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copy_file_range(2): add recommendation to use large "len"

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PR#252358 reported a serious performance problem w.r.t.
cp(1) when copying large non-sparse files.
This problem appears to have been caused by cp(1)
calling copy_file_range(2) with a small "len" argument.

This patch adds a recommendation to use a large "len"
value where possible, for performance reasons.

Reviewed by:	asomers
Differential Revision:	https://reviews.freebsd.org/D27935
This commit is contained in:
Rick Macklem 2021-01-02 17:21:21 -08:00
parent c98a764c68
commit d189a74dfd
1 changed files with 10 additions and 1 deletions
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11
lib/libc/sys/copy_file_range.2
View File

@ -25,7 +25,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd March 30, 2020
.Dd January 2, 2021
.Dt COPY_FILE_RANGE 2
.Os
.Sh NAME
@ -117,6 +117,15 @@ with
.Dv SEEK_DATA
arguments and this system call for the
data ranges found.
.Pp
For best performance, call
.Fn copy_file_range
with the largest
.Fa len
value possible.
It is interruptible on most file systems,
so there is no penalty for using very large len values, even SSIZE_MAX.
.Pp
.Sh RETURN VALUES
If it succeeds, the call returns the number of bytes copied, which can be fewer
than