feat: Phase 3 — performance tuning, iSCSI session refactor, store integration

Phase 3 delivers five checkpoints:

CP1 Engine Tuning: BlockVolConfig tunables, 256-shard DirtyMap, adaptive
group commit (low-watermark immediate flush), WAL pressure handling with
backpressure and ErrWALFull timeout.

CP2 iSCSI Session Refactor: RX/TX goroutine split with respCh (cap 64),
txLoop for serialized response writes, StatSN assignment modes. Login
phase stays single-goroutine; full-duplex after login.

CP3 Store Integration: BlockVolAdapter (iscsi.BlockDevice interface),
BlockVolumeStore management, BlockService in volume_server_block.go,
CLI flags (--block.listen/dir/iqn.prefix), sw-block-attach.sh helper.

CP5 Concurrency Hardening: WAL reuse guard (LSN validation in ReadLBA),
opsOutstanding counter with beginOp/endOp + Close drain, appendWithRetry
shared by WriteLBA and TrimLBA, flusher LSN guard in FlushOnce.

Bug fixes (P3-BUG-2–11): unbounded pending queue cap, Data-Out timeout,
flusher error logging, GroupCommitter panic recovery, Close vs concurrent
ops guard, target shutdown race, WAL-full retry vs Close, WRITE SAME(16)
for XFS, MODE SENSE(10) + VPD 0xB0/0xB2 for Linux kernel compatibility.

797 tests passing (517 engine + 280 iSCSI), go vet clean.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

weed/command/volume.go

@@ -74,6 +74,10 @@ type VolumeServerOptions struct {
 	ldbTimeout                  *int64
 	debug                       *bool
 	debugPort                   *int
+	// Block volume (iSCSI) options
+	blockListen    *string
+	blockDir       *string
+	blockIQNPrefix *string
 }
 
 func init() {
@@ -114,6 +118,9 @@ func init() {
 	v.readBufferSizeMB = cmdVolume.Flag.Int("readBufferSizeMB", 4, "<experimental> larger values can optimize query performance but will increase some memory usage,Use with hasSlowRead normally.")
 	v.debug = cmdVolume.Flag.Bool("debug", false, "serves runtime profiling data via pprof on the port specified by -debug.port")
 	v.debugPort = cmdVolume.Flag.Int("debug.port", 6060, "http port for debugging")
+	v.blockListen = cmdVolume.Flag.String("block.listen", "0.0.0.0:3260", "iSCSI target listen address for block volumes")
+	v.blockDir = cmdVolume.Flag.String("block.dir", "", "directory containing .blk block volume files. Empty disables iSCSI block service.")
+	v.blockIQNPrefix = cmdVolume.Flag.String("block.iqn.prefix", "iqn.2024-01.com.seaweedfs:vol.", "IQN prefix for block volume iSCSI targets")
 }
 
 var cmdVolume = &Command{
@@ -301,6 +308,9 @@ func (v VolumeServerOptions) startVolumeServer(volumeFolders, maxVolumeCounts, v
 	// starting the cluster http server
 	clusterHttpServer := v.startClusterHttpService(volumeMux)
 
+	// Start block volume iSCSI service (disabled if block.dir is empty).
+	blockService := weed_server.StartBlockService(*v.blockListen, *v.blockDir, *v.blockIQNPrefix)
+
 	grace.OnReload(volumeServer.LoadNewVolumes)
 	grace.OnReload(volumeServer.Reload)
 
@@ -315,6 +325,7 @@ func (v VolumeServerOptions) startVolumeServer(volumeFolders, maxVolumeCounts, v
 			time.Sleep(time.Duration(*v.preStopSeconds) * time.Second)
 		}
 
+		blockService.Shutdown()
 		shutdown(publicHttpDown, clusterHttpServer, grpcS, volumeServer)
 		stopChan <- true
 	})
@@ -324,6 +335,7 @@ func (v VolumeServerOptions) startVolumeServer(volumeFolders, maxVolumeCounts, v
 		select {
 		case <-stopChan:
 		case <-ctx.Done():
+			blockService.Shutdown()
 			shutdown(publicHttpDown, clusterHttpServer, grpcS, volumeServer)
 		}
 	} else {