My guess is that mmap () would simply ignore the fact that the file is opened in O_DIRECT mode — that option just affects how the read() and write() calls behave. – Barmar Oct 30 ’15 at 23:20 1, For example, if you want to do an O_DIRECT read into a buffer, there is no reason why it shouldn’t be done in two phases: (1) readahead: allocate pages, and start the IO asynchronously (2) mmap the file with a MAP_UNCACHED flag, which causes read-faults to steal the page from the page cache and make it private to the mapping on page faults.
An interesting example is in mongodb using the mmap engine. O_DIRECT is best used, as others have stated, where the data is unlikely to be read for some time. In mongodb, the database journal is written using O_DIRECT while the data and indexes writes are handled by the page cache mechanism (pdflush) because, although O_DIRECT offers less …
The following Python3 script fails. import mmap import os fd = os.open(path_to_file, os.O_DIRECT | os.O_RDWR) fo = os.fdopen(fd, ‘rb+’) m = mmap . mmap (-1, 4096) fo.readinto(m) But it worked for Python2. It also works for any other multiple of 4K. For example: m = mmap . mmap (-1, 8192) fo.readinto(m) Is fine! msg352403 -, In reply to: Robert Love: Re: mmap vs. O_DIRECT Next in thread: Avi Kivity: Re: mmap vs. O_DIRECT Messages sorted by: On Thu, 2004-11-11 at 10:41 -0500, Robert Love wrote: > There is a difference between being synchronous and not going through > the page cache, although in Linux …
Search results for ‘using O_DIRECT to write mmap ()’ed IO.’ (newsgroups and mailing lists) 126 replies O_DIRECT question. started 2007-01-11 02:58:02 UTC. fa.linux.kernel. 122 replies O_DIRECT question. started 2007-01-11 02:57:06 UTC. linux-kernel@vger.kernel.org. 5.
The logical block size can be determined using the ioctl(2) BLKSSZGET operation or from the shell using the command: blockdev –getss O_DIRECT I/Os should never be run concurrently with the fork(2) system call, if the memory buffer is a private mapping (i.e.
any mapping created with the mmap (2) MAP_PRIVATE flag this includes memory allocated …
