Improve the "minimize writes" feature

[dmach]

There's a currently inactive feature[1] to minimize writes on SSDs.
I recently realized that it could be extended to even skip computing the checksums.
That would improve speed not only on SSDs, but also rotating SMR[2] drives where the performance temporarily degrades with random writes.

What the change would be all about:

_minimize_writes 0

• standard behavior

_minimize_writes 1

• the already implemented behavior when a new file is written only if the checksum differs

_minimize_writes 2

• if the file checksums in the new and old rpm headers matches and mtime+size matches, don't update the file on disk; == skip computing the checksum, rely on header content
• this should be used only if users know what they're doing because rpm wouldn't have to restore all files a user manually changed on disk
• but still, various build tools and anything that runs in an isolated environment could use that and benefit from lower IO load

[1] Commit: 29c48e1
[2] https://en.wikipedia.org/wiki/Shingled_magnetic_recording

[pmatilai]

2. would break just about every promise that rpm ever made about file integrity, I don't really see this as a supportable option. At any rate such a thing could not be an innocent looking second level of minimize_writes, it'd need some long and very scary option name.

[ffesti]

Implementation otoh should be pretty straight forward. A simple if at https://github.com/rpm-software-management/rpm/blob/master/lib/rpmfi.c#L995 that does equal = 1; goto exit; should do the trick. Passing in the option without expanding the macro for every file may be a bit more complicated but not a roadblock.

[ffesti]

While this is easy to implement this is probably not the right trade off. The regular minimize writes feature already saves a lot. We'd rather not allow rpm to take chances when it comes to correctness of the operation for a relatively little speed up.