Illumos 4951 - ZFS administrative commands should use reserved space

4951 ZFS administrative commands should use reserved space, not with ENOSPC
Reviewed by: John Kennedy <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Christopher Siden <[email protected]>
Reviewed by: Dan McDonald <[email protected]>
Approved by: Garrett D'Amore <[email protected]>

References:
  https://www.illumos.org/issues/4373
  https://github.com/illumos/illumos-gate/commit/7d46dc6

Ported by: Brian Behlendorf <[email protected]>

1 files changed, 38 insertions, 0 deletions

diff --git a/module/zfs/spa_misc.c b/module/zfs/spa_misc.c
index c17043657..8e5fa683e 100644
--- a/module/zfs/spa_misc.c
+++ b/module/zfs/spa_misc.c
@@ -312,6 +312,32 @@ int zfs_deadman_enabled = 1;
 int spa_asize_inflation = 24;
 
 /*
+ * Normally, we don't allow the last 3.2% (1/(2^spa_slop_shift)) of space in
+ * the pool to be consumed.  This ensures that we don't run the pool
+ * completely out of space, due to unaccounted changes (e.g. to the MOS).
+ * It also limits the worst-case time to allocate space.  If we have
+ * less than this amount of free space, most ZPL operations (e.g. write,
+ * create) will return ENOSPC.
+ *
+ * Certain operations (e.g. file removal, most administrative actions) can
+ * use half the slop space.  They will only return ENOSPC if less than half
+ * the slop space is free.  Typically, once the pool has less than the slop
+ * space free, the user will use these operations to free up space in the pool.
+ * These are the operations that call dsl_pool_adjustedsize() with the netfree
+ * argument set to TRUE.
+ *
+ * A very restricted set of operations are always permitted, regardless of
+ * the amount of free space.  These are the operations that call
+ * dsl_sync_task(ZFS_SPACE_CHECK_NONE), e.g. "zfs destroy".  If these
+ * operations result in a net increase in the amount of space used,
+ * it is possible to run the pool completely out of space, causing it to
+ * be permanently read-only.
+ *
+ * See also the comments in zfs_space_check_t.
+ */
+int spa_slop_shift = 5;
+
+/*
  * ==========================================================================
  * SPA config locking
  * ==========================================================================
@@ -1564,6 +1590,18 @@ spa_get_asize(spa_t *spa, uint64_t lsize)
 	return (lsize * spa_asize_inflation);
 }
 
+/*
+ * Return the amount of slop space in bytes.  It is 1/32 of the pool (3.2%),
+ * or at least 32MB.
+ *
+ * See the comment above spa_slop_shift for details.
+ */
+uint64_t
+spa_get_slop_space(spa_t *spa) {
+	uint64_t space = spa_get_dspace(spa);
+	return (MAX(space >> spa_slop_shift, SPA_MINDEVSIZE >> 1));
+}
+
 uint64_t
 spa_get_dspace(spa_t *spa)
 {