mirror of
https://github.com/stornic56/debianito-post-install.git
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stornic56
ad0b39e159
- Added `_detect_lang()` helper to `modules/utils.sh` (extracts first 2 chars from `$LANG`, fallback to `en`). - Created `_detect_lang_pkg()` utility for LibreOffice localization: handles regional variants (`es-ar` → `es`) and skips English packages. - Implemented `_check_network()` with cascade methods (ping→wget→curl) in `modules/utils.sh`. Warning-only pre-flight check after `check_sudo` in `debianito.sh`, compatible with `set -euo pipefail`. - Fixed system time validation: changed `2026` threshold to `2025` in `check_system_time()` (replaced self-referential `$(date +%Y)` which always evaluated false). - Refactored LibreOffice language selection in `office.sh`: replaced 10 lines of inline logic with `_detect_lang_pkg "libreoffice-l10n"`. - Overhauled Firefox ESR integration in `internet.sh`: added state detection, mutual exclusion guard (both Firefox variants selected → warning), new `install_firefox_esr()` function with language support. - Updated `install_firefox_mozilla()`: added Debian version guard (`<12`), replaced `_run_install` with `_run_cmd`. - Standardized header in internet menu and improved whiptail UX for Firefox options.
252 lines
12 KiB
Markdown
252 lines
12 KiB
Markdown
# Option 8: ZRAM Configuration & Memory Optimization
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## 1. The Science of ZRAM vs. Traditional Swap
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### Core Concept: CPU Cycles vs. Disk I/O
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Traditional swap storage operates on a fundamental latency gap that becomes critical under memory pressure:
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| Storage Medium | Latency Range | Write Amplification | SSD Wear Impact |
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|---------------|---------------|---------------------|-----------------|
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| **DRAM (RAM)** | ~10–50 nanoseconds | None | Zero |
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| **NVMe SSD** | ~20–70 microseconds | 1.2x–3.0x | Moderate to High |
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| **SATA SSD** | ~100–200 microseconds | 1.5x–4.0x | High |
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| **HDD** | ~5–10 milliseconds | N/A (mechanical) | Irrelevant |
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When a Linux system experiences memory pressure, the kernel must decide what to swap out. Traditional swap writes pages directly to disk storage:
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- **Time Cost**: Each 4 KiB page write takes microseconds (NVMe) to milliseconds (HDD)
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- **Wear Cost**: Every write consumes P/E (Program/Erase) cycles from NAND flash cells, reducing TBW (Terabytes Written) lifespan
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- **System Impact**: High latency causes "thrashing" where the system spends more time waiting for disk I/O than executing actual work
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### ZRAM's Solution: Compression in RAM
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ZRAM creates a compressed block device entirely within physical memory. When pages need to be swapped, they are:
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1. **Compressed on-the-fly** using CPU algorithms (LZ4 or ZSTD)
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2. **Stored in RAM pool** at compressed size (typically 2:1 to 3:1 ratio)
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3. **Decompressed instantly** when needed (microseconds vs milliseconds)
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The trade-off is explicit: **CPU cycles for reduced I/O latency**. Modern CPUs can compress/decompress pages in microseconds, making this far cheaper than any disk operation.
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### Why Only LZ4 and ZSTD?
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The script offers only two algorithms because they represent the optimal balance points:
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| Algorithm | Compression Ratio | Speed | CPU Overhead | Best Use Case |
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|-----------|------------------|-------|--------------|---------------|
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| **LZ4** | ~2:1–3:1 | Fastest | Lowest | Gaming, real-time workloads |
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| **ZSTD** | ~3:1–5:1 | Medium | Moderate | General use, better memory savings |
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- **LZ4**: Prioritizes speed over compression ratio. Ideal for systems where CPU availability is limited or latency-sensitive (gaming servers).
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- **ZSTD**: Offers superior compression ratios with acceptable overhead. Best for systems prioritizing maximum effective RAM capacity.
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The kernel supports additional algorithms (lzo-rle, deflate, lz4hc), but these are either deprecated, slower, or offer diminishing returns compared to LZ4/ZSTD in modern hardware.
