diff --git a/src/blockstore/CMakeLists.txt b/src/blockstore/CMakeLists.txt
index 39511308..a9434117 100644
--- a/src/blockstore/CMakeLists.txt
+++ b/src/blockstore/CMakeLists.txt
@@ -5,7 +5,7 @@ project(vitastor)
 # libvitastor_blk.so
 add_library(vitastor_blk SHARED
 	../util/allocator.cpp ../util/crc32c.c ../util/ringloop.cpp
-	blockstore_heap.cpp blockstore_disk.cpp
+	multilist.cpp blockstore_heap.cpp blockstore_disk.cpp
 	blockstore.cpp blockstore_impl.cpp blockstore_init.cpp blockstore_open.cpp
 	blockstore_flush.cpp blockstore_read.cpp blockstore_stable.cpp blockstore_sync.cpp blockstore_write.cpp
 	v1/flush.cpp v1/impl.cpp v1/init.cpp v1/journal.cpp v1/open.cpp v1/read.cpp v1/rollback.cpp v1/stable.cpp v1/sync.cpp v1/write.cpp
diff --git a/src/blockstore/blockstore_heap.cpp b/src/blockstore/blockstore_heap.cpp
index d8501b06..51c96b05 100644
--- a/src/blockstore/blockstore_heap.cpp
+++ b/src/blockstore/blockstore_heap.cpp
@@ -135,275 +135,6 @@ uint32_t heap_object_t::calc_crc32c()
     return res;
 }
 
-multilist_alloc_t::multilist_alloc_t(uint32_t count, uint32_t maxn):
-    count(count), maxn(maxn)
-{
-    // not-so-memory-efficient: 16 MB memory per 1 GB buffer space, but buffer spaces are small, so OK
-    assert(count > 1 && count < 0x80000000);
-    sizes.resize(count);
-    nexts.resize(count); // nexts[i] = 0 -> area is used; nexts[i] = 1 -> no next; nexts[i] >= 2 -> next item
-    prevs.resize(count);
-    heads.resize(maxn); // heads[i] = 0 -> empty list; heads[i] >= 1 -> list head
-    sizes[0] = count;
-    sizes[count-1] = -count; // end
-    nexts[0] = 1;
-    heads[maxn-1] = 1;
-#ifdef MULTILIST_TRACE
-    print();
-#endif
-}
-
-bool multilist_alloc_t::is_free(uint32_t pos)
-{
-    assert(pos < count);
-    if (sizes[pos] < 0)
-        pos += sizes[pos]+1;
-    while (pos > 0 && !sizes[pos])
-        pos--;
-    return nexts[pos] > 0;
-}
-
-uint32_t multilist_alloc_t::find(uint32_t size)
-{
-    assert(size > 0);
-    assert(size <= maxn);
-    for (uint32_t i = size-1; i < maxn; i++)
-    {
-        if (heads[i])
-        {
-            return heads[i]-1;
-        }
-    }
-    return UINT32_MAX;
-}
-
-void multilist_alloc_t::verify()
-{
-    std::set<uint32_t> reachable;
-    for (int i = 0; i < maxn; i++)
-    {
-        uint32_t cur = heads[i];
-        while (cur)
-        {
-            if (!nexts[cur-1])
-            {
-                fprintf(stderr, "ERROR: item %d from freelist %d is not free\n", cur-1, i);
-                print();
-                abort();
-            }
-            if (nexts[cur-1] >= count+2)
-            {
-                fprintf(stderr, "ERROR: next out of range at %d: %d\n", cur-1, nexts[cur-1]);
-                print();
-                abort();
-            }
-            if (!(i < maxn-1 ? sizes[cur-1] == i+1 : (sizes[cur-1] >= i+1)))
-            {
-                fprintf(stderr, "ERROR: item %d is in wrong freelist: expected size %d, but actual size is %d\n", cur-1, i+1, sizes[cur-1]);
-                print();
-                abort();
-            }
-            if (reachable.find(cur-1) != reachable.end())
-            {
-                fprintf(stderr, "ERROR: doubly-claimed item %d\n", cur-1);
-                print();
-                abort();
-            }
-            reachable.insert(cur-1);
-            cur = nexts[cur-1]-1;
-        }
-    }
-    for (int i = 0; i < count; )
-    {
-        if (sizes[i])
-        {
-            assert(i+sizes[i] <= count);
-            if (sizes[i] > 1 && sizes[i+sizes[i]-1] != -sizes[i])
-            {
-                fprintf(stderr, "ERROR: start/end mismatch at %d: sizes[%d] should be %d, but is %d\n", i, i+sizes[i]-1, -sizes[i], sizes[i+sizes[i]-1]);
-                print();
-                abort();
-            }
-            for (int j = i+1; j < i+sizes[i]-1; j++)
-            {
-                if (sizes[j])
-                {
