diff --git a/sys/kern/vfs_bio.c b/sys/kern/vfs_bio.c
index 34f7acb11d07..fe9b189163f6 100644
--- a/sys/kern/vfs_bio.c
+++ b/sys/kern/vfs_bio.c
@@ -622,8 +622,11 @@ bufinit(void)
 
 	/*
 	 * Note: The 16 MB upper limit for hirunningspace was chosen
-	 * arbitrarily and may need further tuning. The lower 1 MB
-	 * limit is the historical upper limit for hirunningspace.
+	 * arbitrarily and may need further tuning. It corresponds to
+	 * 128 outstanding write IO requests (if IO size is 128 KiB),
+	 * which fits with many RAID controllers' tagged queing limits.
+	 * The lower 1 MB limit is the historical upper limit for
+	 * hirunningspace.
 	 */
 	hirunningspace = lmax(lmin(roundup(hibufspace / 64, MAXBSIZE),
 	    16 * 1024 * 1024), 1024 * 1024);