Tuesday, July 14, 2009

ATA TRIM command for SSD

So far, conventional magnetic disk have NOT have to know about free blocks of file system, since every block on magnetic disks can be overwritten. However, flash memory is different. In order to write something on flash memory, it need to erase the block. Actually, a block consist of many pages, and page can be written separately, but a block should be erased even when it is necessary to erase one page.

TRIM command offers a way for the file system to let the secondary storage device know that a certain block or page no longer contains valid data, so it can make the storage invalidate or free the block or page. It could make more free pages or blocks so improve the performance of the SSD, since SSD has no way to figure out which block or page is not necessary until it is overwritten.

In 2007, Microsoft proposed TRIM command, which carries the information related to erased blocks to device for optimization. In conventional file system, the information about the deleted file only kept in file system, not in device. As a result, both valid and invalid data is same to storage device. Microsoft said that TRIM command could be useful for background defect management and error recovery process for conventinal HDD and merge, wear leveling and erase operations for SSD. If SSD desn't have to keep track of the information about erased block, the performance of SSD will improve and its life expectancy will increase. Actually, OS's process, such as format, defragment, recycle, managing temporary file can produce a large amount of deleted file. So, TRIM command would be really useful for SSD.

So far, SSD manufacturers and device driver programmer know this issue, but I don't think any of them implement this feature yet. Even Intel's latest X25-M doesn't offer this command. Moreover, any OS doesn't offer this feature yet. But, sooner or later, it will be available in market.

In addition, this feature will be implemented in software and firmware, not in device hardware. So, anyone who has SSD needs to keep eye on their update for a time.

For more information:


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