block: Update topology documentation

Update topology comments and sysfs documentation based upon discussions
with Neil Brown.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
index cbbd3e0..5f3beda 100644 (file)
--- a/Documentation/ABI/testing/sysfs-block
+++ b/Documentation/ABI/testing/sysfs-block
@@ -94,28 +94,37 @@ What:               /sys/block/<disk>/queue/physical_block_size
 Date:          May 2009
 Contact:       Martin K. Petersen <martin.petersen@oracle.com>
 Description:
-               This is the smallest unit the storage device can write
-               without resorting to read-modify-write operation.  It is
-               usually the same as the logical block size but may be
-               bigger.  One example is SATA drives with 4KB sectors
-               that expose a 512-byte logical block size to the
-               operating system.
+               This is the smallest unit a physical storage device can
+               write atomically.  It is usually the same as the logical
+               block size but may be bigger.  One example is SATA
+               drives with 4KB sectors that expose a 512-byte logical
+               block size to the operating system.  For stacked block
+               devices the physical_block_size variable contains the
+               maximum physical_block_size of the component devices.
 
 What:          /sys/block/<disk>/queue/minimum_io_size
 Date:          April 2009
 Contact:       Martin K. Petersen <martin.petersen@oracle.com>
 Description:
-               Storage devices may report a preferred minimum I/O size,
-               which is the smallest request the device can perform
-               without incurring a read-modify-write penalty.  For disk
-               drives this is often the physical block size.  For RAID
-               arrays it is often the stripe chunk size.
+               Storage devices may report a granularity or preferred
+               minimum I/O size which is the smallest request the
+               device can perform without incurring a performance
+               penalty.  For disk drives this is often the physical
+               block size.  For RAID arrays it is often the stripe
+               chunk size.  A properly aligned multiple of
+               minimum_io_size is the preferred request size for
+               workloads where a high number of I/O operations is
+               desired.
 
 What:          /sys/block/<disk>/queue/optimal_io_size
 Date:          April 2009
 Contact:       Martin K. Petersen <martin.petersen@oracle.com>
 Description:
                Storage devices may report an optimal I/O size, which is
-               the device's preferred unit of receiving I/O.  This is
-               rarely reported for disk drives.  For RAID devices it is
-               usually the stripe width or the internal block size.
+               the device's preferred unit for sustained I/O.  This is
+               rarely reported for disk drives.  For RAID arrays it is
+               usually the stripe width or the internal track size.  A
+               properly aligned multiple of optimal_io_size is the
+               preferred request size for workloads where sustained
+               throughput is desired.  If no optimal I/O size is
+               reported this file contains 0.

diff --git a/block/blk-settings.c b/block/blk-settings.c
index e1327dd..476d870 100644 (file)
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -413,10 +413,13 @@ EXPORT_SYMBOL(blk_limits_io_min);
  * @min:  smallest I/O size in bytes
  *
  * Description:
- *   Some devices have an internal block size bigger than the reported
- *   hardware sector size.  This function can be used to signal the
- *   smallest I/O the device can perform without incurring a performance
- *   penalty.
+ *   Storage devices may report a granularity or preferred minimum I/O
+ *   size which is the smallest request the device can perform without
+ *   incurring a performance penalty.  For disk drives this is often the
+ *   physical block size.  For RAID arrays it is often the stripe chunk
+ *   size.  A properly aligned multiple of minimum_io_size is the
+ *   preferred request size for workloads where a high number of I/O
+ *   operations is desired.
  */
 void blk_queue_io_min(struct request_queue *q, unsigned int min)
 {
@@ -430,8 +433,12 @@ EXPORT_SYMBOL(blk_queue_io_min);
  * @opt:  optimal request size in bytes
  *
  * Description:
- *   Drivers can call this function to set the preferred I/O request
- *   size for devices that report such a value.
+ *   Storage devices may report an optimal I/O size, which is the
+ *   device's preferred unit for sustained I/O.  This is rarely reported
+ *   for disk drives.  For RAID arrays it is usually the stripe width or
+ *   the internal track size.  A properly aligned multiple of
+ *   optimal_io_size is the preferred request size for workloads where
+ *   sustained throughput is desired.
  */
 void blk_queue_io_opt(struct request_queue *q, unsigned int opt)
 {