Amazon EFS performance tips - Amazon Elastic File System

Amazon EFS performance tips

When using Amazon EFS, keep the following performance tips in mind.

Average I/O size

The distributed nature of Amazon EFS enables high levels of availability, durability, and scalability. This distributed architecture results in a small latency overhead for each file operation. Because of this per-operation latency, overall throughput generally increases as the average I/O size increases, because the overhead is amortized over a larger amount of data.

Request model

If you enable asynchronous writes to your file system, pending write operations are buffered on the Amazon EC2 instance before they're written to Amazon EFS asynchronously. Asynchronous writes typically have lower latencies. When performing asynchronous writes, the kernel uses additional memory for caching.

A file system that has enabled synchronous writes, or one that opens files using an option that bypasses the cache (for example, O_DIRECT), issues synchronous requests to Amazon EFS. Every operation goes through a round trip between the client and Amazon EFS.


Your chosen request model has tradeoffs in consistency (if you're using multiple Amazon EC2 instances) and speed. Using synchronous writes provides increased data consistency by completing each write request transaction before processing the next request. Using asynchronous writes provides increased throughput by buffering pending write operations.

NFS client mount settings

Verify that you're using the recommended mount options as outlined in Mounting EFS file systems and in Additional mounting considerations.

When mounting your file systems on Amazon EC2 instances, Amazon EFS supports the Network File System version 4.0 and 4.1 (NFSv4) protocols. NFSv4.1 provides better performance for parallel small-file read operations (greater than 10,000 files per second) compared to NFSv4.0 (less than 1,000 files per second). For Amazon EC2 macOS instances running macOS Big Sur, only NFSv4.0 is supported.

Don't use the following mount options:

  • noac, actimeo=0, acregmax=0, acdirmax=0 – These options disable the attribute cache, which has a very large performance impact.

  • lookupcache=pos, lookupcache=none – These options disable the file name lookup cache, which has a very large impact on performance.

  • fsc – This option enables local file caching, but does not change NFS cache coherency, and does not reduce latencies.


When you mount your file system, consider increasing the size of the read and write buffers for your NFS client to 1 MB.

Optimizing small-file performance

You can improve small-file performance by minimizing file reopens, increasing parallelism, and bundling reference files where possible.

  • Minimize the number of round trips to the server.

    Don't unnecessarily close files if you will need them later in a workflow. Keeping file descriptors open enables direct access to the local copy in the cache. File open, close, and metadata operations generally cannot be made asynchronously or through a pipeline.

    When reading or writing small files, the two additional round trips are significant.

    Each round trip (file open, file close) can take as much time as reading or writing megabytes of bulk data. It's more efficient to open an input or output file once, at the beginning of your compute job, and hold it open for the entire length of the job.

  • Use parallelism to reduce the impact of round-trip times.

  • Bundle reference files in a .zip file. Some applications use a large set of small, mostly read-only reference files. Bundling these in a .zip file allows you to read many files with one open-close round trip.

    The .zip format allows for random access to individual files.

Optimizing directory performance

When performing a listing (ls) on very large directories (over 100k files) that are being modified concurrently, Linux NFS clients can hang, not returning a response. This issue is fixed in kernel 5.11, which has been ported to Amazon Linux 2 kernels 4.14, 5.4, and 5.10.

We recommend keeping the number of directories on your file system to less than 10,000, if possible. Use nested subdirectories as much as possible.

When listing a directory, avoid getting file attributes if they are not required, because they are not stored in the directory itself.

Optimizing the NFS read_ahead_kb size

The NFS read_ahead_kb attribute defines the number of kilobytes for the Linux kernel to read ahead or prefetch during a sequential read operation.

For Linux kernel versions prior to 5.4.*, the read_ahead_kb value is set by multiplying NFS_MAX_READAHEAD by the value for rsize (the client configured read buffer size set in the mount options). When using the recommended mount options, this formula sets read_ahead_kb to 15 MB.


Starting with Linux kernel versions 5.4.*, the Linux NFS client uses a default read_ahead_kb value of 128 KB. We recommend increasing this value to 15 MB.

The Amazon EFS mount helper that is available in amazon-efs-utils version 1.33.2 and later automatically modifies the read_ahead_kb value to equal 15 * rsize, or 15 MB, after mounting the file system.

For Linux kernels 5.4 or later, if you do not use the mount helper to mount your file systems, consider manually setting read_ahead_kb to 15 MB for improved performance. After mounting the file system, you can reset the read_ahead_kb value by using the following command. Before using this command, replace the following values:

  • Replace read-ahead-value-kb with the desired size in kilobytes.

  • Replace efs-mount-point with the file system's mount point.

device_number=$(stat -c '%d' efs-mount-point) ((major = ($device_number & 0xFFF00) >> 8)) ((minor = ($device_number & 0xFF) | (($device_number >> 12) & 0xFFF00))) sudo bash -c "echo read-ahead-value-kb > /sys/class/bdi/$major:$minor/read_ahead_kb"

The following example sets the read_ahead_kb size to 15 MB.

device_number=$(stat -c '%d' efs) ((major = ($device_number & 0xFFF00) >> 8)) ((minor = ($device_number & 0xFF) | (($device_number >> 12) & 0xFFF00))) sudo bash -c "echo 15000 > /sys/class/bdi/$major:$minor/read_ahead_kb"