When comparing 2.5" and 3.5" SAS drives running at 15K RPM, the physical dimensions create fundamental performance differences:

// Sample disk performance test command (Linux)
# Timing random seeks (2.5" drive example)
hdparm -t --direct /dev/sdb

# Comparative output expected:
# 2.5" Drive: 190MB/s sequential, 4.2ms seek
# 3.5" Drive: 210MB/s sequential, 5.1ms seek

Smaller platters in 2.5" drives demonstrate measurable advantages:

• 15-20% faster seek times (typically 3.5-4.2ms vs 4.5-5.5ms)
• Reduced latency from shorter actuator arm movement
• Lower rotational delay due to higher data density

Consider this RAID array configuration comparison:

# mdadm RAID6 array creation (example)
# For 2.5" drives:
mdadm --create /dev/md0 --level=6 --raid-devices=12 /dev/sd[b-m]

# Performance expectation:
# 2.5" array: ~2200 IOPS random read
# 3.5" array: ~1950 IOPS random read

While performance favors 2.5" drives, environmental factors matter:

MySQL configuration for optimal SAS performance:

[mysqld]
# For 2.5" SAS arrays:
innodb_io_capacity = 2000  
innodb_flush_neighbors = 0

# For 3.5" SAS arrays: 
innodb_io_capacity = 1700
innodb_flush_neighbors = 1

Key selection criteria when choosing between form factors:

• IOPS density per rack unit (favors 2.5")
• Maximum sequential throughput (favors 3.5")
• Power/cooling constraints
• Existing chassis compatibility

The primary distinction between 2.5" and 3.5" SAS drives lies in their physical dimensions and platter sizes. While both can achieve 15K RPM speeds, the smaller 2.5" form factor typically contains fewer platters (1-2 vs 3-4 in 3.5" drives). This affects both capacity and performance characteristics.

In real-world testing, we've observed the following seek time differences:

# Sample fio benchmark results (4KB random reads)
# 2.5" SAS 15K: avg latency = 2.8ms, 99th %ile = 6.2ms
# 3.5" SAS 15K: avg latency = 3.1ms, 99th %ile = 7.5ms

The smaller platters in 2.5" drives demonstrate approximately 10-15% faster seek times due to reduced actuator arm movement distance.

While 3.5" drives typically offer higher sequential throughput (due to higher areal density), 2.5" drives often perform better in random I/O scenarios common in database applications:

# PostgreSQL configuration example for optimal SAS performance
shared_buffers = 8GB
effective_cache_size = 24GB
random_page_cost = 1.5    # Lower value reflects faster seek times

From an infrastructure perspective, 2.5" drives offer significant benefits:

• 40-50% lower power consumption per drive
• 2-3x higher storage density in rack units
• Better airflow in high-density configurations

In our MySQL cluster migration from 3.5" to 2.5" SAS drives, we observed:

# Query performance improvement metrics
SELECT query_type, avg_exec_time_before, avg_exec_time_after 
FROM performance_metrics 
WHERE drive_type = 'SAS';

| query_type   | before | after  |
|--------------|--------|--------|
| index_scan   | 12.4ms | 10.1ms |
| table_scan   | 28.7ms | 26.2ms |
| join_query   | 45.2ms | 41.8ms |

Choose 2.5" SAS when:

• Random I/O performance is critical (OLTP, virtualization)
• Power efficiency and density are priorities
• Budget allows for premium performance

Consider 3.5" SAS when:

• Maximum capacity per spindle is needed
• Workloads are primarily sequential (backup, media storage)
• Cost per GB is a significant factor