Are disk sector writes atomic?

Question

Clarified Question:

When the OS sends the command to write a sector to disk is it atomic? i.e. Write of new data succeeds fully or old data is left intact should the

Answer 1

People don't seem to agree on what happens during a sector write if the power fails. Maybe because it depends on the hardware being used, and even the filesystem.

From wikipedia (http://en.wikipedia.org/wiki/Journaling_file_system):

Some disk drives guarantee write atomicity during a power failure. Others, however, may stop writing midway through a sector after power is lost, leaving it mismatched against its error-correcting code. The sector is thus corrupt and its contents lost. A physical journal guards against such corruption because it holds a complete copy of the sector, which it can replay over the corruption upon next mount.

Seems to suggest that some hard drives will not finish writing the sector, but that a journaling filesystem can protect you from data loss the same way the xlog protects a database.

From the linux kernel mailing list in a discussion on ext3 journaling filesystem:

In any case bad sector checksum is hardware bug. Sector write is supposed to be atomic, it either happens or not.

I'd tend to believe that over the wiki comment. Actually, the very existence of a database (firebird) with no xlog implies that sector write is atomic, that it cannot clobber data you did not mean to change.

There's quite a bit of discussion Here about atomicity of sector writes, and again no agreement. But the people who are disagreeing seem to be talking about multiple-sector writes (which are not atomic on many modern hard-drives.) Those who are saying sector writes are atomic do seem to know more about what they're talking about.