-                    fprintf(stderr, "ERROR: internal non-zero at %d: %d\n", j, sizes[j]);
-                    print();
-                    abort();
-                }
-            }
-            if (nexts[i] && reachable.find(i) == reachable.end())
-            {
-                fprintf(stderr, "ERROR: %d is unreachable from heads\n", i);
-                print();
-                abort();
-            }
-            if (nexts[i] >= 2)
-            {
-                if (nexts[i] >= 2+count)
-                {
-                    fprintf(stderr, "ERROR: next out of range at %d: %d\n", i, nexts[i]);
-                    print();
-                    abort();
-                }
-                if (prevs[nexts[i]-2] != i+1)
-                {
-                    fprintf(stderr, "ERROR: prev[next] (%d) != this (%d) at %d", prevs[nexts[i]-2], i+1, i);
-                    print();
-                    abort();
-                }
-            }
-            i += (sizes[i] > 1 ? sizes[i] : 1);
-        }
-        else
-            i++;
-    }
-}
-
-void multilist_alloc_t::print()
-{
-    printf("heads:");
-    for (int i = 0; i < maxn; i++)
-        if (heads[i])
-            printf(" %u=%u", i, heads[i]);
-    printf("\n");
-    printf("sizes:");
-    for (int i = 0; i < count; i++)
-        if (sizes[i])
-            printf(" %d=%d", i, sizes[i]);
-    printf("\n");
-    printf("prevs:");
-    for (int i = 0; i < count; i++)
-        if (prevs[i])
-            printf(" %d=%d", i, prevs[i]);
-    printf("\n");
-    printf("nexts:");
-    for (int i = 0; i < count; i++)
-        if (nexts[i])
-            printf(" %d=%d", i, nexts[i]);
-    printf("\n");
-    printf("items:");
-    for (int i = 0; i < count; )
-    {
-        if (sizes[i])
-        {
-            printf(" %u=(s:%d,n:%u,p:%u)", i, sizes[i], nexts[i], prevs[i]);
-            assert(i+sizes[i] <= count);
-            i += (sizes[i] > 1 ? sizes[i] : 1);
-        }
-        else
-            i++;
-    }
-    printf("\n");
-}
-
-void multilist_alloc_t::use(uint32_t pos, uint32_t size)
-{
-    assert(pos < count);
-    if (sizes[pos] <= 0)
-    {
-        uint32_t start = pos;
-        if (sizes[start] < 0)
-            start += sizes[start]+1;
-        else
-            while (start > 0 && !sizes[start])
-                start--;
-        assert(sizes[start] >= size);
-        use_full(start);
-        uint32_t full = sizes[start];
-        sizes[pos-1] = -pos+start;
-        sizes[start] = pos-start;
-        free(start);
-        sizes[pos+size-1] = -size;
-        sizes[pos] = size;
-        if (pos+size < start+full)
-        {
-            sizes[start+full-1] = -(start+full-pos-size);
-            sizes[pos+size] = start+full-pos-size;
-            free(pos+size);
-        }
-    }
-    else
-    {
-        assert(sizes[pos] >= size);
-        use_full(pos);
-        if (sizes[pos] > size)
-        {
-            uint32_t full = sizes[pos];
-            sizes[pos+size-1] = -size;
-            sizes[pos] = size;
-            sizes[pos+full-1] = -full+size;
-            sizes[pos+size] = full-size;
-            free(pos+size);
-        }
-    }
-#ifdef MULTILIST_TRACE
-    print();
-#endif
-}
-
-void multilist_alloc_t::use_full(uint32_t pos)
-{
-    uint32_t prevsize = sizes[pos];
-    assert(prevsize);
-    assert(nexts[pos]);
-    uint32_t pi = (prevsize < maxn ? prevsize : maxn)-1;
-    if (heads[pi] == pos+1)
-        heads[pi] = nexts[pos]-1;
-    if (prevs[pos])
-        nexts[prevs[pos]-1] = nexts[pos];
-    if (nexts[pos] >= 2)
-        prevs[nexts[pos]-2] = prevs[pos];
-    prevs[pos] = 0;
-    nexts[pos] = 0;
-}
-
-void multilist_alloc_t::free(uint32_t pos)
-{
-    do_free(pos);
-#ifdef MULTILIST_TRACE
-    print();
-#endif
-}
-
-void multilist_alloc_t::do_free(uint32_t pos)
-{
-    assert(!nexts[pos]);
-    uint32_t size = sizes[pos];
-    assert(size > 0);
-    // merge with previous?
