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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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"