diff --git a/index.js b/index.js
index 256df3fd..5db1cd0e 100644
--- a/index.js
+++ b/index.js
@@ -12,6 +12,7 @@ const Hypercore = require('hypercore')
 const { BLOCK_NOT_AVAILABLE, BAD_ARGUMENT } = require('hypercore-errors')
 const Monitor = require('./lib/monitor')
 const Download = require('./lib/download')
+const header = require('./lib/header')
 
 const keyEncoding = new SubEncoder('files', 'utf-8')
 
@@ -565,7 +566,7 @@ module.exports = class Hyperdrive extends ReadyResource {
     return stream
   }
 
-  createWriteStream(name, { executable = false, metadata = null } = {}) {
+  createWriteStream(name, { executable = false, metadata = null, dedup = false } = {}) {
     const self = this
 
     let destroyed = false
@@ -573,6 +574,8 @@ module.exports = class Hyperdrive extends ReadyResource {
     let ondrain = null
     let onfinish = null
 
+    const blocks = dedup ? { blocks: [], byteLengths: [] } : null
+
     const stream = new Writable({
       open(cb) {
         self.getBlobs().then(onblobs, cb)
@@ -604,6 +607,10 @@ module.exports = class Hyperdrive extends ReadyResource {
         }
       },
       write(data, cb) {
+        if (blocks) {
+          blocks.blocks.push(self.blobs.core.length)
+          blocks.byteLengths.push(data.byteLength)
+        }
         if (ws.write(data) === true) return cb(null)
         ondrain = cb
       },
@@ -626,13 +633,24 @@ module.exports = class Hyperdrive extends ReadyResource {
       onfinish = null
 
       if (err) return cb(err)
-      self.db
-        .put(
-          std(name, false),
-          { executable, linkname: null, blob: ws.id, metadata },
-          { keyEncoding }
-        )
-        .then(() => cb(null), cb)
+
+      flush().then(() => cb(null), cb)
+    }
+
+    async function flush() {
+      let b = null
+
+      if (blocks) {
+        const blks = header.encode(blocks)
+        b = { blockOffset: self.blobs.core.length, blockLength: blks.length }
+        await self.blobs.core.append(blks)
+      }
+
+      await self.db.put(
+        std(name, false),
+        { executable, linkname: null, blob: ws.id, metadata, blocks: b },
+        { keyEncoding }
+      )
     }
 
     function callOndrain(err) {
diff --git a/lib/header.js b/lib/header.js
new file mode 100644
index 00000000..40a1dbde
--- /dev/null
+++ b/lib/header.js
@@ -0,0 +1,48 @@
+const b4a = require('b4a')
+const c = require('compact-encoding')
+
+const SOFT_LIMIT = 128 * 1024
+
+const uints = c.array(c.uint)
+
+exports.encode = function ({ blocks, byteLengths }) {
+  const result = []
+
+  const state = { start: 0, end: 0, buffer: null }
+
+  uints.preencode(state, blocks)
+  uints.preencode(state, byteLengths)
+
+  if (state.end > SOFT_LIMIT) {
+    const result = []
+
+    state.start = state.end = 0
+
+    uints.preencode(state, blocks)
+    state.buffer = b4a.allocUnsafe(state.end)
+    uints.encode(state, blocks)
+
+    result.push(state.buffer)
+
+    state.start = state.end = 0
+
+    uints.preencode(state, byteLengths)
+    state.buffer = b4a.allocUnsafe(state.end)
+    uints.encode(state, byteLengths)
+
+    result.push(state.buffer)
+    return result
+  }
+
+  state.buffer = b4a.allocUnsafe(state.end)
+  uints.encode(state, blocks)
+  uints.encode(state, byteLengths)
+
+  result.push(state.buffer)
+
+  return result
+}
+
+exports.decode = function (buf) {
+
+}