very risky to make the edit in place since the savings become smaller and the time penalty will not decrease proportionally.