On Thu, Jun 18, 2009 at 12:53:17PM -0700, Curt Wohlgemuth wrote: > This might be a question with an obvious answer, but I'd like > verification one way or the other. > > Does the use of O_DIRECT essentially disable delayed allocation for a > given file? > > My simple tests show a larger degree of block fragmentation for files > written using O_DIRECT than without, and on its face, this makes sense > to me. This fragmentation can be removed by using fallocate() on a > file before extending it with writes. > > (Strictly speaking, I guess the use of O_DIRECT wouldn't "disable" > delayed allocation, since the blocks are allocated at the "normal" > time -- when going to disk. But effectively there would be a lot less > block grouping available to build large extents if each write goes to > disk immediately, instead of going through the page cache.) > exactly. So it is possible that we are getting smaller number of block request in O_DIRECT case. But you should still see better block allocation because of mballoc. mballoc normalize the input block request count based on the file size. w.r.t fallocate I have noticed one problem with O_DIRECT which is explained in the url below. So there may be performance impact on using O_DIRECT with fallocate. http://article.gmane.org/gmane.comp.file-systems.ext4/13762 -aneesh -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
