Re: [Bug-ddrescue] Version 1.11 of GNU ddrescue released

[Antonio Diaz Diaz]

John wrote:
> What's the difference between ddrescue and dd_rescue? Or are they the 
> same thing? Seems to be synonymous on the web.

GNU ddrescue and dd_rescue are very different programs. The confusion 
comes from some distros distributing dd_rescue in a package named ddrescue.

See these descriptions from John Gilmore 
http://www.toad.com/gnu/sysadmin/index.html#ddrescue

"Kurt Garloff's dd_rescue was the first attempt to improve on this [dd]. 
It reads and copies bigger blocks until it sees an error, then slows 
down and goes back, and reads single sectors. After a while it speeds up 
again. It can also read backward, and can quit after it gets some 
specified number of errors. It keeps a 3-line display updated in your 
text window so you can see what it's doing. If you run it simply, it 
just does what "dd bs=64k" does until it sees an error, then backs up 
and does "dd bs=512". If it gets an error reading a sector, it doesn't 
write to that sector of the output file, but it skips past it to write 
the next good one, so everything stays in sync. It seeks the input and 
output in parallel so it makes an exact copy of the parts that it can read.

LAB Valentin's dd_rhelp. is a complex shell script that runs dd_rescue 
many times, trying to be strategic about copying the drive. It copies 
forward until it gets errors, then jumps forward by a big jump looking 
for either the end of the drive, or more easy-to-read stuff. Once it 
finds the end of the drive, then it starts working backward, trying to 
close up the "hole" that it hasn't read yet. As it encounters errors, it 
skips around looking for more error-free parts of the drive. It only 
reads each sector once. It reads the logfile output of dd_rescue to see 
what happened and to figure out what to do next.

One problem with dd_rhelp is that it's a shell script, so it's really 
slow and consumes massive resources. On one of my drives that had about 
2900 bad sectors on it, dd_rhelp would waste upwards of 15 minutes 
deciding what blocks to tell dd_rescue to try reading next. During that 
time it makes about 100 new Unix processes every second.

Antonio Diaz Diaz's GNU ddrescue learned from these experiences. It 
combines both dd_rescue's ability to read big blocks and then shift 
gears, with dd_rhelp's ability to remember what parts of the disk have 
been looked at already. It keeps this info in a really simple logfile 
format, and keeps it updated every 30 seconds, or whenever it stops or 
is interrupted. It's written in C++ and it's small and fast.

It starts off running like "dd", blasting through large error-free 
areas. When it gets an error, it writes out any partial data that it 
received during that read, and KEEPS GOING to the next big block. It 
notes in the logfile that a bunch of sectors (the first erroneous one, 
plus whatever ones followed it in the multi-sector read) were skipped. 
And keeps going. So it reads through the entire disk in big blocks 
first. Then it goes back to "split" the skipped parts, trying to read 
each sector individually. The compact logfile always shows which chunks 
of disk have been read OK, have been read with errors, have been read 
one sector at a time with errors, or have never been read yet."


Regards,
Antonio.