13389 After persistent L2ARC import, cache device has constant 8KB/sec load
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Dan McDonald <danmcd@joyent.com>
Reviewed by: Toomas Soome <tsoome@me.com>

@@ -8345,10 +8345,11 @@
         uint64_t                write_asize, write_psize, write_lsize, headroom;
         boolean_t               full;
         l2arc_write_callback_t  *cb = NULL;
         zio_t                   *pio, *wzio;
         uint64_t                guid = spa_load_guid(spa);
+        l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
 
         ASSERT3P(dev->l2ad_vdev, !=, NULL);
 
         pio = NULL;
         write_lsize = write_asize = write_psize = 0;

@@ -8563,10 +8564,11 @@
 
                 /*
                  * Although we did not write any buffers l2ad_evict may
                  * have advanced.
                  */
+                if (dev->l2ad_evict != l2dhdr->dh_evict)
                 l2arc_dev_hdr_update(dev);
 
                 return (0);
         }