Prefetch start and end of volumes

When adding a zvol to the system prefetch zvol_prefetch_bytes from the
start and end of the volume.  Prefetching these regions of the volume is
desirable because they are likely to be accessed immediately by blkid(8),
the kernel scanning for a partition table, or another task which probes
the devices.

Signed-off-by: Richard Yao <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Issue #3659

man/man5/zfs-module-parameters.5

@@ -1631,6 +1631,21 @@ Max number of blocks to discard at once
 Default value: \fB16,384\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzvol_prefetch_bytes\fR (uint)
+.ad
+.RS 12n
+When adding a zvol to the system prefetch \fBzvol_prefetch_bytes\fR
+from the start and end of the volume.  Prefetching these regions
+of the volume is desirable because they are likely to be accessed
+immediately by \fBblkid(8)\fR or by the kernel scanning for a partition
+table.
+.sp
+Default value: \fB131,072\fR.
+.RE
+
 .SH ZFS I/O SCHEDULER
 ZFS issues I/O operations to leaf vdevs to satisfy and complete I/Os.
 The I/O scheduler determines when and in what order those operations are

module/zfs/zvol.c

@@ -50,6 +50,7 @@
 
 unsigned int zvol_inhibit_dev = 0;
 unsigned int zvol_major = ZVOL_MAJOR;
+unsigned int zvol_prefetch_bytes = (128 * 1024);
 unsigned long zvol_max_discard_blocks = 16384;
 
 static kmutex_t zvol_state_lock;
@@ -1296,6 +1297,7 @@ __zvol_create_minor(const char *name, boolean_t ignore_snapdev)
 	objset_t *os;
 	dmu_object_info_t *doi;
 	uint64_t volsize;
+	uint64_t len;
 	unsigned minor = 0;
 	int error = 0;
 
@@ -1369,6 +1371,18 @@ __zvol_create_minor(const char *name, boolean_t ignore_snapdev)
 			zil_replay(os, zv, zvol_replay_vector);
 	}
 
+	/*
+	 * When udev detects the addition of the device it will immediately
+	 * invoke blkid(8) to determine the type of content on the device.
+	 * Prefetching the blocks commonly scanned by blkid(8) will speed
+	 * up this process.
+	 */
+	len = MIN(MAX(zvol_prefetch_bytes, 0), SPA_MAXBLOCKSIZE);
+	if (len > 0) {
+		dmu_prefetch(os, ZVOL_OBJ, 0, len);
+		dmu_prefetch(os, ZVOL_OBJ, volsize - len, len);
+	}
+
 	zv->zv_objset = NULL;
 out_dmu_objset_disown:
 	dmu_objset_disown(os, zvol_tag);
@@ -1625,3 +1639,6 @@ MODULE_PARM_DESC(zvol_major, "Major number for zvol device");
 
 module_param(zvol_max_discard_blocks, ulong, 0444);
 MODULE_PARM_DESC(zvol_max_discard_blocks, "Max number of blocks to discard");
+
+module_param(zvol_prefetch_bytes, uint, 0644);
+MODULE_PARM_DESC(zvol_prefetch_bytes, "Prefetch N bytes at zvol start+end");