Re: [PERFORMANCE]fs: sendfile suffer performance degradation when buffer size have performance impact on underling IO

[David Wang]

At 2023-10-23 06:53:17, "Dave Chinner" <david@xxxxxxxxxxxxx> wrote:

>
>O_DSYNC is the problem here.
>
>This forces an IO to disk for every write IO submission from
>sendfile to the filesystem. For synchronous IO (as in "waiting for
>completion before sending the next IO), a larger IO size will
>*always* move data faster to storage.
>
>FWIW, you'll get the same behaviour if you use O_DIRECT for either
>source or destination file with sendfile - synchronous 64kB IOs are
>a massive performance limitation even without O_DSYNC.
>
>IOWs, don't use sendfile like this. Use buffered IO and
>sendfile(fd); fdatasync(fd); if you need data integrity guarantees
>and you won't see any perf problems resulting from the size of the
>internal sendfile buffer....
>
>-Dave.
>-- 
>Dave Chinner
>david@xxxxxxxxxxxxx

Thanks for the information, and Yes, buffered IO shows no significant 
performance difference.
Feel that this usage caveat should be recorded in the "NOTE" section of man page for sendfile.

Thanks
David