doc update fs.read() documentation for clarity

doc/api/fs.md

@@ -3072,6 +3072,18 @@ extended part is filled with null bytes (`'\0'`):
 
 If `len` is negative then `0` will be used.
 
+The `fs.read()` method reads data from the file specified by the file descriptor (`fd`). The `length` argument indicates the maximum number of bytes that Node.js will attempt to read from the kernel. However, the actual number of bytes read (`bytesRead`) can be lower than the specified `length` for various reasons.
+
+For example:
+- If the file is shorter than the specified `length`, `bytesRead` will be set to the actual number of bytes read.
+- If the file encounters EOF (End of File) before the buffer could be filled, the callback will be called with an error.
+- If the file is longer than the specified `length`, only `length` bytes will be read from the file, and `bytesRead` will be set to `length`.
+- If the file is on a slow network filesystem or encounters any other issue during reading, `bytesRead` can be lower than the specified `length`.
+
+Therefore, when using `fs.read()`, it's important to check the `bytesRead` value to determine how many bytes were actually read from the file. Depending on your application logic, you may need to handle cases where `bytesRead` is lower than the specified `length`, such as by wrapping the read call in a loop if you require a minimum amount of bytes.
+
+This behavior is similar to the POSIX `preadv2` function.
+
 ### `fs.futimes(fd, atime, mtime, callback)`
 
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