High Speed Disc Array (HSDA) Theory

The HSDA utilizes eight (8) Serial ATA (SATA) Hard Disk Drives (HDDs) for saving raw scan data in a redundant format (RAID 5). By utilizing a RAID 5 configuration in the HSDA, a single HDD failure will not cause immediate down time. The HSDA will continue working until such time where the failed HDD can be replaced. (See RAID definitions RAID Definitions.)

The following lists the HSDA HDD specifications:

• • Drive type: Serial ATA

• • Interface – SATA, 3.0 Gb/s or 6.0 Gb/s

• Drives: (8) Seagate – PN#ST3250310NS - Baracuda - 3 Gb/s 250GB or (8) Seagate – PN#ST3500514NS – Constellation – 3 Gb/s 500GB or (8) Seagate – PN#ST500NM0011 – Constellation – 6 Gb/s 500GB or any combination of the three.

Figure 4. HSDA SATA Hard Disk Drive

Each HDD will be mounted in a tray assembly which allows for Hot Swapping of the drives in the HSDA assembly.

The HSDA is equipped with two (2) power supply assemblies in a redundant fashion. Both power supplies are required for normal operation, but the HSDA can function with only one working power supply. The power supply assemblies are Hot Swappable and have independent power switches on each of the PSU modules

The HSDA is equipped with two (2) redundant cooling modules for controlling the temperature of the HSDA assembly. Under normal operation each of the cooling modules will run in a low speed mode. Under the following conditions the cooling modules will increase their fan rotational speed.

• • Component failure: if one cooling fan in a cooling module, a PSU, or a temperature sensor fails, the remaining cooling fan(s) automatically raises its rotation speed.

• • Elevated temperature: if any of the temperature readings breaches the upper threshold set for any of the interior temperature sensors, the cooling fans automatically raise their rotation speed.

• • During the subsystem initialization stage, the cooling fans operate at the high speed and return to low speed once the initialization process is completed and no erroneous condition is detected.

By design each HSDA contains eight Hard Disk Drives (HDDs) of 250 GB or larger capacity (8 X 250 GB = 2 TB). The actual Logical Disk (/dev/sda) created on the HSDA is only 512 GB in size. The Logical Disk Volume of 512 GB is evenly spread across all eight HDDs with an effective scan data capacity of approximately 439 GB (due to RAID 5 utilization).

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The HSDA's RAID Controller Module is responsible for mapping the available hardware (HDDs), and based on the RAID method configured, creates a disk volume of a specific size spanned across the HDD's. This mapping process is optimized to support specific read/write performance specification critical in handling of the raw scan data.

With RAID 5 utilization and limiting the Logical Disk Volume to 512 GB, each HDD is effectively partitioned to approximately 73 GB. This technique of partitioning the HDDs to be smaller than the actual capacity of the HDD is call “destroking”. Destroking allows for slightly better read/write performance since the logical disk space on the HDD is limited to the faster rotating section of the HDD platter/s.

With RAID 5 utilization, one (1) HDD worth of usable storage evenly spread across all eight (8) HDDs is used for storing the Distributed Parity (Ap - Hp) information. This Distributed Parity information is later used, in the event of a single HDD failure, to supply the necessary data needed to replace missing scan data from the failed HDD. The Distributed Parity information is also used to rebuild the logical disk on a replacement drive, when its installed in the array. The following illustration demonstrates how the HDDs are mapped in the array.

Figure 6. HSDA RAID 5 Disk Array Diagram

RAID (Redundant Array of Independent Discs) is a concept in storage subsystems that can deliver higher levels of protection against down-time and data loss than conventional disc drives. RAID refers to a drive architecture designed to safeguard critical data through redundancy.

RAID 5 Definition: Block level striping with parity data distributed across all member disks (utilized within HSDA).

Distributed parity requires all but one drive to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive.

Provides improved performance and additional storage, but provides no fault tolerance. Any disk failure destroys the array, which becomes more likely with more disks in the array. A single disk failure destroys the entire array because when data is written to a RAID 0 drive, the data is broken into fragments.

The number of fragments is dictated by the number of disks in the drive. The fragments are written to their respective disks simultaneously on the same sector. This allows smaller sections of the entire chunk of data to be read off the drive in parallel, giving this type of arrangement huge bandwidth. When one sector on one of the disks fails, however, the corresponding sector on every other disk is rendered useless because part of the data is now corrupted.

There is more to RAID than redundancy. RAID contributes to automatic load balancing. The right choice of RAID level can speed up data transfers or handle more I/O requests per second.