diff --git a/services/history-v1/storage/lib/blob_store/index.js b/services/history-v1/storage/lib/blob_store/index.js
index 9c762202a3..c4151637c3 100644
--- a/services/history-v1/storage/lib/blob_store/index.js
+++ b/services/history-v1/storage/lib/blob_store/index.js
@@ -125,6 +125,34 @@ async function loadGlobalBlobs() {
   }
 }
 
+/**
+ * Return metadata for all blobs in the given project
+ * @param {Array<string|number>} projectIds
+ * @return {Promise<{nBlobs:number, blobs:Map<string,Array<core.Blob>>}>}
+ */
+async function getProjectBlobsBatch(projectIds) {
+  const mongoProjects = []
+  const postgresProjects = []
+  for (const projectId of projectIds) {
+    if (typeof projectId === 'number') {
+      postgresProjects.push(projectId)
+    } else {
+      mongoProjects.push(projectId)
+    }
+  }
+  const [
+    { nBlobs: nBlobsPostgres, blobs: blobsPostgres },
+    { nBlobs: nBlobsMongo, blobs: blobsMongo },
+  ] = await Promise.all([
+    postgresBackend.getProjectBlobsBatch(postgresProjects),
+    mongoBackend.getProjectBlobsBatch(mongoProjects),
+  ])
+  for (const [id, blobs] of blobsPostgres.entries()) {
+    blobsMongo.set(id.toString(), blobs)
+  }
+  return { nBlobs: nBlobsPostgres + nBlobsMongo, blobs: blobsMongo }
+}
+
 /**
  * @classdesc
  * Fetch and store the content of files using content-addressable hashing. The
@@ -366,6 +394,7 @@ class BlobStore {
 
 module.exports = {
   BlobStore,
+  getProjectBlobsBatch,
   loadGlobalBlobs,
   makeProjectKey,
   makeBlobForFile,
diff --git a/services/history-v1/storage/lib/blob_store/mongo.js b/services/history-v1/storage/lib/blob_store/mongo.js
index 276c7f097f..9117382148 100644
--- a/services/history-v1/storage/lib/blob_store/mongo.js
+++ b/services/history-v1/storage/lib/blob_store/mongo.js
@@ -16,13 +16,17 @@
  */
 
 const { Blob } = require('overleaf-editor-core')
-const { ObjectId, Binary, MongoError } = require('mongodb')
+const { ObjectId, Binary, MongoError, ReadPreference } = require('mongodb')
 const assert = require('../assert')
 const mongodb = require('../mongodb')
 
 const MAX_BLOBS_IN_BUCKET = 8
 const DUPLICATE_KEY_ERROR_CODE = 11000
 
+/**
+ * @typedef {import('mongodb').ReadPreferenceLike} ReadPreferenceLike
+ */
+
 /**
  * Set up the data structures for a given project.
  * @param {string} projectId
@@ -246,6 +250,60 @@ async function getProjectBlobs(projectId) {
   return blobs
 }
 
+/**
+ * Return metadata for all blobs in the given project
+ * @param {Array<string>} projectIds
+ * @return {Promise<{ nBlobs: number, blobs: Map<string, Array<Blob>> }>}
+ */
+async function getProjectBlobsBatch(projectIds) {
+  for (const project of projectIds) {
+    assert.mongoId(project, 'bad projectId')
+  }
+  let nBlobs = 0
+  const blobs = new Map()
+  if (projectIds.length === 0) return { nBlobs, blobs }
+
+  // blobs
+  {
+    const cursor = await mongodb.blobs.find(
+      { _id: { $in: projectIds.map(projectId => new ObjectId(projectId)) } },
+      { readPreference: ReadPreference.secondaryPreferred }
+    )
+    for await (const record of cursor) {
+      const projectBlobs = Object.values(record.blobs).flat().map(recordToBlob)
+      blobs.set(record._id.toString(), projectBlobs)
+      nBlobs += projectBlobs.length
+    }
+  }
+
+  // sharded blobs
+  {
+    // @ts-ignore We are using a custom _id here.
+    const cursor = await mongodb.shardedBlobs.find(
+      {
+        _id: {
+          $gte: makeShardedId(projectIds[0], '0'),
+          $lte: makeShardedId(projectIds[projectIds.length - 1], 'f'),
+        },
+      },
+      { readPreference: ReadPreference.secondaryPreferred }
+    )
+    for await (const record of cursor) {
+      const recordIdHex = record._id.toString('hex')
+      const recordProjectId = recordIdHex.slice(0, 24)
+      const projectBlobs = Object.values(record.blobs).flat().map(recordToBlob)
+      const found = blobs.get(recordProjectId)
+      if (found) {
+        found.push(...projectBlobs)
+      } else {
+        blobs.set(recordProjectId, projectBlobs)
+      }
+      nBlobs += projectBlobs.length
+    }
+  }
+  return { nBlobs, blobs }
+}
+
 /**
  * Add a blob's metadata to the blobs collection after it has been uploaded.
  * @param {string} projectId
@@ -373,6 +431,7 @@ module.exports = {
   findBlob,
   findBlobs,
   getProjectBlobs,
+  getProjectBlobsBatch,
   insertBlob,
   deleteBlobs,
 }
diff --git a/services/history-v1/storage/lib/blob_store/postgres.js b/services/history-v1/storage/lib/blob_store/postgres.js
index fc41644138..715a985b5b 100644
--- a/services/history-v1/storage/lib/blob_store/postgres.js
+++ b/services/history-v1/storage/lib/blob_store/postgres.js
@@ -70,6 +70,35 @@ async function getProjectBlobs(projectId) {
   return blobs
 }
 
