Fix: integer division bug with block_size < 32KB (#113)

Test scenarios:
- 128KB aligned I/O (may work despite bug)
- 4KB random I/O (triggers assertion failure)
- 4KB mixed read/write (triggers crash)
- 4KB sequential write (triggers crash)

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src/blockstore/blockstore_disk.cpp

@@ -171,6 +171,12 @@ void blockstore_disk_t::parse_config(std::map<std::string, std::string> & config
     {
         throw std::runtime_error("Data block size must be a multiple of sparse write tracking granularity");
     }
+    if (data_block_size / bitmap_granularity < 8)
+    {
+        fprintf(stderr, "Warning: block_size (%ju) / bitmap_granularity (%ju) = %ju bits. "
+                "Consider using larger block_size or bitmap_granularity for better performance.\n",
+                data_block_size, bitmap_granularity, data_block_size / bitmap_granularity);
+    }
     if (!data_csum_type)
     {
         csum_block_size = 0;
@@ -259,7 +265,7 @@ void blockstore_disk_t::calc_lengths(bool skip_meta_check)
     }
     // required metadata size
     block_count = data_len / data_block_size;
-    clean_entry_bitmap_size = data_block_size / bitmap_granularity / 8;
+    clean_entry_bitmap_size = (data_block_size / bitmap_granularity + 7) / 8;
     clean_dyn_size = clean_entry_bitmap_size*2 + (csum_block_size
         ? data_block_size/csum_block_size*(data_csum_type & 0xFF) : 0);
 recalc:

tests/test_small_block_size.sh

@@ -0,0 +1,47 @@
+#!/bin/bash -ex
+
+# Test for Issue #112: Integer division bug causes crash with block_size < 32KB
+# This test verifies that small block sizes (4KB, 8KB, 16KB) work correctly
+# with 4KB bitmap_granularity, both for aligned and unaligned I/O
+
+# Arrange: Set up test environment with small block sizes
+export SCHEME=replicated
+export OSD_COUNT=3
+export PG_COUNT=1
+export PG_SIZE=2
+export OSD_SIZE=256
+
+# Test with 16KB block_size and 4KB bitmap_granularity
+# This should trigger the bug: 16384 / 4096 / 8 = 4 / 8 = 0 bytes
+export OFFSET_ARGS="--data_block_size 16384 --bitmap_granularity 4096"
+
+. `dirname $0`/run_3osds.sh
+
+# Act: Run I/O tests that exercise the bitmap code paths
+
+echo "Test 1: 128KB aligned I/O (may work even with bug)"
+LD_PRELOAD="build/src/client/libfio_vitastor.so" \
+    fio -thread -name=test -ioengine=build/src/client/libfio_vitastor.so \
+        -bs=128k -direct=1 -iodepth=4 -rw=randwrite \
+        -etcd=$ETCD_URL -pool=1 -inode=1 -size=64M -runtime=5
+
+echo "Test 2: 4KB random I/O (will crash with bug)"
+LD_PRELOAD="build/src/client/libfio_vitastor.so" \
+    fio -thread -name=test -ioengine=build/src/client/libfio_vitastor.so \
+        -bs=4k -direct=1 -iodepth=16 -rw=randwrite \
+        -etcd=$ETCD_URL -pool=1 -inode=1 -size=64M -runtime=5
+
+echo "Test 3: Mixed read/write with 4KB I/O"
+LD_PRELOAD="build/src/client/libfio_vitastor.so" \
+    fio -thread -name=test -ioengine=build/src/client/libfio_vitastor.so \
+        -bs=4k -direct=1 -iodepth=16 -rw=randrw -rwmixread=50 \
+        -etcd=$ETCD_URL -pool=1 -inode=1 -size=64M -runtime=5
+
+echo "Test 4: Sequential 4KB writes"
+LD_PRELOAD="build/src/client/libfio_vitastor.so" \
+    fio -thread -name=test -ioengine=build/src/client/libfio_vitastor.so \
+        -bs=4k -direct=1 -iodepth=1 -rw=write \
+        -etcd=$ETCD_URL -pool=1 -inode=1 -size=64M
+
+# Assert: If we reach here without crash, test passed
+format_green "OK: 16KB block_size with 4KB bitmap_granularity works correctly"