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A Trim command (known as TRIM in the ATA command set, and UNMAP in the SCSI command set) allows an operating system to inform a solid-state drive (SSD) which blocks of data are no longer considered in use and can be wiped internally. Trim was introduced soon after SSDs were introduced. Because low-level operation of SSDs differs significantly from hard drives, the typical way in which operating systems handle operations like deletes and formats resulted in unanticipated progressive performance degradation of write operations on SSDs. Trimming enables the SSD to handle garbage collection overhead, which would otherwise significantly slow down future write operations to the involved blocks, in advance.〔Shimpi, Anand Lal. (2009-03-18). p. 10.〕 Although tools to "reset" some drives to a fresh state were already available before the introduction of trimming, they also delete all data on the drive, which makes them impractical to use for ongoing optimization.〔Shimpi, Anand Lal. (2009-03-18). p. 11.〕 By 2014 many SSDs had internal idle/background garbage collection mechanisms that work independently of trimming; although this successfully maintains their performance even under operating systems that do not support Trim, it has the associated drawbacks of increased write amplification and wear of the flash cells. == Flash drive specific issues == Because of the way that many file systems handle delete operations, by flagging data blocks as "not in use",〔Shimpi, Anand Lal. (2009-03-18). p. 7.〕 storage media (SSDs, but also traditional hard drives) generally do not know which sectors/pages are truly in use and which can be considered free space. Contrary to, for example an overwrite operation, a delete will not involve a physical write to the sectors that contain the data. Since a common SSD has no knowledge of the file system structures, including the list of unused blocks/sectors, the storage medium remains unaware that the blocks have become available. While this often enables undelete tools to recover files from electromechanical hard disks,〔 despite the files being reported as "deleted" by the operating system, it also means that when the operating system later performs a write operation to one of the sectors, which it considers free space, it effectively becomes an overwrite operation from the point of view of the storage medium. For magnetic disks this is no different from writing an empty sector, but because of how some SSDs function at the lowest level, an overwrite produces significant overhead compared to writing data into an empty page, potentially crippling write performance.〔 SSDs store data in flash memory cells that are grouped into pages typically of 4 to 16 kB, grouped together into blocks of typically 128 to 512 pages, e.g., 512 kB per block with 4 kB pages in 128-page blocks.〔〔 NAND flash memory cells can only be directly written to when they are empty. If they may contain data, the contents must be erased before a write operation. An SSD write operation can be done on a single page but, due to hardware limitations, erase commands always affect entire blocks;〔Shimpi, Anand Lal. (2009-03-18). p. 5.〕 consequently writing data to empty pages on an SSD is very fast, but slows down considerably once previously written pages need to be overwritten. Since an erase of the cells in the page is needed before it can be written again, but only entire blocks can be erased, an overwrite will initiate a read-erase-modify-write cycle:〔〔Shimpi, Anand Lal. (2009-03-18). p. 8.〕 the contents of the entire block are stored in cache, then the entire block is erased from the SSD, then the overwritten page is written to the cached block, and only then can the entire updated block be written to the flash medium. This phenomenon is known as write amplification.〔(【引用サイトリンク】title=Write Amplification: Intel's Secret Sauce )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Trim (computing)」の詳細全文を読む スポンサード リンク
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