Consistency guarantee of file system regarding seq

Question

My program (only 1 process and 1 thread) sequentially write n consecutive chunks of data to a file on a HDD (regular kind of HDD) using plain old write system call. It's like some kind of append-only log file.

After a system crash (power failure, not HDD failure), I read back and verified that chunks[i] (0 < i < n) had been entirely written down to disk (by checking length). May be the content of the chunk is not checksum correct but still the whole chunks[i] stably sit on the surface of the magnetic disk.

Is it safe for me to assume all other chunks before chunks[i] are entirely written down too? Or there exists a (or many) chunks[j] (0 < j < i) that is partly (or isn't at all) written down to disk? I know that random writes could be reordered to improve disk throughput but could sequential writes be reordered too?

Answer 1

Yes, writes that appear sequential (to you) can be reordered before being written to disk, primarily because the order seen by your code (or even the OS) may not correspond directly to locations on the disk.

Although IDE disks did (at one time) use addressing based on specifying a track, head and sector that would hold a piece of data, they've long since converted to a system where you just have some number of sectors, and it's up to the disk to arrange those in an order that makes sense. It usually does a pretty good job, but in some cases (especially if a sector has gone bad and been replaced by a spare sector) it may make the most sense to write sectors out of order.