Raw receives must compress metadnode blocks

Currently, the DMU relies on ZIO layer compression to free LO
dnode blocks that no longer have objects in them. However,
raw receives disable all compression, meaning that these blocks
can never be freed. In addition to the obvious space concerns,
this could also cause incremental raw receives to fail to mount
since the MAC of a hole is different from that of a completely
zeroed block.

This patch corrects this issue by adding a special case in
zio_write_compress() which will attempt to compress these blocks
to a hole even if ZIO_FLAG_RAW_ENCRYPT is set. This patch also
removes the zfs_mdcomp_disable tunable, since tuning it could
cause these same issues.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tom Caputi <[email protected]>
Closes #7198 

1 files changed, 6 insertions, 15 deletions

diff --git a/module/zfs/dmu.c b/module/zfs/dmu.c
index b7c5bba15..e0114e659 100644
--- a/module/zfs/dmu.c
+++ b/module/zfs/dmu.c
@@ -2107,8 +2107,6 @@ dmu_object_dirty_raw(objset_t *os, uint64_t object, dmu_tx_t *tx)
 	return (err);
 }
 
-int zfs_mdcomp_disable = 0;
-
 /*
  * When the "redundant_metadata" property is set to "most", only indirect
  * blocks of this level and higher will have an additional ditto block.
@@ -2138,16 +2136,12 @@ dmu_write_policy(objset_t *os, dnode_t *dn, int level, int wp, zio_prop_t *zp)
 	 *	 3. all other level 0 blocks
 	 */
 	if (ismd) {
-		if (zfs_mdcomp_disable) {
-			compress = ZIO_COMPRESS_EMPTY;
-		} else {
-			/*
-			 * XXX -- we should design a compression algorithm
-			 * that specializes in arrays of bps.
-			 */
-			compress = zio_compress_select(os->os_spa,
-			    ZIO_COMPRESS_ON, ZIO_COMPRESS_ON);
-		}
+		/*
+		 * XXX -- we should design a compression algorithm
+		 * that specializes in arrays of bps.
+		 */
+		compress = zio_compress_select(os->os_spa,
+		    ZIO_COMPRESS_ON, ZIO_COMPRESS_ON);
 
 		/*
 		 * Metadata always gets checksummed.  If the data
@@ -2523,9 +2517,6 @@ EXPORT_SYMBOL(dmu_buf_hold);
 EXPORT_SYMBOL(dmu_ot);
 
 /* BEGIN CSTYLED */
-module_param(zfs_mdcomp_disable, int, 0644);
-MODULE_PARM_DESC(zfs_mdcomp_disable, "Disable meta data compression");
-
 module_param(zfs_nopwrite_enabled, int, 0644);
 MODULE_PARM_DESC(zfs_nopwrite_enabled, "Enable NOP writes");