Readahead value for Linux NFS

Author: khdownie

articles/storage/files/nfs-performance.md

@@ -4,7 +4,7 @@ description: Learn ways to improve the performance of NFS Azure file shares at s
 author: khdownie
 ms.service: azure-file-storage
 ms.topic: conceptual
-ms.date: 09/21/2023
+ms.date: 09/25/2023
 ms.author: kendownie
 ---
 
@@ -18,6 +18,31 @@ This article explains how you can improve performance for NFS Azure file shares.
 | Standard file shares (GPv2), GRS/GZRS | ![No, this article doesn't apply to standard SMB Azure file shares GRS/GZRS.](../media/icons/no-icon.png) | ![NFS is only available in premium Azure file shares.](../media/icons/no-icon.png) |
 | Premium file shares (FileStorage), LRS/ZRS | ![No, this article doesn't apply to premium SMB Azure file shares.](../media/icons/no-icon.png) | ![Yes, this article applies to premium NFS Azure file shares.](../media/icons/yes-icon.png) |
 
+## Increase read-ahead size to improve read throughput
+
+The `read_ahead_kb` kernel parameter in Linux represents the amount of data that should be "read ahead" or prefetched during a sequential read operation. Linux kernel versions prior to 5.4 set the read-ahead value to the equivalent of 15 times the mounted file system's `rsize` (the client-side mount option for read buffer size). This sets the read-ahead value high enough to improve client sequential read throughput in most cases.
+
+However, beginning with Linux kernel version 5.4, the Linux NFS client uses a default `read_ahead_kb` value of 128 KiB. This small value might reduce the amount of read throughput for large files. Customers upgrading from Linux releases with the larger read-ahead value to those with the 128 KiB default might experience a decrease in sequential read performance.
+
+For Linux kernels 5.4 or later, we recommend persistently setting the `read_ahead_kb` to 15 MiB for improved performance.
+
+To change this value, set the read-ahead size by adding a rule in udev, a Linux kernel device manager. Follow these steps:
+
+1. In a text editor, create the */etc/udev/rules.d/99-nfs.rules* file by entering and saving the following text:
+
+   ```output
+   SUBSYSTEM=="bdi" \
+   , ACTION=="add" \
+   , PROGRAM="<absolute_path>/awk -v bdi=$kernel 'BEGIN{ret=1} {if ($4 == bdi) {ret=0}} END{exit ret}' /proc/fs/nfsfs/volumes" \
+   , ATTR{read_ahead_kb}="15360"
+   ```
+
+1. In a console, apply the udev rule by running the [udevadm](https://www.man7.org/linux/man-pages/man8/udevadm.8.html) command as a superuser:
+
+   ```bash
+   sudo udevadm control --reload
+   ```
+
 ## `Nconnect`
 
 `Nconnect` is a client-side Linux mount option that increases performance at scale by allowing you to use more TCP connections between the client and the Azure Premium Files service for NFSv4.1, while maintaining the resiliency of platform as a service (PaaS).