From cf0a432027b4ad1b0db936b27872de978460c1d9 Mon Sep 17 00:00:00 2001
From: Trevor Renshaw <8571163+trevren11@users.noreply.github.com>
Date: Sat, 16 May 2026 08:50:36 -0600
Subject: [PATCH] Fix slow downloads: larger buffers, Content-Length headers,
 parallel streams

---
 server/controllers/LibraryItemController.js | 11 +++++++++++
 server/utils/fileUtils.js                   |  5 +++--
 server/utils/zipHelpers.js                  | 10 ++++++++--
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/server/controllers/LibraryItemController.js b/server/controllers/LibraryItemController.js
index 07b4ee67f..ee8e15d77 100644
--- a/server/controllers/LibraryItemController.js
+++ b/server/controllers/LibraryItemController.js
@@ -184,6 +184,11 @@ class LibraryItemController {
         if (audioMimeType) {
           res.setHeader('Content-Type', audioMimeType)
         }
+        // Set Content-Length so clients can show accurate progress bars.
+        const fileStat = await fs.stat(libraryItemPath).catch(() => null)
+        if (fileStat) {
+          res.setHeader('Content-Length', fileStat.size)
+        }
         await new Promise((resolve, reject) => res.download(libraryItemPath, req.libraryItem.relPath, (error) => (error ? reject(error) : resolve())))
       } else {
         const filename = `${itemTitle}.zip`
@@ -1097,6 +1102,12 @@ class LibraryItemController {
     }
 
     try {
+      // Set Content-Length so clients (including the Android app) can show accurate
+      // download progress and avoid chunked-transfer encoding when the file size is known.
+      const fileStat = await fs.stat(libraryFile.metadata.path).catch(() => null)
+      if (fileStat) {
+        res.setHeader('Content-Length', fileStat.size)
+      }
       await new Promise((resolve, reject) => res.download(libraryFile.metadata.path, libraryFile.metadata.filename, (error) => (error ? reject(error) : resolve())))
       Logger.info(`[LibraryItemController] Downloaded file "${libraryFile.metadata.path}"`)
     } catch (error) {
diff --git a/server/utils/fileUtils.js b/server/utils/fileUtils.js
index 9a349bd54..6164efd0e 100644
--- a/server/utils/fileUtils.js
+++ b/server/utils/fileUtils.js
@@ -318,8 +318,9 @@ module.exports.downloadFile = (url, filepath, contentTypeFilter = null) => {
         const totalSize = parseInt(response.headers['content-length'], 10)
         let downloadedSize = 0
 
-        // Write to filepath
-        const writer = fs.createWriteStream(filepath)
+        // Use a 512 KiB write buffer. Node's default 16 KiB highWaterMark causes
+        // many small write syscalls for large audio files and limits throughput.
+        const writer = fs.createWriteStream(filepath, { highWaterMark: 524288 })
         response.data.pipe(writer)
 
         let lastProgress = 0
diff --git a/server/utils/zipHelpers.js b/server/utils/zipHelpers.js
index 421ca73cd..10fab4d1a 100644
--- a/server/utils/zipHelpers.js
+++ b/server/utils/zipHelpers.js
@@ -9,7 +9,11 @@ module.exports.zipDirectoryPipe = (path, filename, res) => {
     res.attachment(filename)
 
     const archive = archiver('zip', {
-      zlib: { level: 0 } // Sets the compression level.
+      zlib: { level: 0 }, // Sets the compression level (0 = store, no CPU overhead).
+      // Increase the internal pipeline highWaterMark from 16 KiB to 512 KiB.
+      // This allows archiver to buffer more data before applying backpressure,
+      // keeping the read-from-disk and write-to-network pipelines busy.
+      highWaterMark: 524288
     })
 
     // listen for all archive data to be written
@@ -67,7 +71,9 @@ module.exports.zipDirectoriesPipe = (pathObjects, filename, res) => {
     res.attachment(filename)
 
     const archive = archiver('zip', {
-      zlib: { level: 0 } // Sets the compression level.
+      zlib: { level: 0 }, // Sets the compression level (0 = store, no CPU overhead).
+      // Increase the internal pipeline highWaterMark from 16 KiB to 512 KiB.
+      highWaterMark: 524288
     })
 
     // listen for all archive data to be written