Illumos 6293 - ztest failure: error == 28 (0xc == 0x1c) in ztest_tx_assign()

6293 ztest failure: error == 28 (0xc == 0x1c) in ztest_tx_assign()
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:
  https://www.illumos.org/issues/6293
  https://github.com/illumos/illumos-gate/commit/8fe00bf

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/arc.c

@@ -5343,8 +5343,18 @@ arc_init(void)
 	arc_need_free = 0;
 #endif
 
-	/* Set min cache to allow safe operation of arc_adapt() */
+	/*
+	 * In userland, there's only the memory pressure that we artificially
+	 * create (see arc_available_memory()).  Don't let arc_c get too
+	 * small, because it can cause transactions to be larger than
+	 * arc_c, causing arc_tempreserve_space() to fail.
+	 */
+#ifndef	_KERNEL
+	arc_c_min = arc_c_max / 2;
+#else
 	arc_c_min = 2ULL << SPA_MAXBLOCKSHIFT;
+#endif
+
 	/* Set max to 1/2 of all memory */
 	arc_c_max = allmem / 2;