-    if (pos > 0 && nexts[pos+(sizes[pos-1] == 1 ? -1 : sizes[pos-1])] > 0)
-    {
-        assert(sizes[pos-1] < 0 || sizes[pos-1] == 1);
-        uint32_t prevsize = sizes[pos-1] < 0 ? -sizes[pos-1] : 1;
-        use_full(pos-prevsize);
-        sizes[pos] = 0;
-        sizes[pos-1] = 0;
-        size += prevsize;
-        pos -= prevsize;
-        sizes[pos+size-1] = -size;
-        sizes[pos] = size;
-    }
-    // merge with next?
-    if (pos+size < count && nexts[pos+size] >= 1)
-    {
-        uint32_t nextsize = sizes[pos+size];
-        use_full(pos+size);
-        sizes[pos+size] = 0;
-        sizes[pos+size-1] = 0;
-        size += nextsize;
-        sizes[pos+size-1] = -size;
-        sizes[pos] = size;
-    }
-    uint32_t ni = (size < maxn ? size : maxn)-1; // FIXME ni -> nb (next bucket)
-    nexts[pos] = heads[ni]+1;
-    prevs[pos] = 0;
-    if (heads[ni])
-        prevs[heads[ni]-1] = pos+1;
-    heads[ni] = pos+1;
-}
-
 uint64_t blockstore_heap_t::get_pg_id(inode_t inode, uint64_t stripe)
 {
     uint64_t pg_num = 0;
diff --git a/src/blockstore/blockstore_heap.h b/src/blockstore/blockstore_heap.h
index 2c0bf417..eb874d92 100644
--- a/src/blockstore/blockstore_heap.h
+++ b/src/blockstore/blockstore_heap.h
@@ -12,6 +12,7 @@
 #include "../client/object_id.h"
 #include "../../cpp-btree/btree_map.h"
 #include "blockstore_disk.h"
+#include "multilist.h"
 
 struct pool_shard_settings_t
 {
@@ -110,23 +111,6 @@ struct heap_inflight_lsn_t
     uint64_t flags;
 };
 
-struct multilist_alloc_t
-{
-    const uint32_t count, maxn;
-    std::vector<int32_t> sizes;
-    std::vector<uint32_t> nexts, prevs, heads;
-
-    multilist_alloc_t(uint32_t count, uint32_t maxn);
-    bool is_free(uint32_t pos);
-    uint32_t find(uint32_t size);
-    void use_full(uint32_t pos);
-    void use(uint32_t pos, uint32_t size);
-    void do_free(uint32_t pos);
-    void free(uint32_t pos);
-    void verify();
-    void print();
-};
-
 class blockstore_heap_t
 {
     friend class heap_write_t;
diff --git a/src/blockstore/multilist.cpp b/src/blockstore/multilist.cpp
new file mode 100644
index 00000000..8027f526
--- /dev/null
+++ b/src/blockstore/multilist.cpp
@@ -0,0 +1,278 @@
+// Variable-length O(1) disk space allocator
+// Copyright (c) Vitaliy Filippov, 2025+
+// License: VNPL-1.1 (see README.md for details)
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <set>
+#include "multilist.h"
+
+multilist_alloc_t::multilist_alloc_t(uint32_t count, uint32_t maxn):
+    count(count), maxn(maxn)
+{
+    // not-so-memory-efficient: 16 MB memory per 1 GB buffer space, but buffer spaces are small, so OK
+    assert(count > 1 && count < 0x80000000);
+    sizes.resize(count);
+    nexts.resize(count); // nexts[i] = 0 -> area is used; nexts[i] = 1 -> no next; nexts[i] >= 2 -> next item
+    prevs.resize(count);
+    heads.resize(maxn); // heads[i] = 0 -> empty list; heads[i] >= 1 -> list head
+    sizes[0] = count;
+    sizes[count-1] = -count; // end
+    nexts[0] = 1;
+    heads[maxn-1] = 1;
+#ifdef MULTILIST_TRACE
+    print();
+#endif
+}
+
+bool multilist_alloc_t::is_free(uint32_t pos)
+{
+    assert(pos < count);
+    if (sizes[pos] < 0)
+        pos += sizes[pos]+1;
+    while (pos > 0 && !sizes[pos])
+        pos--;
+    return nexts[pos] > 0;
+}
+
+uint32_t multilist_alloc_t::find(uint32_t size)
+{
+    assert(size > 0);
+    assert(size <= maxn);
+    for (uint32_t i = size-1; i < maxn; i++)
+    {
+        if (heads[i])
+        {
+            return heads[i]-1;
+        }
+    }
+    return UINT32_MAX;
+}
+
+void multilist_alloc_t::verify()
+{
+    std::set<uint32_t> reachable;
+    for (int i = 0; i < maxn; i++)
+    {
+        uint32_t cur = heads[i];
+        while (cur)
+        {
+            if (!nexts[cur-1])
+            {
