[SOLVED] How to write data into an offset which is not 512*n bytes using linux native AIO?

How to write data into an offset which is not 512*n bytes using linux native AIO?

I'm writing some app like Bittorrent client to download file from net and write it to local file. I will get partial data and write to the file.

For example, I will download a 1GB file, I will get offset 100, data: 312 bytes, offset 1000000, data: 12345, offset 4000000, data: 888 bytes.

I'm using Linux native AIO(io_setup, io_submit, io_getevents), I found this

When using linux kernel AIO, files are required to be opened in O_DIRECT mode. This introduces further requirements of all read and write operations to have their file offset, memory buffer and size be aligned to 512 bytes.

So how can I write data into some offset which is not 512 aligned?

For example, first I write 4 bytes to a file, so I have to do something like this:

fd = open("a.txt", O_CREAT | O_RDWR | O_DIRECT, 0666);
struct iocb cb;
char data[512] = "asdf";
cb.aio_buf = ALIGN(data, 512);
cb.aio_offset = 512;
cb.aio_nbytes = 512;

Then I would like to append data after asdf:

struct iocb cb2;
char data2[512] = "ghij";
cb2.aio_buf = ALIGN(data2, 512);
cb2.aio_offset = 5;
cb2.aio_nbytes = 512;

It will give error when write

Invalid argument (-22)

So how to do it?

Solution

You have to do what the driver would do if you weren't using O_DIRECT. That is, read the whole block, update the portion you want, and write it back. Block devices simply don't allow smaller accesses.

Doing it yourself can be more efficient (for example, you can update a number of disconnected sequences in the same block for the cost of one read and write). However, since you aren't letting the driver do the work you also aren't getting any atomicity guarantees across the read-modify-write operation.