Chapter 33. Factors affecting I/O and file system performance

The block is the unit of work for the file system. The block size determines how much data can be stored in a single block, and therefore the smallest amount of data that is written or read at one time.

The default block size is appropriate for most use cases. However, your file system performs better and stores data more efficiently if the block size or the size of multiple blocks is the same as or slightly larger than the amount of data that is typically read or written at one time. A small file still uses an entire block. Files can be spread across multiple blocks, but this can create additional runtime overhead.

Additionally, some file systems are limited to a certain number of blocks, which in turn limits the maximum size of the file system. Block size is specified as part of the file system options when formatting a device with the mkfs command. The parameter that specifies the block size varies with the file system.

File system geometry is concerned with the distribution of data across a file system. If your system uses striped storage, like RAID, you can improve performance by aligning data and metadata with the underlying storage geometry when you format the device.

Many devices export recommended geometry, which is then set automatically when the devices are formatted with a particular file system. If your device does not export these recommendations, or you want to change the recommended settings, you must specify geometry manually when you format the device with the mkfs command.

The parameters that specify file system geometry vary with the file system.

Every time a file is read, its metadata is updated with the time at which access occurred (atime). This involves additional write I/O. The relatime is the default atime setting for most file systems.

However, if updating this metadata is time consuming, and if accurate access time data is not required, you can mount the file system with the noatime mount option. This disables updates to metadata when a file is read. It also enables nodiratime behavior, which disables updates to metadata when a directory is read.

Read-ahead behavior speeds up file access by pre-fetching data that is likely to be needed soon and loading it into the page cache, where it can be retrieved more quickly than if it were on disk. The higher the read-ahead value, the further ahead the system pre-fetches data.

Red Hat Enterprise Linux attempts to set an appropriate read-ahead value based on what it detects about your file system. However, accurate detection is not always possible. For example, if a storage array presents itself to the system as a single LUN, the system detects the single LUN, and does not set the appropriate read-ahead value for an array.

Workloads that involve heavy streaming of sequential I/O often benefit from high read-ahead values. The storage-related tuned profiles provided with Red Hat Enterprise Linux raise the read-ahead value, as does using LVM striping, but these adjustments are not always sufficient for all workloads.

This type of discard operation is configured at mount time with the discard option, and runs in real time without user intervention. However, it only discards blocks that are transitioning from used to free. Red Hat Enterprise Linux 8 supports online discard on XFS and ext4 formatted devices.

Red Hat recommends batch discard, except where online discard is required to maintain performance, or where batch discard is not feasible for the system’s workload.

Any I/O scheduler is expected to perform well with most SSDs. However, as with any other storage type, Red Hat recommends benchmarking to determine the optimal configuration for a given workload. When using SSDs, Red Hat advises changing the I/O scheduler only for benchmarking particular workloads. For instructions on how to switch between I/O schedulers, see the /usr/share/doc/kernel-version/Documentation/block/switching-sched.txt file.

For single queue HBA, the default I/O scheduler is deadline. For multiple queue HBA, the default I/O scheduler is none. For information about how to set the I/O scheduler, see Setting the disk scheduler.

By default, iostats is enabled and the default value is 1. Setting iostats value to 0 disables the gathering of I/O statistics for the device, which removes a small amount of overhead with the I/O path. Setting iostats to 0 might slightly improve performance for very high performance devices, such as certain NVMe solid-state storage devices. It is recommended to leave iostats enabled unless otherwise specified for the given storage model by the vendor.

If you disable iostats, the I/O statistics for the device are no longer present within the /proc/diskstats file. The content of /sys/diskstats file is the source of I/O information for monitoring I/O tools, such as sar or iostats. Therefore, if you disable the iostats parameter for a device, the device is no longer present in the output of I/O monitoring tools.

Specifies the maximum size of an I/O request in kilobytes. The default value is 512 KB. The minimum value for this parameter is determined by the logical block size of the storage device. The maximum value for this parameter is determined by the value of the max_hw_sectors_kb.

Red Hat recommends max_sectors_kb to always be a multiple of the optimal I/O size and the internal erase block size. Use a value of logical_block_size for either parameter if they are zero or not specified by the storage device.

Defines the maximum number of kilobytes that the operating system may read ahead during a sequential read operation. As a result, the necessary information is already present within the kernel page cache for the next sequential read, which improves read I/O performance.

Device mappers often benefit from a high read_ahead_kb value. 128 KB for each device to be mapped is a good starting point, but increasing the read_ahead_kb value up to request queue’s max_sectors_kb of the disk might improve performance in application environments where sequential reading of large files takes place.