Disk Options and Comparisons

We have 3 disk options that conform with the low power requirement (< 2-3 Watts?):

• Solid State Drives
• Flash Memory Cards
• Hybrid Drives

To compare options amongst these drives, we need common parameters that give us a decent and standard metric. Some parameters are:

• Power Consumption (Idle and Peak)
• Read/Write Speed (Data Transfer Rate)
  • Sequential Data
  • Random Data
• Cost per Gigabyte
• I/O Interface Support and Bottlenecks
  • PCIe
  • USB 2.0/3.0
  • SD/MMC

RAID0

RAID0 gives the option to provide better read/write performance. Although it is not the best option for reliability.

The SSD used is OCZ Vertex2 120GB (Measured 230 MB/s write and 275 MB/s read throughput on Atom D510MO motherboard)

RAID0 was built both on Core 2 and Atom D510.

Core 2

2 disks RAID0 was installed on Core2 and achieved measured 469 MB/s write throughput. It is the value close to double of single disk performance.

Atom D510

• SATA was treated as IDE

However 2 disks RAID0 was installed on Atom D510 but only 304 MB/s read throughput when testing md0. The results shows far less than expected double performance.

Atom D510 uses DMI bus which has theoretical limit 10Gb/s. It needs at least four SSDs to saturate and is not the bottleneck.

Interesting result through testing read throughput from reading from two disks at them same time, shows a rough read throughput over 450 MB/s. This test indicate that RAID0 throughput was limited by internal software bottleneck.

• Change SATA settings to ACHI without rebuild RAID0

Speed was not obviously affected.

• Interrupts

cat /proc/interrupts 

I found that all interrupts go to processor 0. Run following command to balance interrupts

sudo bash -c "echo '3'>  /proc/irq/19/smp_affinity " 

Read throughput on RAID0 /dev/md0 achieves 348 MB/s

• Create 10GB large file and read directly

Read throughput on RAID0 achieves 438 MB/s

• SATA was treated as Advanced Host Controller Interface (AHCI) and RAID0 was rebuilt

Read throughput on md1 achieves 520 MB/s and write achieves 318 MB/s, which are within the expectation.

RAID1

Read throughput achieved on Atom D510 platform was only 189 MB/s, far less than expected more than 400 MB/s.

Write throughput was 177 MB/s.