O_DIRECT and delayed allocation question

O_DIRECT and delayed allocation question

[Curt Wohlgemuth]

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.)

Thanks,
Curt

Re: O_DIRECT and delayed allocation question

[Aneesh Kumar K.V]

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