Chapter 26. 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. It therefore sets the smallest data amount written or read at one time.

The default block size is appropriate for most use cases. However, your file system performs better if the block size matches the typical read or write amount. Optimal performance occurs when the block size equals or slightly exceeds the data typically accessed at once.

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 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, align data and metadata with the underlying storage geometry when formatting. This improves performance.

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 them, specify geometry manually when formatting with mkfs.

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.

Any I/O scheduler is expected to perform well with most SSDs. However, as with any other storage type, benchmark to determine the optimal configuration for a given workload. When using SSDs, change the I/O scheduler only to benchmark 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 Host Bus Adapter (HBA), the default I/O scheduler is deadline. For multiple queue HBA, the default I/O scheduler is none.

By default, iostats is enabled and the default value is 1. Setting iostats to 0 disables gathering of I/O statistics for the device. This removes a small amount of overhead with the I/O path.

Setting iostats to 0 might improve performance for high performance devices, such as certain Non-volatile Memory Express (NVMe) storage devices. It is preferable 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, it 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.

max_sectors_kb must 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. This 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. Increasing the read_ahead_kb value up to request queue’s max_sectors_kb of the disk might improve performance where sequential reading of large files occur.