+/**
+ * Return metadata for all blobs in the given project
+ * @param {Array<number>} projectIds
+ * @return {Promise<{ nBlobs: number, blobs: Map<number, Array<Blob>> }>}
+ */
+async function getProjectBlobsBatch(projectIds) {
+  for (const projectId of projectIds) {
+    assert.integer(projectId, 'bad projectId')
+  }
+  let nBlobs = 0
+  const blobs = new Map()
+  if (projectIds.length === 0) return { nBlobs, blobs }
+
+  const records = await knex('project_blobs')
+    .select('project_id', 'hash_bytes', 'byte_length', 'string_length')
+    .whereIn('project_id', projectIds)
+
+  for (const record of records) {
+    const found = blobs.get(record.project_id)
+    if (found) {
+      found.push(recordToBlob(record))
+    } else {
+      blobs.set(record.project_id, [recordToBlob(record)])
+    }
+    nBlobs++
+  }
+  return { nBlobs, blobs }
+}
+
 /**
  * Add a blob's metadata to the blobs table after it has been uploaded.
  */
@@ -126,6 +155,7 @@ module.exports = {
   findBlob,
   findBlobs,
   getProjectBlobs,
+  getProjectBlobsBatch,
   insertBlob,
   deleteBlobs,
 }
diff --git a/services/history-v1/storage/scripts/back_fill_file_hash.mjs b/services/history-v1/storage/scripts/back_fill_file_hash.mjs
index 3e919018f4..c1d76a8bcd 100644
--- a/services/history-v1/storage/scripts/back_fill_file_hash.mjs
+++ b/services/history-v1/storage/scripts/back_fill_file_hash.mjs
@@ -25,11 +25,12 @@ import {
   BlobStore,
   GLOBAL_BLOBS,
   loadGlobalBlobs,
+  getProjectBlobsBatch,
   getStringLengthOfFile,
   makeBlobForFile,
   makeProjectKey,
 } from '../lib/blob_store/index.js'
-import { backedUpBlobs, db } from '../lib/mongodb.js'
+import { backedUpBlobs as backedUpBlobsCollection, db } from '../lib/mongodb.js'
 import filestorePersistor from '../lib/persistor.js'
 
 // Silence warning.
@@ -68,10 +69,7 @@ ObjectId.cacheHexString = true
  * @typedef {Object} Project
  * @property {ObjectId} _id
  * @property {Array<Folder>} rootFolder
- * @property {Array<string>} deletedFileIds
- * @property {Array<Blob>} blobs
- * @property {{history: {id: string}}} overleaf
- * @property {Array<string>} [backedUpBlobs]
+ * @property {{history: {id: (number|string)}}} overleaf
  */
 
