java - Can multiple threads write data into a file at the same time?

Question

If you have ever used a p2p downloading software, they can download a file with multi-threading, and they created only one file, So I wonder how the threads write data into that file. Sequentially or in parallel?

Imagine that you want to dump a big database table to a file, and how to make this job faster?

Answer 1

You can use multiple threads writing a to a file e.g. a log file. but you have to co-ordinate your threads as @Thilo points out. Either you need to synchronize file access and only write whole record/lines, or you need to have a strategy for allocating regions of the file to different threads e.g. re-building a file with known offsets and sizes.

This is rarely done for performance reasons as most disk subsystems perform best when being written to sequentially and disk IO is the bottleneck. If CPU to create the record or line of text (or network IO) is the bottleneck it can help.

Image that you want to dump a big database table to a file, and how to make this job faster?

Writing it sequentially is likely to be the fastest.