Avoid small buffer copying on write

It is wrong for arc_write_ready() to use zfs_abd_scatter_enabled to
decide whether to reallocate/copy the buffer, because the answer is
OS-specific and depends on the buffer size.  Instead of that use
abd_size_alloc_linear(), moved into public header.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Brian Atkinson <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Closes #12425 

1 files changed, 1 insertions, 0 deletions

diff --git a/include/sys/abd.h b/include/sys/abd.h
index 6903e0c0e..5c6bd0c27 100644
--- a/include/sys/abd.h
+++ b/include/sys/abd.h
@@ -91,6 +91,7 @@ abd_t *abd_alloc_linear(size_t, boolean_t);
 abd_t *abd_alloc_gang(void);
 abd_t *abd_alloc_for_io(size_t, boolean_t);
 abd_t *abd_alloc_sametype(abd_t *, size_t);
+boolean_t abd_size_alloc_linear(size_t);
 void abd_gang_add(abd_t *, abd_t *, boolean_t);
 void abd_free(abd_t *);
 abd_t *abd_get_offset(abd_t *, size_t);