Q 1 : Four differences between SATA and SCSI drives?

Ans-

SCSI(sku4zē) Short for small computer system interface, a parallel interface standard used by Apple Macintosh computers, PCs and many UNIX systems for attaching peripheral devices to computers. Nearly all Apple Macintosh computers, excluding only the earliest Macs and the recent iMac, come with a SCSI port for attaching devices such as disk drives and printers. SCSI interfaces provide for data transmission rates (up to 80 megabytes per second). In addition, you can attach multiple devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.

Serial ATA (SATA)

Often abbreviated as SATA, Serial ATA is an evolution of the Parallel ATA physical storage interface. Serial ATA is a serial link - a single cable with a minimum of four wires creates a point-to-point connection between devices. Transfer rates for Serial ATA begin at 150MB/s.

Starting with SATA, it extends the capabilities of ATA and offers transfer rates starting at 150MB/s and, after years of development, has moved to the mainstream of disk interfaces. The successor the SCSI interface is SAS at speeds of up to 3Gb/s. Additionally, it also addresses parallel interface issues such as drive addressability and limitations on the number of device per port connection.

SAS devices can communicate with both SATA and SCSI devices (the backplanes of SAS devices are identical to SATA devices). A key difference between SCSI and SAS devices is the addition in SAS devices of two data ports, each of which resides in a different SAS domain. This enables complete failover redundancy. If one path fails, there is still communication along a separate and independent path.

SATA vs. SCSI

There are many differences in the interface technology and in the commands executed.

SCSI has a robust tagged command queuing implementation to allow multiple commands to be outstanding which provides significant performance gains for drives or controllers by being able to order the commands in an optimal execution manner. ATA only allows a bus release that is mistakenly called command queuing. The implementation on the class of disk drives that support ATA is different as well.

ATA drives are cheaper than SCSI or Fibre Channel drives and there's a reason. SCSI and FC drives use a processor for executing the commands and handling the interface and a separate processor controlling the head positioning through servos. ATA drives use a single processor for both which means that if the rotational positioning requires more adjustments due to factors such as rotational vibration or wear, more processor time will be dedicated for that which can affect the performance of the drive. Because of this, you will usually see a lower RPM for the ATA disk drive and also a shorter

warranty period. But it is cheaper and can be a very good, economical solution for many environments and applications.

SATA configurations perform admirably well in file server and streaming media scenarios, while SCSI configurations (dependent on the specific type of adapter and harddisks) are king of the hill in database applications.

With the arrival of Serial ATA II all shortcomings of the ATA-interface concerning performance and reliability have been solved. A new generation of hard disks, supporting native command queuing, will be able to offer even better performance than the Raptor WD740GD? in this comparison. At the same time, the arrival of Serial Attached SCSI and future SAS-harddisks with spindle speeds of 22,000rpm ensures that the SCSI interface will consolidate its unique feature set and its performance advantage in certain workloads. SCSI, thanks to its higher level market position, will always be the technology to profit first from technological advancements. This technological advantage will not only affect harddisk performance, but will also have its effect on RAID adapter performance. The higher priced SAS RAID solutions will be equipped with the latest generation I/O processors and larger amounts of cache memory, for instance. In the low-end server market - single and dual processors systems using direct attached storage - Serial ATA will become a mature and powerful alternative to SCSI.

Q 2 : Detailed note on Indexed Allocation?

A partial way around this is to keep all block numbers in a contiguous table for each file.

Pros

• Good for direct access.

• Information for each file is kept in one area.

• No external fragmentation.

Cons

• File size limited by the number of indices in the index table. But we can extend this in a number of ways.

• If we lose an index block we have lost access to a whole chunk of the file.

Extending index blocks

If the file has more blocks than we can reference from the index block we need to have some way of connecting to more index blocks. We can link index blocks together (like the linked allocation of files). Similar pros and cons.

We can have multiple levels of index blocks.

• The first level points to index blocks.

• The second level points to actual blocks.

e.g. If we have blocks of size 8K, a block address of 4 bytes and we have a two level system we can address files of up to 32 gigabytes. The indirect index block points to 2048 index blocks. Each index block points to 2048 actual file blocks. So we can have 4194304 actual blocks in a file.