Talk:Clarifying Direct IO's Semantics

The Solaris mount_ufs man page suggests:

            If forcedirectio is specified [...] data  is transferred
            directly between user address space and the disk. 

            forcedirectio is a performance option that  is
            of  benefit only in large sequential data transfers.
            The default behavior is noforcedirectio.

[That was a quote: a paraphrase follows at the end of this page]

Note the mention of large sequential I/O: in a recent project we were pleased (and the customer was a little surprised) to find that Solaris UFS was coalescing many contiguous logical writes into a substantially smaller number of large physical writes. This improved their performance when doing full-table scans and large updates.

There is more discussion in the directio man page, where they note that buffered I/O is used if the buffer is misaligned or mmap'd, and:

     Large  sequential   I/O   generally   performs   best   with
    DIRECTIO_ON,  except  when  a  file  is  sparse  or is being
    extended  and  is  opened  with  O_SYNC  or   O_DSYNC   

[Another quote]

Again, they recommend direct i/o for large read or writes.

To paraphrase for copyright purposes, one might say:

Solaris provides "forcedirectio" as a mount option, and when it
is applied, data is transferred without being copied to the
buffer cache. It is recommended as a performance optimization
when large amounts of data are transferred sequentially, unlike
other discussion of direct I/O. 

In practice, forecedirectio indeed does appear to
coalesce multiple contiguous logical writes into a substantially 
smaller number of larger physical writes. This improves 
performance when doing full-table scans or other large
I/O operations. 

See man mount_ufs(1M), directio(3C)

--dave