add reclaim api to append_blk_allocator for GC

src/lib/blkalloc/append_blk_allocator.cpp

@@ -123,6 +123,18 @@ BlkAllocStatus AppendBlkAllocator::mark_blk_allocated(BlkId const&) { return Blk
 
 void AppendBlkAllocator::free_on_disk(BlkId const&) {}
 
+// note that this function will force to reclaim nblks from the tail, so make sure the nblks at tail are not in use
+// or have been copied to another chunk for later access;
+void AppendBlkAllocator::reclaim_tail_nblks(blk_count_t nblks) {
+    std::unique_lock lk(m_mtx);
+    auto cur_cp = hs()->cp_mgr().cp_guard();
+    m_last_append_offset -= nblks;
+    m_freeable_nblks += nblks;
+    set_dirty_offset(cur_cp->id() % MAX_CP_COUNT);
+
+    COUNTER_INCREMENT(m_metrics, num_reclaimed, nblks);
+}
+
 bool AppendBlkAllocator::is_blk_alloced_on_disk(BlkId const&, bool) const { return false; }
 
 //

src/lib/blkalloc/append_blk_allocator.h

@@ -49,6 +49,7 @@ class AppendBlkAllocMetrics : public sisl::MetricsGroup {
     explicit AppendBlkAllocMetrics(const char* inst_name) : sisl::MetricsGroup("AppendBlkAlloc", inst_name) {
         REGISTER_COUNTER(num_alloc, "Number of blks alloc attempts");
         REGISTER_COUNTER(num_alloc_failure, "Number of blk alloc failures");
+        REGISTER_COUNTER(num_reclaimed, "Number of blk reclaimed from tail");
 
         register_me_to_farm();
     }
@@ -87,6 +88,7 @@ class AppendBlkAllocator : public BlkAllocator {
     BlkAllocStatus mark_blk_allocated(BlkId const& b) override;
 
     void free_on_disk(BlkId const& b) override;
+    void reclaim_tail_nblks(blk_count_t nblks) override;
     bool is_blk_alloced_on_disk(BlkId const& b, bool use_lock = false) const override;
 
     /**

src/lib/blkalloc/bitmap_blk_allocator.h

@@ -74,6 +74,7 @@ class BitmapBlkAllocator : public BlkAllocator {
     virtual void load() = 0;
     BlkAllocStatus alloc_on_disk(BlkId const& in_bid) override;
     void free_on_disk(BlkId const& b) override;
+    void reclaim_tail_nblks(blk_count_t nblks) override{};
     bool is_blk_alloced_on_disk(BlkId const& b, bool use_lock = false) const override;
     void cp_flush(CP* cp) override;
 

src/lib/blkalloc/blk_allocator.h

@@ -157,6 +157,9 @@ class BlkAllocator {
     virtual void free(BlkId const& id) = 0;
     virtual void free_on_disk(BlkId const& bid) = 0;
 
+    // Reclaim nblks from the tail of the chunk
+    virtual void reclaim_tail_nblks(blk_count_t nblks) = 0;
+
     virtual blk_num_t available_blks() const = 0;
     virtual blk_num_t get_freeable_nblks() const = 0;
     virtual blk_num_t get_defrag_nblks() const = 0;