+                fprintf(stderr, "ERROR: item %d from freelist %d is not free\n", cur-1, i);
+                print();
+                abort();
+            }
+            if (nexts[cur-1] >= count+2)
+            {
+                fprintf(stderr, "ERROR: next out of range at %d: %d\n", cur-1, nexts[cur-1]);
+                print();
+                abort();
+            }
+            if (!(i < maxn-1 ? sizes[cur-1] == i+1 : (sizes[cur-1] >= i+1)))
+            {
+                fprintf(stderr, "ERROR: item %d is in wrong freelist: expected size %d, but actual size is %d\n", cur-1, i+1, sizes[cur-1]);
+                print();
+                abort();
+            }
+            if (reachable.find(cur-1) != reachable.end())
+            {
+                fprintf(stderr, "ERROR: doubly-claimed item %d\n", cur-1);
+                print();
+                abort();
+            }
+            reachable.insert(cur-1);
+            cur = nexts[cur-1]-1;
+        }
+    }
+    for (int i = 0; i < count; )
+    {
+        if (sizes[i])
+        {
+            assert(i+sizes[i] <= count);
+            if (sizes[i] > 1 && sizes[i+sizes[i]-1] != -sizes[i])
+            {
+                fprintf(stderr, "ERROR: start/end mismatch at %d: sizes[%d] should be %d, but is %d\n", i, i+sizes[i]-1, -sizes[i], sizes[i+sizes[i]-1]);
+                print();
+                abort();
+            }
+            for (int j = i+1; j < i+sizes[i]-1; j++)
+            {
+                if (sizes[j])
+                {
+                    fprintf(stderr, "ERROR: internal non-zero at %d: %d\n", j, sizes[j]);
+                    print();
+                    abort();
+                }
+            }
+            if (nexts[i] && reachable.find(i) == reachable.end())
+            {
+                fprintf(stderr, "ERROR: %d is unreachable from heads\n", i);
+                print();
+                abort();
+            }
+            if (nexts[i] >= 2)
+            {
+                if (nexts[i] >= 2+count)
+                {
+                    fprintf(stderr, "ERROR: next out of range at %d: %d\n", i, nexts[i]);
+                    print();
+                    abort();
+                }
+                if (prevs[nexts[i]-2] != i+1)
+                {
+                    fprintf(stderr, "ERROR: prev[next] (%d) != this (%d) at %d", prevs[nexts[i]-2], i+1, i);
+                    print();
+                    abort();
+                }
+            }
+            i += (sizes[i] > 1 ? sizes[i] : 1);
+        }
+        else
+            i++;
+    }
+}
+
+void multilist_alloc_t::print()
+{
+    printf("heads:");
+    for (int i = 0; i < maxn; i++)
+        if (heads[i])
+            printf(" %u=%u", i, heads[i]);
+    printf("\n");
+    printf("sizes:");
+    for (int i = 0; i < count; i++)
+        if (sizes[i])
+            printf(" %d=%d", i, sizes[i]);
+    printf("\n");
+    printf("prevs:");
+    for (int i = 0; i < count; i++)
+        if (prevs[i])
+            printf(" %d=%d", i, prevs[i]);
+    printf("\n");
+    printf("nexts:");
+    for (int i = 0; i < count; i++)
+        if (nexts[i])
+            printf(" %d=%d", i, nexts[i]);
+    printf("\n");
+    printf("items:");
+    for (int i = 0; i < count; )
+    {
+        if (sizes[i])
+        {
+            printf(" %u=(s:%d,n:%u,p:%u)", i, sizes[i], nexts[i], prevs[i]);
+            assert(i+sizes[i] <= count);
+            i += (sizes[i] > 1 ? sizes[i] : 1);
+        }
+        else
+            i++;
+    }
+    printf("\n");
+}
+
+void multilist_alloc_t::use(uint32_t pos, uint32_t size)
+{
+    assert(pos < count);
+    if (sizes[pos] <= 0)
+    {
+        uint32_t start = pos;
+        if (sizes[start] < 0)
+            start += sizes[start]+1;
+        else
+            while (start > 0 && !sizes[start])
