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+# Test Case: LZ4 Compressed Physical Cluster Mapping (64KB)
+
+**Test ID**: TC090-lz4-pcluster-64k  
+
+**G5 fixture contract**: Use [G5-MANUAL-SETUP.md](G5-MANUAL-SETUP.md).
+Its generated paths, source comparisons, structured corruption offsets, and
+cleanup rules supersede placeholder examples in this file.
+**Category**: Compression  
+**Priority**: High  
+**Regression**: None
+
+## Objective
+Verify correct physical cluster mapping for LZ4 compression with 64KB pcluster size.
+
+## Preconditions
+- EROFS image with LZ4, 64KB pcluster: `test-lz4-pcluster-64k.erofs`
+- Test file: `/files/test-64k.dat` (1MB, highly compressible)
+- Mount point: `/mnt/test`
+- Generate with `mkfs.erofs -zlz4 -C65536 test-lz4-pcluster-64k.erofs source`
+  and confirm `compr_cfgs big_pcluster` with `dump.erofs -s`.
+
+## Test Steps
+1. Mount the image:
+   ```
+   mount -t erofs /dev/md0 /mnt/test
+   ```
+
+2. Read first pcluster (64KB):
+   ```
+   dd if=/mnt/test/files/test-64k.dat of=/tmp/out1 bs=64K count=1
+   ```
+
+3. Read small portion requiring full pcluster decompression:
+   ```
+   dd if=/mnt/test/files/test-64k.dat of=/tmp/out2 bs=4K count=1 skip=1
+   ```
+
+4. Read crossing pcluster boundary:
+   ```
+   dd if=/mnt/test/files/test-64k.dat of=/tmp/cross bs=1 skip=65504 count=64
+   dd if=/root/reference/test-64k.dat of=/tmp/cross.expected bs=1 skip=65504 count=64
+   cmp /tmp/cross /tmp/cross.expected
+   ```
+
+5. Record the physical extent layout:
+   ```
+   dump.erofs --path=/files/test-64k.dat -e test-lz4-pcluster-64k.erofs
+   ```
+
+## Expected Results
+- Step 2: Full 64KB pcluster decompressed correctly
+- Step 3: Reading 4KB requires decompressing entire 64KB pcluster
+- Step 4: Boundary crossing handled correctly
+- Step 5: The fixture contains a real compressed big-pcluster extent
+
+## Verification Method
+- Verify large pcluster decompression works
+- Confirm small reads still trigger full pcluster decompression
+- Test memory usage reasonable for 64KB decompression
+- Do not require a better compression ratio than 4KB for every corpus
+
+## Cleanup
+```
+umount /mnt/test
+mdconfig -d -u md0
+rm /tmp/out* /tmp/cross /tmp/cross.expected
+```
+
+## Notes
+- 64KB pcluster provides better compression at cost of random access
+- Common pcluster size for read-heavy workloads
+- Related tests: TC089 (4KB pcluster), TC088 (config handling)