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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
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@ -25,7 +25,7 @@
.\" .\"
.\" $FreeBSD$ .\" $FreeBSD$
.\" .\"
.Dd March 30, 2020 .Dd January 2, 2021
.Dt COPY_FILE_RANGE 2 .Dt COPY_FILE_RANGE 2
.Os .Os
.Sh NAME .Sh NAME
@ -117,6 +117,15 @@ with
.Dv SEEK_DATA .Dv SEEK_DATA
arguments and this system call for the arguments and this system call for the
data ranges found. 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 .Sh RETURN VALUES
If it succeeds, the call returns the number of bytes copied, which can be fewer If it succeeds, the call returns the number of bytes copied, which can be fewer
than than