 /**
@@ -499,22 +497,16 @@ async function processFiles(files) {
  * @return {Promise<void>}
  */
 async function handleLiveTreeBatch(batch, prefix = 'rootFolder.0') {
-  let nBackedUpBlobs = 0
-  if (process.argv.includes('collectBackedUpBlobs')) {
-    nBackedUpBlobs = await collectBackedUpBlobs(batch)
-  }
-  if (process.argv.includes('deletedFiles')) {
-    await collectDeletedFiles(batch)
-  }
-  let blobs = 0
-  if (COLLECT_BLOBS) {
-    blobs = await collectBlobs(batch)
-  }
-  const files = Array.from(findFileInBatch(batch, prefix))
+  const deletedFiles = await collectDeletedFiles(batch)
+  const { nBlobs, blobs } = await collectProjectBlobs(batch)
+  const { nBackedUpBlobs, backedUpBlobs } = await collectBackedUpBlobs(batch)
+  const files = Array.from(
+    findFileInBatch(batch, prefix, deletedFiles, blobs, backedUpBlobs)
+  )
   STATS.projects += batch.length
-  STATS.blobs += blobs
+  STATS.blobs += nBlobs
   STATS.backedUpBlobs += nBackedUpBlobs
-  STATS.filesWithoutHash += files.length - (blobs - nBackedUpBlobs)
+  STATS.filesWithoutHash += files.length - (nBlobs - nBackedUpBlobs)
   batch.length = 0 // GC
   // The files are currently ordered by project-id.
   // Order them by file-id ASC then blobs ASC to
@@ -709,17 +701,36 @@ function* findFiles(ctx, folder, path) {
 /**
  * @param {Array<Project>} projects
  * @param {string} prefix
+ * @param {Map<string,Array<string>>} deletedFiles
+ * @param {Map<string,Array<Blob>>} blobs
+ * @param {Map<string,Array<string>>} backedUpBlobs
  * @return Generator<QueueEntry>
  */
-function* findFileInBatch(projects, prefix) {
+function* findFileInBatch(
+  projects,
+  prefix,
+  deletedFiles,
+  blobs,
+  backedUpBlobs
+) {
   for (const project of projects) {
-    const ctx = new ProjectContext(project)
+    const projectIdS = project._id.toString()
+    const historyIdS = project.overleaf.history.id.toString()
+    const projectBlobs = blobs.get(historyIdS) || []
+    const projectBackedUpBlobs = new Set(backedUpBlobs.get(projectIdS) || [])
+    const projectDeletedFiles = deletedFiles.get(projectIdS) || []
+    const ctx = new ProjectContext(
+      project._id,
+      historyIdS,
+      projectBlobs,
+      projectBackedUpBlobs
+    )
     yield* findFiles(ctx, project.rootFolder[0], prefix)
-    for (const fileId of project.deletedFileIds || []) {
+    for (const fileId of projectDeletedFiles) {
       yield { ctx, cacheKey: fileId, fileId, path: '' }
     }
-    for (const blob of project.blobs || []) {
-      if (ctx.hasBackedUpBlob(blob.getHash())) continue
+    for (const blob of projectBlobs) {
+      if (projectBackedUpBlobs.has(blob.getHash())) continue
       yield {
         ctx,
         cacheKey: blob.getHash(),
@@ -732,25 +743,22 @@ function* findFileInBatch(projects, prefix) {
 }
 
 /**
- * @param {Array<Project>} projects
- * @return {Promise<number>}
+ * @param {Array<Project>} batch
+ * @return {Promise<{nBlobs: number, blobs: Map<string, Array<Blob>>}>}
  */
-async function collectBlobs(projects) {
-  let blobs = 0
-  for (const project of projects) {
-    const historyId = project.overleaf.history.id.toString()
-    const blobStore = new BlobStore(historyId)
-    project.blobs = await blobStore.getProjectBlobs()
-    blobs += project.blobs.length
-  }
-  return blobs
+async function collectProjectBlobs(batch) {
+  if (!COLLECT_BLOBS) return { nBlobs: 0, blobs: new Map() }
+  return await getProjectBlobsBatch(batch.map(p => p.overleaf.history.id))
 }
 
 /**
  * @param {Array<Project>} projects
- * @return {Promise<void>}
+ * @return {Promise<Map<string, Array<string>>>}
  */
 async function collectDeletedFiles(projects) {
+  const deletedFiles = new Map()
+  if (!process.argv.includes('deletedFiles')) return deletedFiles
+
   const cursor = deletedFilesCollection.find(
     {
       projectId: { $in: projects.map(p => p._id) },
@@ -762,52 +770,42 @@ async function collectDeletedFiles(projects) {
       sort: { projectId: 1 },
     }
   )
-  const processed = projects.slice()
   for await (const deletedFileRef of cursor) {
-    const idx = processed.findIndex(
-      p => p._id.toString() === deletedFileRef.projectId.toString()
-    )
-    if (idx === -1) {
-      throw new Error(
-        `bug: order of deletedFiles mongo records does not match batch of projects (${deletedFileRef.projectId} out of order)`
-      )
+    const projectId = deletedFileRef.projectId.toString()
+    const fileId = deletedFileRef._id.toString()
+    const found = deletedFiles.get(projectId)
+    if (found) {
+      found.push(fileId)
+    } else {
+      deletedFiles.set(projectId, [fileId])
     }
-    processed.splice(0, idx)
-    const project = processed[0]
-    project.deletedFileIds = project.deletedFileIds || []
-    project.deletedFileIds.push(deletedFileRef._id.toString())
   }
+  return deletedFiles
 }
 
