diff --git a/lib/libspl/include/umem.h b/lib/libspl/include/umem.h
index 0ed55ae5af..87db1f451e 100644
--- a/lib/libspl/include/umem.h
+++ b/lib/libspl/include/umem.h
@@ -37,6 +37,7 @@
  */
 
 #include <stdlib.h>
+#include <stdio.h>
 
 #ifdef  __cplusplus
 extern "C" {
@@ -82,9 +83,30 @@ umem_alloc(size_t size, int flags)
 {
 	void *ptr;
 
-	ptr = malloc(size);
-	while (ptr == NULL && (flags & UMEM_NOFAIL))
+	do {
 		ptr = malloc(size);
+	} while (ptr == NULL && (flags & UMEM_NOFAIL));
+
+	return ptr;
+}
+
+static inline void *
+umem_alloc_aligned(size_t size, size_t align, int flags)
+{
+	void *ptr;
+	int rc;
+
+	do {
+		rc = posix_memalign(&ptr, align, size);
+	} while (rc == ENOMEM && (flags & UMEM_NOFAIL));
+
+	if (rc == EINVAL) {
+		fprintf(stderr, "%s: invalid memory alignment (%zd)\n",
+		    __func__, align);
+		if (flags & UMEM_NOFAIL)
+			abort();
+		return NULL;
+	}
 
 	return ptr;
 }
@@ -146,7 +168,11 @@ umem_cache_alloc(umem_cache_t *cp, int flags)
 {
 	void *ptr;
 
-	ptr = umem_alloc(cp->cache_bufsize, flags);
+	if (cp->cache_align != 0)
+		ptr = umem_alloc_aligned(cp->cache_bufsize, cp->cache_align, flags);
+	else
+		ptr = umem_alloc(cp->cache_bufsize, flags);
+
 	if (ptr && cp->cache_constructor)
 		cp->cache_constructor(ptr, cp->cache_private, UMEM_DEFAULT);
 
diff --git a/lib/libzpool/kernel.c b/lib/libzpool/kernel.c
index 5f767e5949..d65c90a731 100644
--- a/lib/libzpool/kernel.c
+++ b/lib/libzpool/kernel.c
@@ -37,6 +37,7 @@
 #include <sys/zfs_context.h>
 #include <sys/utsname.h>
 #include <sys/time.h>
+#include <sys/mount.h> /* for BLKGETSIZE64 */
 #include <sys/systeminfo.h>
 
 /*
@@ -552,13 +553,13 @@ vn_open(char *path, int x1, int flags, int mode, vnode_t **vpp, int x2, int x3)
 			return (errno);
 	}
 
-#ifdef __linux__
 	if (!(flags & FCREAT) && S_ISBLK(st.st_mode)) {
+#ifdef __linux__
 		flags |= O_DIRECT;
-		if (flags & FWRITE)
-			flags |= O_EXCL;
-	}
 #endif
+		/* We shouldn't be writing to block devices in userspace */
+		VERIFY(!(flags & FWRITE));
+	}
 
 	if (flags & FCREAT)
 		old_umask = umask(0);
@@ -581,6 +582,16 @@ vn_open(char *path, int x1, int flags, int mode, vnode_t **vpp, int x2, int x3)
 		return (err);
 	}
 
+#ifdef __linux__
+	/* In Linux, use an ioctl to get the size of a block device. */
+	if (S_ISBLK(st.st_mode)) {
+		if (ioctl(fd, BLKGETSIZE64, &st.st_size) != 0) {
+			err = errno;
+			close(fd);
+			return (err);
+		}
+	}
+#endif
 	(void) fcntl(fd, F_SETFD, FD_CLOEXEC);
 
 	*vpp = vp = umem_zalloc(sizeof (vnode_t), UMEM_NOFAIL);
@@ -634,6 +645,16 @@ vn_rdwr(int uio, vnode_t *vp, void *addr, ssize_t len, offset_t offset,
 		}
 	}
 
+#ifdef __linux__
+	if (rc == -1 && errno == EINVAL) {
+		/*
+		 * Under Linux, this most likely means an alignment issue
+		 * (memory or disk) due to O_DIRECT, so we abort() in order to
+		 * catch the offender.
+		 */
+		 abort();
+	}
+#endif
 	if (rc == -1)
 		return (errno);