Bug 607345

The release note Bug 580851 - "4.1. Storage Input/Output Limits" is confusing

correctly indicates that the term "limits" is misleading - "sounds like more general meaning, such as the device size limits, and/or some kind of QoS feature".

To address this, we would like to change the phrase "I/O limits" to the more specific term "I/O Alignment and Size", and when the context is clear, "I/O Parameters". This should be done in both the release notes and in this document. 

I would suggest replacing the existing text at the beginning of Chapter 20 as follows:

Chapter 20. [NEW!] Storage Input/Output (I/O) Alignment and Size 

Recent enhancements to the SCSI and ATA standards allow storage devices to
indicate their preferred (and in some cases, required) I/O alignment and I/O
size. This information is particularly useful with newer disk drives that
increase the physical sector size from 512 byes to 4K bytes. This information
may also be beneficial for RAID devices, where the chunk size and stripe size
may impact performance. 

The Linux I/O stack has been enhanced to process vendor-provided I/O alignment and size information, allowing storage management tools (parted, lvm, mkfs.*, and the like) to optimize data placement and access. If a legacy device does not export I/O alignment and I/O size data, then the storage management tools will conservatively align I/O on a 4k (or larger power of 2) boundary. This will ensure that 4K-sector devices operate correctly even if they do not implement this feature.  

Note: As of this release, 4K sector devices are supported as data disks, not as boot disks. Boot support for 4K sector devices is planned for a later release.  

Refer to Section 20.2, “Userspace Access” to learn how to determine the information that the operating system obtained from the device. This data is subsequently used by the storage management tools to determine data placement.  

20.1. Parameters for Storage Access

The information that the operating system uses to determine I/O alignment and size is as follows: 

• physical_block_size - smallest internal unit the device can operate on
• logical_block_size - used externally to address a location on the device
• alignment_offset - the number of bytes the beginning of the Linux block device
  (partition/MD/LVM device) is offset from the underlying physical alignment
• minimum_io_size - device’s preferred minimum unit for random I/O
• optimal_io_size - device’s preferred unit for streaming I/O

As an example, certain 4K sector devices may use a 4K physical_block_size internally but expose a more granular 512-byte logical_block_size to Linux. This discrepancy introduces potential for misaligned I/O.
To address this, the Red Hat Enterprise Linux 6 I/O stack will attempt to start all data areas on a naturally aligned boundary (physical_block_size) by making sure it accounts for any alignment_offset if the beginning of the block device is offset from the underlying physical alignment.

Storage vendors can also supply I/O hints about the preferred minimum unit for random I/O (minimum_io_size) and streaming I/O (optimal_io_size) of a device. For example, minimum_io_size and optimal_io_size may correspond to a RAID device's chunk size and stripe size respectively. 


< For the remainder of Chapter 20, just replace "I/O limits" with "I/O parameters" throughout. >

Mike, please review.