web spider: programs that automatically fetch information from websites, a email spam program is a example (though bad one) about web spiders/crawlers. but there are some good ones, such as RSS, a HTTP web spider that feeds the user news.
Ubuntu tools that can act as web spider:
wget - web downloading command line tool that automatically fetch the file indicated
snarf - simple web resource fetcher, just like wget
Linux Flash Drive File system
unlike disk file system, flash drives are made of NAND memory blocks and have some different characteristics:
1. although faster in reading and writing, flash drives memory's rewritable times are smaller than conventional hard disk, writing optimization algorithm is needed to lengthen the life span of flash drives, diverging programs from constantly writing to a frequently used part of the flash drive.
2. flash drive's writing is different, instead of being able to write 1's and 0's, flash drive writing only consist of 1's. thus the writing of a block requires the flash driver head to first erase everything into 0, reset the entire block once, then write the 1's. it also means whenever just a single bit has changed in a block of flash drive, the entire block must be rewritten, further decrease the life span of flash drives
3. memory dissapation after frequent reading also happens to flash drive. so after a certain amount of time of reading for a block of flash drive, the block must be rewritten to charge the NAND transisters in order to keep the memory, further increasing the frequency of rewriting
linux uses JFFS (journalling flash file system) and YAFFS for flash drive. they both employ writing frequency prevention algorithms.
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