quota: disable quota check for ZVOL

The quota for ZVOLs is set to the size of the volume. When the quota
reaches the maximum, there isn't an excellent way to check if the new
writers are overwriting the data or if they are inserting a new one.
Because of that, when we reach the maximum quota, we wait till txg is
flushed. This is causing a significant fluctuation in bandwidth.

In the case of ZVOL, the quota is enforced by the volsize, so we
can omit it.

This commit adds a sysctl thats allow to control if the quota mechanism
should be enforced or not.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mariusz Zaborski <mariusz.zaborski@klarasystems.com>
Sponsored-by: Zededa Inc.
Sponsored-by: Klara Inc.
Closes #13838

man/man4/zfs.4

@@ -15,7 +15,7 @@
 .\" own identifying information:
 .\" Portions Copyright [yyyy] [name of copyright owner]
 .\"
-.Dd June 1, 2021
+.Dd November 7, 2022
 .Dt ZFS 4
 .Os
 .
@@ -2386,6 +2386,10 @@ Defines zvol block devices behaviour when
 .It Sy 3
 .No equivalent to Sy none
 .El
+.
+.It Sy zvol_enforce_quotas Ns = Ns Sy 0 Ns | Ns 1 Pq uint
+Enable strict ZVOL quota enforcement.
+The strict quota enforcement may have a performance impact.
 .El
 .
 .Sh ZFS I/O SCHEDULER

module/zfs/dsl_dir.c

@@ -53,6 +53,15 @@
 #include "zfs_namecheck.h"
 #include "zfs_prop.h"
 
+/*
+ * This controls if we verify the ZVOL quota or not.
+ * Currently, quotas are not implemented for ZVOLs.
+ * The quota size is the size of the ZVOL.
+ * The size of the volume already implies the ZVOL size quota.
+ * The quota mechanism can introduce a significant performance drop.
+ */
+static int zvol_enforce_quotas = B_TRUE;
+
 /*
  * Filesystem and Snapshot Limits
  * ------------------------------
@@ -1311,7 +1320,9 @@ dsl_dir_tempreserve_impl(dsl_dir_t *dd, uint64_t asize, boolean_t netfree,
 	 * If this transaction will result in a net free of space,
 	 * we want to let it through.
 	 */
-	if (ignorequota || netfree || dsl_dir_phys(dd)->dd_quota == 0)
+	if (ignorequota || netfree || dsl_dir_phys(dd)->dd_quota == 0 ||
+	    (dmu_objset_type(tx->tx_objset) == DMU_OST_ZVOL &&
+	    zvol_enforce_quotas == B_FALSE))
 		quota = UINT64_MAX;
 	else
 		quota = dsl_dir_phys(dd)->dd_quota;
@@ -1399,10 +1410,9 @@ dsl_dir_tempreserve_impl(dsl_dir_t *dd, uint64_t asize, boolean_t netfree,
 		ignorequota = (dsl_dir_phys(dd)->dd_head_dataset_obj == 0);
 		first = B_FALSE;
 		goto top_of_function;
-
-	} else {
-		return (0);
 	}
+
+	return (0);
 }
 
 /*
@@ -2483,3 +2493,7 @@ dsl_dir_cancel_waiters(dsl_dir_t *dd)
 EXPORT_SYMBOL(dsl_dir_set_quota);
 EXPORT_SYMBOL(dsl_dir_set_reservation);
 #endif
+
+/* CSTYLED */
+ZFS_MODULE_PARAM(zfs, , zvol_enforce_quotas, INT, ZMOD_RW,
+	"Enable strict ZVOL quota enforcment");