the linux page cache flush order

Question

There is page cache before we write data to disk.

So if I have two operations.

write(fileA)
write(fileB)

Then if the system is suddenly shutdown. We don't initiative call the sync() call.

I want to know if it is possible that the data we wrote to fileB has flush to the disk, while the data we wrote to fileA haven't been flush to the disk?

Answer 1

I believe that it is possible for fileB to be written to disk before fileA, as the writes will be bundled into block I/O requests and can be reordered at the block device layer by the I/O scheduler in an attempt to minimise disk seeking.

See the kernel documentation for more info about the I/O scheduler (elevator): http://lxr.free-electrons.com/source/Documentation/block/biodoc.txt#L885