[PATCH]doc: Added explaination for splice system call

From: Ram Gupta
Date: Fri Sep 29 2006 - 12:27:43 EST


This patch adds documentation for splice system call. Please apply

Signed-off-by: Ram Gupta<r.gupta@xxxxxxxxxxxxxxxx>
-------
diff -uprN -X linux-2.6.18-doc/Documentation/dontdiff
linux-2.6.18/Documentation/splice.txt
linux-2.6.18-doc/Documentation/splice.txt
--- linux-2.6.18/Documentation/splice.txt 1969-12-31
19:00:00.000000000 -0500
+++ linux-2.6.18-doc/Documentation/splice.txt 2006-09-29
10:54:11.000000000 -0500
@@ -0,0 +1,57 @@
+ Splice system call
+ ----------------------------
+A splice() is a system call mechanism to do i/o from one file to another file
+in kernel space without doing copying from/to user space.It is a way of
+improving I/O performance. The splice system call avoids all data copy from
+user space to kernel space & vice versa. It reads from the specified offset
+from the input file & writes to a pipe in the kernel space. There is no copying
+of data to user space.Then it can be called to write the data from the pipe
+to the output file at the specified/current offset.
+
+splice() works by using the pipe buffer mechanism to open a file descriptor
+for a data source and another for a data sink then by using splice() it can
+join the two together. In other words, splice() work on a kernel buffer that
+the user has control over and moves data to/from the buffer from/to an
+arbitrary file descriptor. Specifying offset with pipe is an error as usual.
+If no offset is specified with an input/output file descriptor then the
+current offset will be assumed to be the offset specified. Currently one of
+the file descriptor must be pipe otherwise it is an error.
+This system call was added in 2.6.17 kernel
+
+########################################
+Overview of reading from a file to pipe
+########################################
+* It does the read ahead if there is more than one page
+
+* It tries to find the nr_pages from the input file mapping (contiguous
+ if possible)
+
+* If we got less number of contiguous pages than needed then find the other
+ pages or allocate new page.
+
+* If the page is not uptodate then we may need to start io on that page. But
+ the page may be under i/o already.
+
+* If needed start io if we are not in non-block mode.
+
+* Set up the pages in the buffers of the pipe.
+
+################################################
+Overview of writing from pipe to the output file
+#################################################
+
+* Make sure the page is uptodate
+
+* Steal the page from the page cache if the flag SPLICE_F_MOVE is
set & add the
+ page to the page cache of the file mapping.
+
+* Otherwise find the page of the mapping for the offset & make sure that page
+ is uptodate.
+
+* Prepare the page to be written in the file
+
+* balance dirty memory state
+
+* If file or inode is SYNC and we actually wrote some data, sync it.
+
+Ram Gupta<r.gupta@xxxxxxxxxxxxxxxx>

Thanks
Ram Gupta
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