 /**
  * @param {Array<Project>} projects
- * @return {Promise<number>}
+ * @return {Promise<{nBackedUpBlobs:number,backedUpBlobs:Map<string,Array<string>>}>}
  */
 async function collectBackedUpBlobs(projects) {
-  const cursor = backedUpBlobs.find(
+  let nBackedUpBlobs = 0
+  const backedUpBlobs = new Map()
+  if (!process.argv.includes('collectBackedUpBlobs')) {
+    return { nBackedUpBlobs, backedUpBlobs }
+  }
+  const cursor = backedUpBlobsCollection.find(
     { _id: { $in: projects.map(p => p._id) } },
     {
       readPreference: READ_PREFERENCE_SECONDARY,
       sort: { _id: 1 },
     }
   )
-  let nBackedUpBlobs = 0
-  const processed = projects.slice()
   for await (const record of cursor) {
-    const idx = processed.findIndex(
-      p => p._id.toString() === record._id.toString()
-    )
-    if (idx === -1) {
-      throw new Error(
-        `bug: order of backedUpBlobs mongo records does not match batch of projects (${record._id} out of order)`
-      )
-    }
-    processed.splice(0, idx)
-    const project = processed[0]
-    project.backedUpBlobs = record.blobs.map(b => b.toString('hex'))
-    nBackedUpBlobs += record.blobs.length
+    const blobs = record.blobs.map(b => b.toString('hex'))
+    backedUpBlobs.set(record._id.toString(), blobs)
+    nBackedUpBlobs += blobs.length
   }
-  return nBackedUpBlobs
+  return { nBackedUpBlobs, backedUpBlobs }
 }
 
 const BATCH_HASH_WRITES = 1_000
@@ -824,13 +822,16 @@ class ProjectContext {
   #historyBlobs
 
   /**
-   * @param {Project} project
+   * @param {ObjectId} projectId
+   * @param {string} historyId
+   * @param {Array<Blob>} blobs
+   * @param {Set<string>} backedUpBlobs
    */
-  constructor(project) {
-    this.projectId = project._id
-    this.historyId = project.overleaf.history.id.toString()
-    this.#backedUpBlobs = new Set(project.backedUpBlobs || [])
-    this.#historyBlobs = new Set((project.blobs || []).map(b => b.getHash()))
+  constructor(projectId, historyId, blobs, backedUpBlobs) {
+    this.projectId = projectId
+    this.historyId = historyId
+    this.#backedUpBlobs = backedUpBlobs
+    this.#historyBlobs = new Set(blobs.map(b => b.getHash()))
   }
 
   hasHistoryBlob(hash) {
@@ -901,7 +902,7 @@ class ProjectContext {
     )
     this.#completedBlobs.clear()
     STATS.mongoUpdates++
-    await backedUpBlobs.updateOne(
+    await backedUpBlobsCollection.updateOne(
       { _id: this.projectId },
       { $addToSet: { blobs: { $each: blobs } } },
       { upsert: true }
diff --git a/services/history-v1/test/acceptance/js/storage/blob_store.test.js b/services/history-v1/test/acceptance/js/storage/blob_store.test.js
index 3e4a19a204..f61c940356 100644
--- a/services/history-v1/test/acceptance/js/storage/blob_store.test.js
+++ b/services/history-v1/test/acceptance/js/storage/blob_store.test.js
@@ -22,6 +22,7 @@ const {
 } = require('../../../../storage')
 const mongoBackend = require('../../../../storage/lib/blob_store/mongo')
 const postgresBackend = require('../../../../storage/lib/blob_store/postgres')
+const { getProjectBlobsBatch } = require('../../../../storage/lib/blob_store')
 
 const mkTmpDir = promisify(temp.mkdir)
 
@@ -327,6 +328,31 @@ describe('BlobStore', function () {
             'expected Blob.NotFoundError when calling blobStore.getStream()'
           )
         })
+
+        if (scenario.backend !== mongoBackend) {
+          // mongo backend has its own test for this, covering sharding
+          it('getProjectBlobsBatch() returns blobs per project', async function () {
+            const projects = [
+              parseInt(scenario.projectId, 10),
+              parseInt(scenario.projectId2, 10),
+            ]
+            const { nBlobs, blobs } =
+              await postgresBackend.getProjectBlobsBatch(projects)
+            expect(nBlobs).to.equal(2)
+            expect(Object.fromEntries(blobs.entries())).to.deep.equal({
+              [parseInt(scenario.projectId, 10)]: [
+                new Blob(helloWorldHash, 11, 11),
+              ],
+              [parseInt(scenario.projectId2, 10)]: [
+                new Blob(
+                  testFiles.GRAPH_PNG_HASH,
+                  testFiles.GRAPH_PNG_BYTE_LENGTH,
+                  null
+                ),
+              ],
+            })
+          })
+        }
       })
 
