3 ms·

My understanding with flash, having talked to someone who had to write his own wear-leveling code so his hardware could use cheap memory, is it's not so much th

My understanding with flash, having talked to someone who had to write his own wear-leveling code so his hardware could use cheap memory, is it's not so much the pure throughout that can kill a flash cell, but the write pattern that matters. My memory of the conversation is a bit hazy, but it's something like, in order to update even a single bit in a cell, it needs to be deleted and written to, and each cell only has so many write statistically before it's flagged no good. So to detroy a disk, you can strategically flip bits all over your disk, or with a disk with naive wear-leveling maybe just flip the same bit over and over. Good disks will make this harder with tricks like buffers (hopefully backed by batt/cap in the event of power outtage), and very clever wear-leveling algos, but you can definitely wear out flash memory with a fraction of the data and time you might expect if you have a truly pathological workload.

astrodust 8y ago
Flash memory is different from a regular drive, and it's to do with how it writes. You can't alter a single bit, you must rewrite the whole block and those can be anywhere from 256KB to 4MB or more. http://codecapsule.com/2014/02/12/coding-for-ssds-part-2-architecture-of-an-ssd-and-benchmarking/ http://codecapsule.com/2014/02/12/coding-for-ssds-part-2-arc... Adding new bits is cheap, though, so if you can come up with an append-only strategy for your system and periodically combine those into a new, pristine version you'll get far more life out of your SSD than if you go around randomly flipping bits. Is Redis using memory-mapped files the problem here? Treating a file like memory is all fine and good until your random bit-flipping has to be directly persisted. I wonder if future SSD firmware implementations will avoid wear by having a small buffer reserved for little post-write patches that can absorb a few small changes before having to commit to rebuilding the whole block. There's talk of "annealing" as a way of restoring cells to full life: https://www.extremetech.com/computing/142096-self-healing-self-heating-flash-memory-survives-more-than-100-million-cycles https://www.extremetech.com/computing/142096-self-healing-se... Has anyone tried reflowing a worn out drive?
planteen 8y ago
Flash must be erased on a page boundary, the size of which varies, but ranges from 512 bytes up. The act of erasing a flash page turns all bits to 1s (0xFF for the entire page). When the flash IC breaks, you can usually tell by doing a blank check after erasing. Instead of being 0xFF all over, some bits will be "stuck" to 0. A common reason for a discrete IC flash part wearing out is that charge pumps are used to raise a low voltage like 3.3 V to 12-15 V which is required for erasing. These charge pumps are touchy and can die first, from what I have heard.

astrodust 8y ago

Flash memory is different from a regular drive, and it's to do with how it writes. You can't alter a single bit, you must rewrite the whole block and those can be anywhere from 256KB to 4MB or more. http://codecapsule.com/2014/02/12/coding-for-ssds-part-2-architecture-of-an-ssd-and-benchmarking/ http://codecapsule.com/2014/02/12/coding-for-ssds-part-2-arc... Adding new bits is cheap, though, so if you can come up with an append-only strategy for your system and periodically combine those into a new, pristine version you'll get far more life out of your SSD than if you go around randomly flipping bits. Is Redis using memory-mapped files the problem here? Treating a file like memory is all fine and good until your random bit-flipping has to be directly persisted. I wonder if future SSD firmware implementations will avoid wear by having a small buffer reserved for little post-write patches that can absorb a few small changes before having to commit to rebuilding the whole block. There's talk of "annealing" as a way of restoring cells to full life: https://www.extremetech.com/computing/142096-self-healing-self-heating-flash-memory-survives-more-than-100-million-cycles https://www.extremetech.com/computing/142096-self-healing-se... Has anyone tried reflowing a worn out drive?

planteen 8y ago

Flash must be erased on a page boundary, the size of which varies, but ranges from 512 bytes up. The act of erasing a flash page turns all bits to 1s (0xFF for the entire page). When the flash IC breaks, you can usually tell by doing a blank check after erasing. Instead of being 0xFF all over, some bits will be "stuck" to 0. A common reason for a discrete IC flash part wearing out is that charge pumps are used to raise a low voltage like 3.3 V to 12-15 V which is required for erasing. These charge pumps are touchy and can die first, from what I have heard.