diff --git a/src/blockstore_sync.cpp b/src/blockstore_sync.cpp
index 50891a13..d29035b5 100644
--- a/src/blockstore_sync.cpp
+++ b/src/blockstore_sync.cpp
@@ -76,6 +76,7 @@ int blockstore_impl_t::continue_sync(blockstore_op_t *op)
         // 2nd step: Data device is synced, prepare & write journal entries
         // Check space in the journal and journal memory buffers
         blockstore_journal_check_t space_check(this);
+        auto reservation = (unstable_writes.size()+unstable_unsynced+PRIV(op)->sync_big_writes.size())*journal.block_size;
         if (dsk.csum_block_size)
         {
             // More complex check because all journal entries have different lengths
@@ -85,15 +86,14 @@ int blockstore_impl_t::continue_sync(blockstore_op_t *op)
                 left--;
                 auto & dirty_entry = dirty_db.at(sbw);
                 uint64_t dyn_size = dsk.dirty_dyn_size(dirty_entry.offset, dirty_entry.len);
-                if (!space_check.check_available(op, 1, sizeof(journal_entry_big_write) + dyn_size, 0))
+                if (!space_check.check_available(op, 1, sizeof(journal_entry_big_write) + dyn_size, left ? 0 : reservation))
                 {
                     return 0;
                 }
             }
         }
         else if (!space_check.check_available(op, PRIV(op)->sync_big_writes.size(),
-            sizeof(journal_entry_big_write) + dsk.clean_entry_bitmap_size,
-            (unstable_writes.size()+unstable_unsynced)*journal.block_size))
+            sizeof(journal_entry_big_write) + dsk.clean_entry_bitmap_size, reservation))
         {
             return 0;
         }
diff --git a/src/blockstore_write.cpp b/src/blockstore_write.cpp
index 8f314ccf..fe768f88 100644
--- a/src/blockstore_write.cpp
+++ b/src/blockstore_write.cpp
@@ -320,7 +320,7 @@ int blockstore_impl_t::dequeue_write(blockstore_op_t *op)
         blockstore_journal_check_t space_check(this);
         if (!space_check.check_available(op, unsynced_big_write_count + 1,
             sizeof(journal_entry_big_write) + dsk.clean_dyn_size,
-            (unstable_writes.size()+unstable_unsynced)*journal.block_size))
+            (unstable_writes.size()+unstable_unsynced+((dirty_it->second.state & BS_ST_INSTANT) ? 0 : 1))*journal.block_size))
         {
             return 0;
         }
@@ -412,7 +412,7 @@ int blockstore_impl_t::dequeue_write(blockstore_op_t *op)
                 sizeof(journal_entry_big_write) + dsk.clean_dyn_size, 0)
             || !space_check.check_available(op, 1,
                 sizeof(journal_entry_small_write) + dyn_size,
-                op->len + (unstable_writes.size()+unstable_unsynced)*journal.block_size))
+                op->len + (unstable_writes.size()+unstable_unsynced+((dirty_it->second.state & BS_ST_INSTANT) ? 0 : 1))*journal.block_size))
         {
             return 0;
         }
@@ -549,7 +549,7 @@ resume_2:
         uint64_t dyn_size = dsk.dirty_dyn_size(op->offset, op->len);
         blockstore_journal_check_t space_check(this);
         if (!space_check.check_available(op, 1, sizeof(journal_entry_big_write) + dyn_size,
-            (unstable_writes.size()+unstable_unsynced)*journal.block_size))
+            (unstable_writes.size()+unstable_unsynced+((dirty_it->second.state & BS_ST_INSTANT) ? 0 : 1))*journal.block_size))
         {
             return 0;
         }