       describe('a global blob', function () {
@@ -454,4 +480,42 @@ describe('BlobStore', function () {
       })
     })
   }
+
+  it('getProjectBlobsBatch() with mixed projects', async function () {
+    for (const scenario of scenarios) {
+      const blobStore = new BlobStore(scenario.projectId)
+      const blobStore2 = new BlobStore(scenario.projectId2)
+      await blobStore.initialize()
+      await blobStore.putString(helloWorldString)
+      await blobStore2.initialize()
+      await blobStore2.putFile(testFiles.path('graph.png'))
+    }
+
+    const projects = [
+      parseInt(scenarios[0].projectId, 10),
+      scenarios[1].projectId,
+      parseInt(scenarios[0].projectId2, 10),
+      scenarios[1].projectId2,
+    ]
+    const { nBlobs, blobs } = await getProjectBlobsBatch(projects)
+    expect(nBlobs).to.equal(4)
+    expect(Object.fromEntries(blobs.entries())).to.deep.equal({
+      [scenarios[0].projectId]: [new Blob(helloWorldHash, 11, 11)],
+      [scenarios[1].projectId]: [new Blob(helloWorldHash, 11, 11)],
+      [scenarios[0].projectId2]: [
+        new Blob(
+          testFiles.GRAPH_PNG_HASH,
+          testFiles.GRAPH_PNG_BYTE_LENGTH,
+          null
+        ),
+      ],
+      [scenarios[1].projectId2]: [
+        new Blob(
+          testFiles.GRAPH_PNG_HASH,
+          testFiles.GRAPH_PNG_BYTE_LENGTH,
+          null
+        ),
+      ],
+    })
+  })
 })
diff --git a/services/history-v1/test/acceptance/js/storage/blob_store_mongo.test.js b/services/history-v1/test/acceptance/js/storage/blob_store_mongo.test.js
index d2695681ab..319827856e 100644
--- a/services/history-v1/test/acceptance/js/storage/blob_store_mongo.test.js
+++ b/services/history-v1/test/acceptance/js/storage/blob_store_mongo.test.js
@@ -22,6 +22,7 @@ describe('BlobStore Mongo backend', function () {
       'abcdaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa',
     ],
     1234: ['1234000000000000000000000000000000000000'],
+    1337: ['1337000000000000000000000000000000000000'],
   }
 
   beforeEach('clean up', cleanup.everything)
@@ -77,6 +78,42 @@ describe('BlobStore Mongo backend', function () {
     })
   })
 
+  describe('getProjectBlobsBatch', function () {
+    it('finds all the blobs', async function () {
+      const projectId0 = new ObjectId().toString()
+      const hashesProject0 = hashes[1234].concat(hashes.abcd)
+      const projectId1 = new ObjectId().toString()
+      const hashesProject1 = hashes[1337].concat(hashes.abcd)
+      const projectId2 = new ObjectId().toString()
+      const hashesProject2 = [] // no hashes
+      const projectId3 = new ObjectId().toString()
+      const hashesProject3 = hashes[1337]
+      const projectBlobs = {
+        [projectId0]: hashesProject0,
+        [projectId1]: hashesProject1,
+        [projectId2]: hashesProject2,
+        [projectId3]: hashesProject3,
+      }
+      for (const [projectId, hashes] of Object.entries(projectBlobs)) {
+        for (const hash of hashes) {
+          const blob = new Blob(hash, 123, 99)
+          await mongoBackend.insertBlob(projectId, blob)
+        }
+      }
+      const projects = [projectId0, projectId1, projectId2, projectId3]
+      const { nBlobs, blobs } =
+        await mongoBackend.getProjectBlobsBatch(projects)
+      expect(nBlobs).to.equal(
+        hashesProject0.length + hashesProject1.length + hashesProject3.length
+      )
+      expect(Object.fromEntries(blobs.entries())).to.deep.equal({
+        [projectId0]: hashesProject0.map(hash => new Blob(hash, 123, 99)),
+        [projectId1]: hashesProject1.map(hash => new Blob(hash, 123, 99)),
+        [projectId3]: hashesProject3.map(hash => new Blob(hash, 123, 99)),
+      })
+    })
+  })
+
   describe('with existing blobs', function () {
     beforeEach(async function () {
       for (const hash of hashes.abcd.concat(hashes[1234])) {