+                start--;
+        assert(sizes[start] >= size);
+        use_full(start);
+        uint32_t full = sizes[start];
+        sizes[pos-1] = -pos+start;
+        sizes[start] = pos-start;
+        free(start);
+        sizes[pos+size-1] = -size;
+        sizes[pos] = size;
+        if (pos+size < start+full)
+        {
+            sizes[start+full-1] = -(start+full-pos-size);
+            sizes[pos+size] = start+full-pos-size;
+            free(pos+size);
+        }
+    }
+    else
+    {
+        assert(sizes[pos] >= size);
+        use_full(pos);
+        if (sizes[pos] > size)
+        {
+            uint32_t full = sizes[pos];
+            sizes[pos+size-1] = -size;
+            sizes[pos] = size;
+            sizes[pos+full-1] = -full+size;
+            sizes[pos+size] = full-size;
+            free(pos+size);
+        }
+    }
+#ifdef MULTILIST_TRACE
+    print();
+#endif
+}
+
+void multilist_alloc_t::use_full(uint32_t pos)
+{
+    uint32_t prevsize = sizes[pos];
+    assert(prevsize);
+    assert(nexts[pos]);
+    uint32_t pi = (prevsize < maxn ? prevsize : maxn)-1;
+    if (heads[pi] == pos+1)
+        heads[pi] = nexts[pos]-1;
+    if (prevs[pos])
+        nexts[prevs[pos]-1] = nexts[pos];
+    if (nexts[pos] >= 2)
+        prevs[nexts[pos]-2] = prevs[pos];
+    prevs[pos] = 0;
+    nexts[pos] = 0;
+}
+
+void multilist_alloc_t::free(uint32_t pos)
+{
+    do_free(pos);
+#ifdef MULTILIST_TRACE
+    print();
+#endif
+}
+
+void multilist_alloc_t::do_free(uint32_t pos)
+{
+    assert(!nexts[pos]);
+    uint32_t size = sizes[pos];
+    assert(size > 0);
+    // merge with previous?
+    if (pos > 0 && nexts[pos+(sizes[pos-1] == 1 ? -1 : sizes[pos-1])] > 0)
+    {
+        assert(sizes[pos-1] < 0 || sizes[pos-1] == 1);
+        uint32_t prevsize = sizes[pos-1] < 0 ? -sizes[pos-1] : 1;
+        use_full(pos-prevsize);
+        sizes[pos] = 0;
+        sizes[pos-1] = 0;
+        size += prevsize;
+        pos -= prevsize;
+        sizes[pos+size-1] = -size;
+        sizes[pos] = size;
+    }
+    // merge with next?
+    if (pos+size < count && nexts[pos+size] >= 1)
+    {
+        uint32_t nextsize = sizes[pos+size];
+        use_full(pos+size);
+        sizes[pos+size] = 0;
+        sizes[pos+size-1] = 0;
+        size += nextsize;
+        sizes[pos+size-1] = -size;
+        sizes[pos] = size;
+    }
+    uint32_t ni = (size < maxn ? size : maxn)-1; // FIXME ni -> nb (next bucket)
+    nexts[pos] = heads[ni]+1;
+    prevs[pos] = 0;
+    if (heads[ni])
+        prevs[heads[ni]-1] = pos+1;
+    heads[ni] = pos+1;
+}
diff --git a/src/blockstore/multilist.h b/src/blockstore/multilist.h
new file mode 100644
index 00000000..8079faa1
--- /dev/null
+++ b/src/blockstore/multilist.h
@@ -0,0 +1,25 @@
+// Variable-length O(1) disk space allocator
+// Copyright (c) Vitaliy Filippov, 2025+
+// License: VNPL-1.1 (see README.md for details)
+
+#pragma once
+
+#include <stdint.h>
+#include <vector>
+
+struct multilist_alloc_t
+{
+    const uint32_t count, maxn;
+    std::vector<int32_t> sizes;
+    std::vector<uint32_t> nexts, prevs, heads;
+
+    multilist_alloc_t(uint32_t count, uint32_t maxn);
+    bool is_free(uint32_t pos);
+    uint32_t find(uint32_t size);
+    void use_full(uint32_t pos);
+    void use(uint32_t pos, uint32_t size);
+    void do_free(uint32_t pos);
+    void free(uint32_t pos);
+    void verify();
+    void print();
+};
diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
index 59a74fbd..a40ca535 100644
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -37,6 +37,7 @@ add_test(NAME test_allocator COMMAND test_allocator)
 # test_heap
 add_executable(test_heap
 	test_heap.cpp
+	../blockstore/multilist.cpp
 	../blockstore/blockstore_heap.cpp
 	../util/crc32c.c
 	../util/allocator.cpp