add prefetch property

ZFS prefetch is currently governed by the zfs_prefetch_disable
tunable. However, this is a module-wide settings - if a specific
dataset benefits from prefetch, while others have issue with it,
an optimal solution does not exists.

This commit introduce the "prefetch" property, which enable
granular control (at dataset/volume level) for prefetching.

This patch does not remove the zfs_prefetch_disable, which reimains
a system-wide switch for enable/disable prefetch. However, to avoid
duplication, it would be preferable to deprecate and then remove
the module tunable.

Signed-off-by: Gionatan Danti <[email protected]>

include/sys/dmu_objset.h

@@ -197,6 +197,9 @@ struct objset {
  dmu_objset_upgrade_cb_t os_upgrade_cb;
  boolean_t os_upgrade_exit;
  int os_upgrade_status;
+
+ /* prefetch */
+ boolean_t os_prefetch;
 };
 
 #define DMU_META_OBJSET 0

include/sys/fs/zfs.h

@@ -191,6 +191,7 @@ typedef enum {
  ZFS_PROP_REDACTED,
  ZFS_PROP_REDACT_SNAPS,
  ZFS_PROP_SNAPSHOTS_CHANGED,
+ ZFS_PROP_PREFETCH,
  ZFS_NUM_PROPS
 } zfs_prop_t;
 

module/zcommon/zfs_prop.c

@@ -746,6 +746,10 @@ zfs_prop_init(void)
  ZFS_TYPE_VOLUME, "<date>", "SNAPSHOTS_CHANGED", B_FALSE, B_TRUE,
  B_TRUE, NULL, sfeatures);
 
+ zprop_register_index(ZFS_PROP_PREFETCH, "prefetch", 1, PROP_INHERIT,
+ ZFS_TYPE_FILESYSTEM | ZFS_TYPE_SNAPSHOT | ZFS_TYPE_VOLUME,
+ "on | off", "PREFETCH", boolean_table, sfeatures);
+
  zfs_mod_list_supported_free(sfeatures);
 }
 

module/zfs/dmu_objset.c

@@ -263,6 +263,18 @@ secondary_cache_changed_cb(void *arg, uint64_t newval)
  os->os_secondary_cache = newval;
 }
 
+static void
+prefetch_changed_cb(void *arg, uint64_t newval)
+{
+ objset_t *os = arg;
+
+ /*
+ * Inheritance should have been done by now.
+ */
+ ASSERT(newval == B_TRUE || newval == B_FALSE);
+ os->os_prefetch = newval;
+}
+
 static void
 sync_changed_cb(void *arg, uint64_t newval)
 {
@@ -562,6 +574,11 @@ dmu_objset_open_impl(spa_t *spa, dsl_dataset_t *ds, blkptr_t *bp,
  zfs_prop_to_name(ZFS_PROP_SECONDARYCACHE),
  secondary_cache_changed_cb, os);
  }
+ if (err == 0) {
+ err = dsl_prop_register(ds,
+ zfs_prop_to_name(ZFS_PROP_PREFETCH),
+ prefetch_changed_cb, os);
+ }
  if (!ds->ds_is_snapshot) {
  if (err == 0) {
  err = dsl_prop_register(ds,

module/zfs/dmu_zfetch.c

@@ -329,9 +329,11 @@ dmu_zfetch_prepare(zfetch_t *zf, uint64_t blkid, uint64_t nblks,
 {
  zstream_t *zs;
  spa_t *spa = zf->zf_dnode->dn_objset->os_spa;
+ boolean_t os_prefetch = zf->zf_dnode->dn_objset->os_prefetch;
 
- if (zfs_prefetch_disable)
+ if (zfs_prefetch_disable || !os_prefetch)
  return (NULL);
+
  /*
  * If we haven't yet loaded the indirect vdevs' mappings, we
  * can only read from blocks that we carefully ensure are on