Optimize DiskV2 Deduplication (#878)

* Revert "Lowercase `tart.app` (#751)"

This reverts commit a9e2a19.

* Optimize DiskV2 deduplication logic

In case we cloned `disk.img` from a local image, check if data at offset has the expected contents already.

* Hole punch only if needed

* Calculate hash only if needed

* subdataChunks optimization

* Reapply "Lowercase `tart.app` (#751)"

This reverts commit e74e9c8.

* format

* Save at least 1GB on deduplication logic

* Build separately

* Revert "subdataChunks optimization"

This reverts commit e59382a.

* Another optimization

* Removed debug log

* reformat

* Revert "Hole punch only if needed"

This reverts commit 8c569fc

.cirrus.yml

@@ -8,11 +8,11 @@ task:
       name: dev-mini
     resources:
       tart-vms: 1
+  build_script:
+    - swift build
   test_script:
     - swift test
   integration_test_script:
-    # Build Tart
-    - swift build
     - codesign --sign - --entitlements Resources/tart-dev.entitlements --force .build/debug/tart
     - export PATH=$(pwd)/.build/arm64-apple-macosx/debug:$PATH
     # Run integration tests

Sources/tart/LocalLayerCache.swift

@@ -1,12 +1,19 @@
 import Foundation
 
 struct LocalLayerCache {
+  struct DigestInfo {
+    let range: Range<Data.Index>
+    let compressedDigest: String
+    let uncompressedContentDigest: String?
+  }
+
   let name: String
   let deduplicatedBytes: UInt64
   let diskURL: URL
 
   private let mappedDisk: Data
-  private var digestToRange: [String : Range<Data.Index>] = [:]
+  private var digestToRange: [String: DigestInfo] = [:]
+  private var offsetToRange: [UInt64: DigestInfo] = [:]
 
   init?(_ name: String, _ deduplicatedBytes: UInt64, _ diskURL: URL, _ manifest: OCIManifest) throws {
     self.name = name
@@ -24,17 +31,27 @@ struct LocalLayerCache {
         return nil
       }
 
-      self.digestToRange[layer.digest] = Int(offset)..<Int(offset+uncompressedSize)
+      let info = DigestInfo(
+        range: Int(offset)..<Int(offset + uncompressedSize),
+        compressedDigest: layer.digest,
+        uncompressedContentDigest: layer.uncompressedContentDigest()!
+      )
+      self.digestToRange[layer.digest] = info
+      self.offsetToRange[offset] = info
 
       offset += uncompressedSize
     }
   }
 
-  func find(_ digest: String) -> Data? {
-    guard let foundRange = self.digestToRange[digest] else {
-      return nil
+  func findInfo(digest: String, offsetHint: UInt64) -> DigestInfo? {
+    // Layers can have the same digests, for example, empty ones. Let's use the offset hint to make a better guess.
+    if let info = self.offsetToRange[offsetHint], info.compressedDigest == digest {
+      return info
     }
+    return self.digestToRange[digest]
+  }
 
-    return self.mappedDisk.subdata(in: foundRange)
+  func subdata(_ range: Range<Data.Index>) -> Data {
+    return self.mappedDisk.subdata(in: range)
   }
 }

Sources/tart/OCI/Layerizer/DiskV2.swift

@@ -121,11 +121,14 @@ class DiskV2: Disk {
         // Launch a fetching and decompression task
         group.addTask {
           // No need to fetch and decompress anything if we've already done so
-          if try pullResumed && Digest.hash(diskURL, offset: diskWritingOffset, size: uncompressedLayerSize) == uncompressedLayerContentDigest {
-            // Update the progress
-            progress.completedUnitCount += Int64(diskLayer.size)
+          if pullResumed {
+            // do not check hash in the condition above to make it lazy e.g. only do expensive calculations if needed
+            if try Digest.hash(diskURL, offset: diskWritingOffset, size: uncompressedLayerSize) == uncompressedLayerContentDigest {
+              // Update the progress
+              progress.completedUnitCount += Int64(diskLayer.size)
 
-            return
+              return
+            }
           }
 
           // Open the disk file for writing
@@ -140,9 +143,17 @@ class DiskV2: Disk {
           }
 
           // Check if we already have this layer contents in the local layer cache
-          if let localLayerCache = localLayerCache, let data = localLayerCache.find(diskLayer.digest), Digest.hash(data) == uncompressedLayerContentDigest {
-            // Fulfil the layer contents from the local blob cache
-            _ = try zeroSkippingWrite(disk, rdisk, fsBlockSize, diskWritingOffset, data)
+          if let localLayerCache = localLayerCache, let localLayerInfo = localLayerCache.findInfo(digest: diskLayer.digest, offsetHint: diskWritingOffset) {
+            // indicates that the locally cloned disk image has the same content at the given offset
+            let localHit = localLayerInfo.uncompressedContentDigest == uncompressedLayerContentDigest
+              && localLayerInfo.range.lowerBound == diskWritingOffset
+            // doesn't seem that localHit can ever be false if the localLayerCache is not nil
+            // but let's just add extra safety here and check it
+            if !localHit {
+              // Fulfil the layer contents from the local blob cache
+              let data = localLayerCache.subdata(localLayerInfo.range)
+              _ = try zeroSkippingWrite(disk, rdisk, fsBlockSize, diskWritingOffset, data)
+            }
             try disk.close()
 
             // Update the progress

Sources/tart/URL+Prunable.swift

@@ -17,7 +17,6 @@ extension URL: Prunable {
   func deduplicatedSizeBytes() throws -> Int {
     let values = try resourceValues(forKeys: [.totalFileAllocatedSizeKey, .mayShareFileContentKey])
     // make sure the file's origin file is there and duplication works
-    var dedublicatedSize = 0
     if values.mayShareFileContent == true {
       return Int(deduplicatedBytes())
     }

Sources/tart/VMStorageOCI.swift

@@ -297,7 +297,7 @@ class VMStorageOCI: PrunableStorage {
 
     // Now, find the best match based on how many bytes we'll deduplicate
     let choosen = candidates.filter {
-      $0.deduplicatedBytes > 0
+      $0.deduplicatedBytes > 1024 * 1024 * 1024 // save at least 1GB
     }.max { left, right in
       return left.deduplicatedBytes < right.deduplicatedBytes
     }