Re: Is concurrent file read/write with O_DIRECT flag atomic?

[Liang Chen <liangc8367@xxxxxxxxx>]

On Sun, Nov 12, 2017 at 9:59 AM, Leo Chen <liangc8367@xxxxxxxxx> wrote:
>
>
> I wrote a program to verify my thought. Surprisingly, the readers did
> occasionally read back mixed data. For example, in the first pwrite(),
> the writer writes all 0x11, and in the 2nd pwrite(), it writes all
> 0x22, and in the 3rd write, it writes all 0x33... Occasionally, a
> reader can read back data like "0x11, 0x11, .... 0x11, 0x22, 0x22....
> 0x22". The data appears to be from two consecutive pwrite() calls. I
> checked the offset where the broken starts. The offset seems to be
> sector-aligned (512-byte-aligned).
>
Just for clarification purpose, the writer loop looks like

    while(true) {
        memset(aligned_buffer, seed, 128KB);
        pwrite(fd, aligned_buffer, 128KB);
        seed++;
    }

the reader loop looks like:
    while(true) {
        pread(fd, aligned_reader_buffer, 128KB);
        verify_aligned_reader_buffer();
    }

Thanks,
Leo