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Today we will focus on part two of three, of the ATTO ExpressPCI UL3D RAID Kit. Our area of focus will be:
  1. Frequently asked questions and technical updates not contained in Part 1
  2. Windows2000 Performance vs. Windows Millennium.
  3. Windows2000 performance between ATTO’s UL3D and Adaptec’s 2100S.
Most Frequently asked questions about the UL3D:
  1. Is the UL3D SCSI3 compliant?
  2. Is the UL3D a RAID controller?
  3. Is the UL3D RAID hardware or software supported?
  4. Is the ATTO RAID software exclusive to the UL3D’s chipset?
  5. What are the differences between the UL3D RAID Kit and UL3D?
  6. What are the overhead and performance differences between Win9x and Windows2000?
  7. Software vs. Hardware RAID?
The UL3D is a SCSI3 compliant Ultra3 device. ASPI stands for Advanced SCSI Programming Interface. It is an Adaptec-developed interface specification for sending commands to SCSI host adapters. According to this standard, three specifications must be featured in order to be Ultra3 compliant. The features that are:
  1. Quick Arbitration
  2. Packetized SCSI
  3. Double Transition Clocking
  4. Cyclical Redundancy Checks
  5. Domain Validation
The UL3D features DTC, CRC, and Domain Validation. Therefore, the UL3D complies with the SCSI3 standard.

RAID level 0 does not provide any protection for data. RAID 0 takes multiple drives and stripes the data across all of the drives in the array. A typical RAID 0 set will have a stripe size of 64 KB, though most RAID management software allows this value to be changed by the user. With a 64 KB stripe size and two drives in the set, the initial 64 KB of data will be written to the first drive, the second 64 KB will be on the second drive, and the third 64 KB block will be placed back on the first drive. RAID 0 does not provide any data protection, if one drive fails the entire array is destroyed. However, RAID 0 does provide a significant boost in performance since multiple drives can be simultaneously seeking portions of the data being requested. RAID 0 is ideal for applications that need high performance and a large amount of storage, but do not need data protection beyond that of a periodic backup. If ten 36 GB drives are used in a RAID 0 array, the capacity of the array will be 10 * 36 GB = 360 GB.

RAID level 1 (MAC only), is also commonly called mirroring. With RAID 1, a secondary drive is kept as an identical copy of the primary drive. If one drive in a mirrored set fails, then the other drive is used exclusively until the failed drive is replaced. RAID 1 provides the maximum level of protection, though it is the most expensive RAID level because two drives must be purchased to provide the capacity of one. RAID 1 pays a small price in performance on a write since the data must be written twice; however, it does gain the benefit of RAID 0 on a read operation since different portions of the data can be read from separate drives in the mirrored set. In addition, for added reliability, most RAID systems allow the user to install multiple mirrors for each drive (e.g. one drive with two drives acting as a copy). If a 36 GB drive is used in a RAID 1 set, the drive will be mirrored with another 36 GB drive and the array’s capacity will be 36 GB. Many workstation and low-end server users prefer RAID level 1 because it is easy to implement and can provide very high redundancy.

The UL3D is both hardware and software RAID supported. The UL3D contains a mini-processor on the board itself (micro-code BIOS), which controls the RAID during boot-up or within DOS. Upon entering the operating system, the ATTO driver then regulates the responsibilities.

ATTO’s striping software is closely tied to ATTO’s hardware; therefore, it will not work with any other adapters, even other adapters based on the same chipset. On the Mac side, however, the software will work with just about any SCSI or SCSI-like interface (IDE, Fibre Channel, etc; even firewire, as ATTO’s software ships with the SANcube from Micronet.

The differences between the UL3D RAID Kit and the UL3D are cables only. The ATTO Kit includes two external SCSI cables.

Theoretically, the overhead or performance differences should favor Windows2000 because its programming API is designed as such to accommodate RAID. Therefore, the overhead to run the RAID “group” should be less. These functions take place below the SCSI sub-system.

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