OpenZFS 9290 - device removal reduces redundancy of mirrors

Mirrors are supposed to provide redundancy in the face of whole-disk
failure and silent damage (e.g. some data on disk is not right, but ZFS
hasn't detected the whole device as being broken). However, the current
device removal implementation bypasses some of the mirror's redundancy.
Note that in no case is incorrect data returned, but we might get a
checksum error when we should have been able to find the right data.

There are two underlying problems:

1. When we remove a mirror device, we only read one side of the mirror.
Since we can't verify the checksum, this side may be silently bad, but
the good data is on the other side of the mirror (which we didn't read).
This can cause the removal to "bake in" the busted data – all copies of
the data in the new location are the same, busted version, while we left
the good version behind.

The fix for this is to read and copy both sides of the mirror. If the
old and new vdevs are mirrors, we will read both sides of the old
mirror, and write each copy to the corresponding side of the new mirror.
(If the old and new vdevs have a different number of children, we will
do this as best as possible.) Even though we aren't verifying checksums,
this ensures that as long as there's a good copy of the data, we'll have
a good copy after the removal, even if there's silent damage to one side
of the mirror. If we're removing a mirror that has some silent damage,
we'll have exactly the same damage in the new location (assuming that
the new location is also a mirror).

2. When we read from an indirect vdev that points to a mirror vdev, we
only consider one copy of the data. This can lead to reduced effective
redundancy, because we might read a bad copy of the data from one side
of the mirror, and not retry the other, good side of the mirror.

Note that the problem is not with the removal process, but rather after
the removal has completed (having copied correct data to both sides of
the mirror), if one side of the new mirror is silently damaged, we
encounter the problem when reading the relocated data via the indirect
vdev. Also note that the problem doesn't occur when ZFS knows that one
side of the mirror is bad, e.g. when a disk entirely fails or is
offlined.

The impact is that reads (from indirect vdevs that point to mirrors) may
return a checksum error even though the good data exists on one side of
the mirror, and scrub doesn't repair all data on the mirror (if some of
it is pointed to via an indirect vdev).

The fix for this is complicated by "split blocks" - one logical block
may be split into two (or more) pieces with each piece moved to a
different new location. In this case we need to read all versions of
each split (one from each side of the mirror), and figure out which
combination of versions results in the correct checksum, and then repair
the incorrect versions.

This ensures that we supply the same redundancy whether you use device
removal or not. For example, if a mirror has small silent errors on all
of its children, we can still reconstruct the correct data, as long as
those errors are at sufficiently-separated offsets (specifically,
separated by the largest block size - default of 128KB, but up to 16MB).

Porting notes:

* A new indirect vdev check was moved from dsl_scan_needs_resilver_cb()
  to dsl_scan_needs_resilver(), which was added to ZoL as part of the
  sequential scrub work.

* Passed NULL for zfs_ereport_post_checksum()'s zbookmark_phys_t
  parameter.  The extra parameter is unique to ZoL.

* When posting indirect checksum errors the ABD can be passed directly,
  zfs_ereport_post_checksum() is not yet ABD-aware in OpenZFS.

Authored by: Matthew Ahrens <[email protected]>
Reviewed by: Tim Chase <[email protected]>
Reviewed by: Brian Behlendorf <[email protected]>
Ported-by: Tim Chase <[email protected]>

OpenZFS-issue: https://illumos.org/issues/9290
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/591
Closes #6900

1 files changed, 55 insertions, 3 deletions

diff --git a/cmd/ztest/ztest.c b/cmd/ztest/ztest.c
index 0e3459d32..c81d446a5 100644
--- a/cmd/ztest/ztest.c
+++ b/cmd/ztest/ztest.c
@@ -445,6 +445,7 @@ static spa_t *ztest_spa = NULL;
 static ztest_ds_t *ztest_ds;
 
 static kmutex_t ztest_vdev_lock;
+static boolean_t ztest_device_removal_active = B_FALSE;
 