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### Extending SSD Lifespan Through Reduced Writes
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By intercepting swap writes before they reach physical storage:
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- **Write Reduction**: Pages that would write to disk now compress in RAM
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- **TBW Conservation**: Each avoided write preserves P/E cycles on NAND flash cells
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- **System Longevity**: Critical for systems with limited SSD endurance ratings (e.g., 100 TBW consumer drives)
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As the Linux kernel documentation states: *"Users with SSDs as swap devices can extend device lifespan by drastically reducing writes that shorten its life."*
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---
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## 2. Injection Flow and Configuration Logic (`zram-tools`)
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### Pipeline Execution Sequence
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The script follows a deterministic flow to ensure safe, reproducible configuration:
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```
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┌─────────────────────────────────────────────────────────────┐
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│ install_zram() Function │
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├─────────────────────────────────────────────────────────────┤
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│ 1. Validate RAM Detection │
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│ └─ Check if RAM_KB is available and non-zero │
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│ │
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│ 2. Compression Algorithm Selection │
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│ ├─ Present menu: LZ4 (fast) vs ZSTD (better ratio) │
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│ └─ User choice stored in $algo variable │
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│ │
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│ 3. Size Calculation Logic │
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│ ┌──────────────────────────────────────────────┐ │
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│ │ half_ram_mb = ((RAM_KB / 1024 / 1024 + 1) │ │
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│ │ / 2) * 1024 │ │
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│ └──────────────────────────────────────────────┘ │
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│ └─ Result: ~50% of total physical RAM in MB │
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│ │
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│ 4. Configuration Confirmation │
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│ ├─ Display summary with algorithm, size, priority=100 │
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│ └─ User must confirm before applying │
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│ │
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│ 5. Package Installation │
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│ sudo apt install -y zram-tools │
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│ │
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│ 6. Configuration File Write │
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│ /etc/default/zramswap │
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│ ALGO=$algo │
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│ SIZE=$zram_size │
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│ PRIORITY=100 │
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│ │
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│ 7. Service Restart │
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│ sudo systemctl restart zramswap │
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└─────────────────────────────────────────────────────────────┘
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```
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### Mathematical Size Calculation
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The script uses this formula to determine ZRAM size:
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```bash
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half_ram_mb=$(( ((RAM_KB / 1024 / 1024 + 1) / 2) * 1024 ))
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```
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**Breakdown:**
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- `RAM_KB`: Total RAM in kilobytes from `/proc/meminfo`
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- `/ 1024 / 1024`: Convert KB to MB
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- `+ 1`: Add rounding buffer for odd values
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- `/ 2`: Target approximately 50% of total RAM
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- `* 1024`: Round back to nearest MB
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**Example:**
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```
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System with 8 GB (8388608 KB) RAM:
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half_ram_mb = ((8388608 / 1024 / 1024 + 1) / 2) * 1024
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= ((8 + 1) / 2) * 1024
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= (9 / 2) * 1024
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= 4.5 * 1024
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= 4608 MB (~4.5 GB)
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```
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### Priority Configuration (`PRIORITY=100`)
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The `swapon` priority determines which swap device the kernel prefers when multiple devices exist:
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- **Higher number** = Higher preference (used first by kernel)
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- **Default system swap**: Typically 0–60
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- **ZRAM with PRIORITY=100**: Ensures ZRAM is used before physical disk swap
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This prevents thrashing where pages bounce between slow disk swap and fast RAM-based ZRAM.
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---
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## 3. Kernel Parameter Tuning (`sysctl`)
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### Essential VM Parameters for Aggressive ZRAM Usage
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While the current script focuses on `zram-tools` configuration, optimal performance requires complementary kernel parameter tuning:
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```bash
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# Recommended sysctl configuration for ZRAM systems
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vm.swappiness = 180
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vm.watermark_boost_factor = 0
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vm.watermark_scale_factor = 125
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vm.page-cluster = 0
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```
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### Parameter Explanations
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| Parameter | Value | Purpose |
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|-----------|-------|---------|
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| **`vm.swappiness`** | `180–200` | Aggressively prefer swap over keeping pages in RAM. Higher values (up to 200) are ideal for ZRAM because it's faster than disk swap. Default 60 is too conservative for memory-constrained systems. |
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| **`vm.watermark_boost_factor`** | `0` | Disable additional watermark boosting that could cause premature page reclaim |
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| **`vm.watermark_scale_factor`** | `125` | Adjust low-memory watermark thresholds to trigger swap earlier when RAM is constrained |
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| **`vm.page-cluster`** | `0` | Disable page clustering. Research shows this reduces unnecessary sequential reads during swap operations, improving ZRAM efficiency by ~15% in gaming workloads |
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### Why High Swappiness for ZRAM?
