zio_compress: use min sector size as threshold

Now default compression is lz4, which can stop
compression process by itself on incompressible data.
If there is additional size checks -
we will only make our compressratio worse.
So we can check only for common sector size
(which can be changed by module parameter).

Signed-off-by: George Melikov <[email protected]>

Author: gmelikov

man/man5/zfs-module-parameters.5

@@ -2,6 +2,7 @@
 .\" Copyright (c) 2013 by Turbo Fredriksson <[email protected]>. All rights reserved.
 .\" Copyright (c) 2019 by Delphix. All rights reserved.
 .\" Copyright (c) 2019 Datto Inc.
+.\" Copyright (c) 2019 by George Melikov <[email protected]>. All rights reserved.
 .\" The contents of this file are subject to the terms of the Common Development
 .\" and Distribution License (the "License").  You may not use this file except
 .\" in compliance with the License. You can obtain a copy of the license at
@@ -1097,6 +1098,22 @@ transaction record (itx).
 Default value: \fB5\fR%.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_compress_threshold_bytes\fR (int)
+.ad
+.RS 12n
+This controls the threshold of block compressing ratio which must be exceeded to
+write a block on disk compressed, otherwise it will be writted uncompressed
+to save processor time on decompress.
+Because of disk sector size there is a minimal block size, so if we can compress
+data smaller than this size, full sector block will be allocated against 
+compression gains.
+.sp
+Default value: \fB512\fR.
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/zio_compress.c

@@ -22,13 +22,9 @@
 /*
  * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
- */
-/*
  * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
- */
-
-/*
  * Copyright (c) 2013, 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2019 by George Melikov. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -43,6 +39,9 @@
  */
 unsigned long zio_decompress_fail_fraction = 0;
 
+/* Compression threshold, default size is a common disk sector size. */
+unsigned long zfs_compress_threshold_bytes = 512;
+
 /*
  * Compression vectors.
  */
@@ -120,8 +119,12 @@ zio_compress_data(enum zio_compress c, abd_t *src, void *dst, size_t s_len)
 	if (c == ZIO_COMPRESS_EMPTY)
 		return (s_len);
 
-	/* Compress at least 12.5% */
-	d_len = s_len - (s_len >> 3);
+	/* Data is already less or equal to compress threshold */
+	if (s_len <= zfs_compress_threshold_bytes)
+		return (s_len);
+
+	/* Write compressed only if there is at least one sector compressed */
+	d_len = s_len - MIN(s_len, zfs_compress_threshold_bytes);
 
 	/* No compression algorithms can read from ABDs directly */
 	void *tmp = abd_borrow_buf_copy(src, s_len);
@@ -131,7 +134,6 @@ zio_compress_data(enum zio_compress c, abd_t *src, void *dst, size_t s_len)
 	if (c_len > d_len)
 		return (s_len);
 
-	ASSERT3U(c_len, <=, d_len);
 	return (c_len);
 }
 
@@ -166,3 +168,10 @@ zio_decompress_data(enum zio_compress c, abd_t *src, void *dst,
 
 	return (ret);
 }
+
+#if defined(_KERNEL)
+/* BEGIN CSTYLED */
+module_param(zfs_compress_threshold_bytes, ulong, 0644);
+MODULE_PARM_DESC(zfs_compress_threshold_bytes, "Compression threshold, bytes.");
+/* END CSTYLED */
+#endif