| Storage media | Descriptions | Examples | Advantages |
Magnetic disk  (ii) movable head 
| (i) A fixed magnetic head is an escapement mechanism, that is coupled to the rotary spindle which supports the disk, is free to rotate about the motor axis, causing the disk to precess while the disk is rotating. Thus, there is relative radial movement between the precessing disk and the fixed head, enabling accessing to different data tracks. (ii) A locking mechanism operable during on-times of the magnetic disk | • Floppy disk • Flexible disk cartridge (FDC) | • Short access time combined with high storage capacities. • Recording is performed in concentric tracks unlike a record where there is only one “track” in a spiral line. • Higher data transfer rate due to the higher in-track recording density in magnetic recording. |
Optical disk 
| A plastic-coated disk that stores digital data, such as music or text, as tiny pits etched into the surface and is read with a laser scanning the surface. Also called laser disk. | • HD • DVD • Blue Ray | • Mass storage capacity (gigabytes) • Mountable/ un-mountable storage units • Low cost per bit of storage • Long media life • High data stability • High speed data recording and playback is disclosed. |
Flash memory 
| • Flash memory refers to a particular type of EEPROM, or Electronically Erasable Programmable Read Only Memory that maintains stored information without requiring a power source. • Flash memory differs from EEPROM in that EEPROM erases its content one byte at a time. This makes it slow to update. Flash memory can erase its data in entire blocks, making it a preferable technology for applications that require frequent updating of large amounts of data as in the case of a memory stick. • Inside the flash chip, information is stored in cells. A floating gate protects the data written in each cell. Tunneling electrons pass through a low conductive material to change the electronic charge of the gate in "a flash," clearing the cell of its contents so that it can be rewritten. This is how flash memory gets its name. | • USB Flash drives • Flash cards | • Compare to traditional hard drive, flash memory is nonvolatile or solid state, meaning there are no moving parts. It's also silent, much smaller than a traditional hard drive, and highly portable with a much faster access time. |
Magneto optical disk 
| • Magneto-optical disks, such as the rewritable optical disk and the recordable disk used with the Mini Disc player, have a special layer, as of barium ferrite, that can be magnetically polarized by a recording head when heated with a laser. • Data or sound may be recorded to and erased from any portion of a magneto-optical disk multiple times. • When writing, a laser is used to heat the material to this temperature and a magnetic head is used to change the polarity of the material to write the bit, like a conventional computer hard drive. | • HP 4.8GB Rewritable Magneto-Optical disk • Magento Optical Disc for Data (RW) | • This system yields precise servo signals, and can eliminate the tracking error signal (TES) and focusing error signal (FES) independently. • Enables integrated head to increase the coupling efficiency of a collimator without the deterioration of optical characteristics. • Longer shelf life • Reliable long term - Supports millions of reads and writes without losing performance. • Enhanced polycarbonate substrate. |
Tuesday, September 16, 2008
SEB070011-Tutorial 5
Tuesday, September 2, 2008
SEB070011-TUTORIAL 4
| Windows | UNIX/Linux |
| Windows sets amount of virtual memory at boot time, so, it is based on the amount of free physical RAM and responsible for managing the physical pages. The pages that map to the Win32 is based on the application's code and data that are reserved at application initialization time. This will decrease the available virtual memory but not available for RAM. | Unix and its related systems use very sophisticated memory management algorithms to make the use of memory resources efficiently. The total real memory is calculated by subtracting the kernel memory from the amount of RAM but the total virtual memory is depended on the degree to which the processes use mapped files. The limitation for data and stack space is the amount of real and swap memory. |
Windows handles the page faults by investigating the following program-related problems: 7.Hard disk fragmentation or errors |
CPU is tried to access a page that is not in main memory when page faults is occurred and thus forcing the CPU to wait for the page to be swapped in. The goal of the memory manager is to minimize the number of page faults especially when we moving data to and from disks takes a significant amount of time. The Linux page faults handler acquiring a read-write semaphore (mmap_sem) and a spin lock, the page_table_lock for synchronization between multiple threads of a task. Firstly, it acquires a read lock on mmap_sem (which alone would allow other threads to continue processing page faults) and a spin lock on the page_table_lock (which serializes access to the page table and important data structures) before acquiring a free page from the page allocator. The page is then cleared by overwriting the contents with zeros and is assigned to the process by creating a corresponding page table entry in the page table of the process. Cache line bouncing has a critical influence on page fault performance in SMP systems and particularly significant for large applications (like huge databases or computational applications) that try to minimize the startup time by having multiple threads of a process running on different processors. |
Windows chooses a page size of 4K because that was the only page size supported by that architecture at the time the operating system was designed. (4MB pages were then added to the CPU later in the Pentium but clearly that is too large for everyday use.) |
A page is a fixed length block of main memory, which contiguous in both physical memory addressing and virtual memory addressing. Kernel swap and allocates memory using pages. The page size in Linux is fixed to 4kb/4mb/2mb. If page size is 4kb and a file has a size of 13kb then file will consume four pages, which three pages of 4kb each and last page will have only 1kb of data in it. This will result in waste of memory of 3kb in the last page (internal fragmentation). |
| Thrashing is caused mainly by the file indexing but not with page file. Indexing usually happens in the first 15mins after we boot up. To avoid thrashing issues in windows, the first thing to do is trying to make the page file at a fixed size of about 1024Mb on a second HD if possible. Inserting a spare USB memory stick for ready boost helps in reduce thrashing . | Under allocation of the minimum number of pages required by a process, thrashing happens and forcing it to continuously page fault. The system can detect thrashing by evaluating the level of CPU utilization as compared to the level of multiprogramming and can be eliminated by reducing the level of multiprogramming. |