 /*
  * The ztest_name_lock protects the pool and dataset namespace used by
@@ -3203,7 +3204,7 @@ ztest_vdev_attach_detach(ztest_ds_t *zd, uint64_t id)
 	 * value.  Don't bother trying to attach while we are in the middle
 	 * of removal.
 	 */
-	if (spa->spa_vdev_removal != NULL) {
+	if (ztest_device_removal_active) {
 		spa_config_exit(spa, SCL_ALL, FTAG);
 		mutex_exit(&ztest_vdev_lock);
 		return;
@@ -3375,16 +3376,49 @@ ztest_device_removal(ztest_ds_t *zd, uint64_t id)
 	spa_t *spa = ztest_spa;
 	vdev_t *vd;
 	uint64_t guid;
+	int error;
 
 	mutex_enter(&ztest_vdev_lock);
 
+	if (ztest_device_removal_active) {
+		mutex_exit(&ztest_vdev_lock);
+		return;
+	}
+
+	/*
+	 * Remove a random top-level vdev and wait for removal to finish.
+	 */
 	spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
 	vd = vdev_lookup_top(spa, ztest_random_vdev_top(spa, B_FALSE));
 	guid = vd->vdev_guid;
 	spa_config_exit(spa, SCL_VDEV, FTAG);
 
-	(void) spa_vdev_remove(spa, guid, B_FALSE);
+	error = spa_vdev_remove(spa, guid, B_FALSE);
+	if (error == 0) {
+		ztest_device_removal_active = B_TRUE;
+		mutex_exit(&ztest_vdev_lock);
+
+		while (spa->spa_vdev_removal != NULL)
+			txg_wait_synced(spa_get_dsl(spa), 0);
+	} else {
+		mutex_exit(&ztest_vdev_lock);
+		return;
+	}
 
+	/*
+	 * The pool needs to be scrubbed after completing device removal.
+	 * Failure to do so may result in checksum errors due to the
+	 * strategy employed by ztest_fault_inject() when selecting which
+	 * offset are redundant and can be damaged.
+	 */
+	error = spa_scan(spa, POOL_SCAN_SCRUB);
+	if (error == 0) {
+		while (dsl_scan_scrubbing(spa_get_dsl(spa)))
+			txg_wait_synced(spa_get_dsl(spa), 0);
+	}
+
+	mutex_enter(&ztest_vdev_lock);
+	ztest_device_removal_active = B_FALSE;
 	mutex_exit(&ztest_vdev_lock);
 }
 
@@ -3524,7 +3558,7 @@ ztest_vdev_LUN_growth(ztest_ds_t *zd, uint64_t id)
 	 * that the metaslab_class space increased (because it decreases
 	 * when the device removal completes).
 	 */
-	if (spa->spa_vdev_removal != NULL) {
+	if (ztest_device_removal_active) {
 		spa_config_exit(spa, SCL_STATE, FTAG);
 		mutex_exit(&ztest_vdev_lock);
 		return;
@@ -5520,6 +5554,18 @@ ztest_fault_inject(ztest_ds_t *zd, uint64_t id)
 	pathrand = umem_alloc(MAXPATHLEN, UMEM_NOFAIL);
 
 	mutex_enter(&ztest_vdev_lock);
+
+	/*
+	 * Device removal is in progress, fault injection must be disabled
+	 * until it completes and the pool is scrubbed.  The fault injection
+	 * strategy for damaging blocks does not take in to account evacuated
+	 * blocks which may have already been damaged.
+	 */
+	if (ztest_device_removal_active) {
+		mutex_exit(&ztest_vdev_lock);
+		goto out;
+	}
+
 	maxfaults = MAXFAULTS(zs);
 	leaves = MAX(zs->zs_mirrors, 1) * ztest_opts.zo_raidz;
 	mirror_save = zs->zs_mirrors;
@@ -5875,6 +5921,12 @@ ztest_scrub(ztest_ds_t *zd, uint64_t id)
 {
 	spa_t *spa = ztest_spa;
 
+	/*
+	 * Scrub in progress by device removal.
+	 */
+	if (ztest_device_removal_active)
+		return;
+
 	(void) spa_scan(spa, POOL_SCAN_SCRUB);
 	(void) poll(NULL, 0, 100); /* wait a moment, then force a restart */
 	(void) spa_scan(spa, POOL_SCAN_SCRUB);