Chapter 23. Storage I/O Alignment and Size

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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 bytes 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 I/O 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 size data, then storage management tools in Red Hat Enterprise Linux 6 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 indicate any required/preferred I/O alignment and size.
Refer to Section 23.2, “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.
The IO scheduler has changed for Red Hat Enterprise Linux 7. Default IO Scheduler is now Deadline, except for SATA drives. CFQ is the default IO scheduler for SATA drives. For faster storage, Deadline outperforms CFQ and when it is used there is a performance increase without the need of special tuning.
If default is not right for some disks (for example, SAS rotational disks), then change the IO scheduler to CFQ. This instance will depend on the workload.

23.1. Parameters for Storage Access

The operating system uses the following information to determine I/O alignment and size:
Smallest internal unit on which the device can operate
Used externally to address a location on the device
The number of bytes that the beginning of the Linux block device (partition/MD/LVM device) is offset from the underlying physical alignment
The device’s preferred minimum unit for random I/O
The device’s preferred unit for streaming I/O
For 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.
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