When dealing with traditional server storage configurations, RAID rebuild time becomes a critical factor in maintenance planning. The rebuild duration depends on multiple variables including:

• Disk interface speed (SATA III 6Gbps vs SAS 12Gbps)
• Rotational speed (5400/7200/10k/15k RPM)
• Disk capacity (in sectors, not just TB)
• Controller overhead (software RAID vs hardware RAID)
• Array activity during rebuild

Here's a fundamental formula to estimate rebuild time:

Rebuild Time (hours) = (Disk Capacity in Bytes × 8) / 
                      (Effective Rebuild Speed in bits/sec × 3600)

Example calculation for a 10TB SATA disk:

// 10TB disk with effective rebuild speed of 150MB/s
diskCapacity = 10 * 1000^4 * 8; // Convert to bits
rebuildSpeed = 150 * 1000^2 * 8; // Convert to bits/sec
rebuildTime = diskCapacity / (rebuildSpeed * 3600);
// Result: ~20.2 hours for single disk rebuild

For different RAID levels, we need additional factors:

// RAID 5 adjustment factor (N-1 disks participating)
function raid5RebuildTime(baseTime, diskCount) {
    return baseTime * (diskCount / (diskCount - 1));
}

// RAID 6 adjustment factor (N-2 disks participating)
function raid6RebuildTime(baseTime, diskCount) {
    return baseTime * (diskCount / (diskCount - 2));
}

Here's a complete JavaScript implementation for rebuild estimation:

class RaidRebuildCalculator {
  constructor() {
    this.diskTypes = {
      'SATA_5400': { baseSpeed: 80, variance: 0.3 },
      'SATA_7200': { baseSpeed: 120, variance: 0.25 },
      'SAS_10k': { baseSpeed: 180, variance: 0.2 },
      'SAS_15k': { baseSpeed: 250, variance: 0.15 },
      'SSD': { baseSpeed: 400, variance: 0.1 }
    };
  }

  calculate(diskSizeTB, diskType, raidLevel, diskCount) {
    const type = this.diskTypes[diskType];
    const effectiveSpeed = type.baseSpeed * (1 - type.variance) * 1e6 * 8; // MB/s to bits/s
    
    const rawTime = (diskSizeTB * 1e12 * 8) / (effectiveSpeed * 3600);
    
    switch(raidLevel) {
      case 'RAID5': return rawTime * (diskCount / (diskCount - 1));
      case 'RAID6': return rawTime * (diskCount / (diskCount - 2));
      default: return rawTime;
    }
  }
}

// Example usage:
const calculator = new RaidRebuildCalculator();
console.log(calculator.calculate(10, 'SATA_7200', 'RAID6', 10));
// Output: ~168 hours (7 days) for 10x10TB RAID6

For software RAID implementations, these techniques can help:

1. Limit array activity during rebuilds
2. Increase rebuild priority in mdadm: echo 10000 > /proc/sys/dev/raid/speed_limit_min
3. Stagger rebuilds if replacing multiple disks
4. Consider filesystem - XFS may rebuild faster than ext4 on large arrays

Always add 15-30% overhead to theoretical calculations for real-world scenarios.

When managing storage arrays with conventional servers (using software RAID), administrators often underestimate the operational impact of RAID rebuild operations. A 10-disk RAID6 array with 10TB SATA drives can indeed take 5-7 days to rebuild, during which:

• Performance degradation can reach 50-70%
• Some enterprise policies mandate workload suspension
• The array remains vulnerable to additional disk failures

The rebuild time calculation depends on multiple variables:

# Pseudocode for basic rebuild time estimation
def estimate_raid_rebuild(disk_size_gb, disk_type, rpm, raid_level, disk_count):
    # Base throughput coefficients (MB/s)
    sata_coeff = 80 if rpm == 7200 else 120  # 7200rpm vs 10Krpm
    sas_coeff = 140 if rpm == 10000 else 200 # 10Krpm vs 15Krpm
    ssd_coeff = 500                          # Typical SSD throughput
    
    # RAID penalty factors
    raid_factor = {
        'RAID5': 1.0,
        'RAID6': 0.7,
        'RAID5+1': 0.8,
        'RAID6+1': 0.6
    }
    
    throughput = locals()[f"{disk_type.lower()}_coeff"] * raid_factor[raid_level]
    raw_hours = (disk_size_gb * 1024) / (throughput * 3600)
    
    # Adjust for disk count (more disks = parallel rebuild)
    return raw_hours * (1 + (0.1 * (disk_count - 4)))  # Empirical adjustment

Let's compare three common scenarios:

# Example 1: 8x4TB SATA RAID5 (7200rpm)
time = estimate_raid_rebuild(4000, 'SATA', 7200, 'RAID5', 8)
# Result: ~14.2 hours

# Example 2: 12x8TB SAS RAID6 (10Krpm) 
time = estimate_raid_rebuild(8000, 'SAS', 10000, 'RAID6', 12)
# Result: ~28.5 hours

# Example 3: 6x2TB SSD RAID5+1
time = estimate_raid_rebuild(2000, 'SSD', 0, 'RAID5+1', 6)
# Result: ~2.1 hours

For software RAID implementations (mdadm, ZFS, etc.), consider these tuning parameters:

# Linux mdadm tuning example
echo 500000 > /proc/sys/dev/raid/speed_limit_min
echo 2000000 > /proc/sys/dev/raid/speed_limit_max

# ZFS rebuild priority adjustment
zfs set rebuild_priority=high tank/raid6_pool

# Windows Storage Spaces (PowerShell)
Set-StorageSubSystem -FriendlyName "Storage*" -RebuildPerformancePriority High

Implement proper monitoring during rebuild operations:

# Bash script for mdadm progress monitoring
while true; do
    cat /proc/mdstat | grep -A 1 'md0'
    sync && echo 1 > /proc/sys/vm/drop_caches
    sleep 60
done

# PowerShell alternative for Storage Spaces
Get-StorageJob | Where-Object {$_.Name -like "*rebuild*"} | 
    Select-Object Name, PercentComplete, BytesProcessed, BytesTotal

For arrays exceeding 50TB raw capacity, traditional RAID rebuild times become prohibitive. Consider:

• Erasure coding implementations (like Ceph's EC pools)
• Declustered RAID (used in ZFS draid)
• Hot spare strategies with smaller failure domains

# ZFS draid configuration example for faster rebuilds
zpool create tank draid2:4d:6s:0c /dev/sd[b-m]
# 4 data disks, 6 distributed spares, 0 cache devices