[Qemu-devel] [PATCH 3/5] qcow2: Options to enable discard for freed clusters

[Kevin Wolf <kwolf@redhat.com>]

Deleted snapshots are discarded in the image file by default, discard
requests take their default from the -drive discard=... option and other
places that free clusters must always be enabled explicitly.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-refcount.c |  5 +++++
 block/qcow2.c          | 26 ++++++++++++++++++++++++++
 block/qcow2.h          |  5 +++++
 3 files changed, 36 insertions(+)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 6d35e49..7488988 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -488,6 +488,11 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
             s->free_cluster_index = cluster_index;
         }
         refcount_block[block_index] = cpu_to_be16(refcount);
+        if (refcount == 0 && s->discard_passthrough[type]) {
+            /* Try discarding, ignore errors */
+            /* FIXME Doing this cluster by cluster will be painfully slow */
+            bdrv_discard(bs->file, cluster_offset, 1);
+        }
     }
 
     ret = 0;
diff --git a/block/qcow2.c b/block/qcow2.c
index e28ea47..62e6753 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -295,6 +295,22 @@ static QemuOptsList qcow2_runtime_opts = {
             .type = QEMU_OPT_BOOL,
             .help = "Postpone refcount updates",
         },
+        {
+            .name = QCOW2_OPT_DISCARD_REQUEST,
+            .type = QEMU_OPT_BOOL,
+            .help = "Pass guest discard requests to the layer below",
+        },
+        {
+            .name = QCOW2_OPT_DISCARD_SNAPSHOT,
+            .type = QEMU_OPT_BOOL,
+            .help = "Generate discard requests when snapshot related space "
+                    "is freed",
+        },
+        {
+            .name = QCOW2_OPT_DISCARD_OTHER,
+            .type = QEMU_OPT_BOOL,
+            .help = "Generate discard requests when other clusters are freed",
+        },
         { /* end of list */ }
     },
 };
@@ -532,6 +548,16 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags)
     s->use_lazy_refcounts = qemu_opt_get_bool(opts, QCOW2_OPT_LAZY_REFCOUNTS,
         (s->compatible_features & QCOW2_COMPAT_LAZY_REFCOUNTS));
 
+    s->discard_passthrough[QCOW2_DISCARD_NEVER] = false,
+    s->discard_passthrough[QCOW2_DISCARD_ALWAYS] = true,
+    s->discard_passthrough[QCOW2_DISCARD_REQUEST] =
+        qemu_opt_get_bool(opts, QCOW2_OPT_DISCARD_REQUEST,
+                          flags & BDRV_O_UNMAP),
+    s->discard_passthrough[QCOW2_DISCARD_SNAPSHOT] =
+        qemu_opt_get_bool(opts, QCOW2_OPT_DISCARD_SNAPSHOT, true),
+    s->discard_passthrough[QCOW2_DISCARD_OTHER] =
+        qemu_opt_get_bool(opts, QCOW2_OPT_DISCARD_OTHER, false),
+
     qemu_opts_del(opts);
 
     if (s->use_lazy_refcounts && s->qcow_version < 3) {
diff --git a/block/qcow2.h b/block/qcow2.h
index 64a6479..6f91b9a 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -60,6 +60,9 @@
 
 
 #define QCOW2_OPT_LAZY_REFCOUNTS "lazy_refcounts"
+#define QCOW2_OPT_DISCARD_REQUEST "pass_discard_request"
+#define QCOW2_OPT_DISCARD_SNAPSHOT "pass_discard_snapshot"
+#define QCOW2_OPT_DISCARD_OTHER "pass_discard_other"
 
 typedef struct QCowHeader {
     uint32_t magic;
@@ -187,6 +190,8 @@ typedef struct BDRVQcowState {
     int qcow_version;
     bool use_lazy_refcounts;
 
+    bool discard_passthrough[QCOW2_DISCARD_MAX];
+
     uint64_t incompatible_features;
     uint64_t compatible_features;
     uint64_t autoclear_features;
-- 
1.8.1.4