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Traditional wisdom suggests keeping swappiness low (20–40) to avoid swapping frequently. However:
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- **ZRAM is faster than disk**: Microseconds vs milliseconds
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- **Thrashing prevention**: Higher swappiness moves pages to ZRAM before they hit slow disk swap
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- **Effective RAM expansion**: Compressed pages in ZRAM can store 2–3x more data, effectively increasing available memory
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The Pop!_OS project and Linux kernel documentation both recommend values beyond 100 for in-memory swap scenarios like ZRAM/ZSWAP.
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---
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## 4. Service Lifecycle and Validation
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### Safe Service Initialization
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```bash
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sudo systemctl restart zramswap
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```
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**Why `restart` instead of `start`:**
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- Ensures previous configuration is cleanly terminated
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- Prevents orphaned processes from conflicting with new settings
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- Reloads systemd unit files if they were modified during installation
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### User Verification Commands
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#### Primary: `zramctl` (util-linux)
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```bash
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sudo zramctl
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```
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**Output Interpretation:**
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```
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NAME ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
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/dev/zram0 lz4 4G 2.1G 318.6M 424.9M [SWAP]
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```
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| Column | Meaning |
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|--------|---------|
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| **NAME** | Device identifier (/dev/zram0) |
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| **ALGORITHM** | Active compression algorithm (lz4, zstd, etc.) |
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| **DISKSIZE** | Maximum uncompressed data capacity configured |
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| **DATA** | Currently stored uncompressed pages in ZRAM |
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| **COMPR** | Actual compressed size using physical RAM |
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| **TOTAL** | Total memory used including metadata overhead |
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| **STREAMS** | Number of active swap streams (typically 4) |
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#### Secondary: `swapon --show`
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```bash
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sudo swapon --show
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```
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Shows all active swap devices with priority levels. ZRAM should appear with priority matching the configured value (100 in this script).
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### Real-Time Monitoring
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For continuous monitoring of compression effectiveness:
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```bash
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# Watch compression ratio changes over time
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watch -n 5 'zramctl | grep /dev/zram'
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# Monitor memory pressure and swap usage
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watch -n 5 'free -h && zramctl'
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```
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### Troubleshooting Indicators
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| Symptom | Likely Cause | Solution |
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|---------|--------------|----------|
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| `DATA` equals `DISKSIZE` but `COMPR` is near zero | System under memory pressure, ZRAM not being used | Increase `vm.swappiness` or check if physical swap has lower priority |
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| High CPU usage with low compression ratio | Incompressible data (e.g., encrypted files) | Consider backing device for incompressible pages |
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| Service fails to start | Missing dependencies (`zram-tools`, kernel module) | Run `sudo apt install zram-tools` and verify `modprobe zram` |
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### Permanent Configuration
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To ensure ZRAM persists across reboots, the script writes configuration to `/etc/default/zramswap`. This file is read by systemd's `zramswap.service` unit at boot time. Additionally, adding the following ensures the kernel module loads:
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```bash
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echo "zram" | sudo tee /etc/modules-load.d/zram.conf
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```
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### References:
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- [https://docs.kernel.org/admin-guide/blockdev/zram.html](https://docs.kernel.org/admin-guide/blockdev/zram.html)
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- [https://wiki.debian.org/ZRam](https://wiki.debian.org/ZRam)
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- [https://wiki.archlinux.org/title/Zram](https://wiki.archlinux.org/title/Zram)
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- [https://wiki.gentoo.org/wiki/Zram](https://wiki.gentoo.org/wiki/Zram)
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