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Communications of the ACM


A File System All Its Own

A File System All Its Own, illustration

Credit: Joel Ormsby

Flash memory has come a long way and it is time for software to catch up.

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Melissa Cayer

"...In addition, each write or erase operation wears out NAND flash. Chips are rated for a certain number of such operationsanywhere from 50050,000 cycles today depending on the type and quality, and those numbers are shrinking as the chips themselves shrink..."

How would I, the end user, notice the wear out? I make periodic backups of some of my files.

-Melissa Cayer

Adam Leventhal

Hi Melissa,

Depending on the file system and the SSD you might notice the wear in different ways. High quality SSDs will turn on the "predict fail" flag far in advance of actual data loss, recovering at-risk data, and moving it to safe locations. After a failure, SSDs may percolate bad data back to the file system. A modern file system (like ZFS) will identify the bad data and report an error; a typical consumer file system such as Apple's HSFS+ will just use the bad data leading to system crashes or visible data corruption.

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