2012-06-23, 02:57
Oh boy, I'm not sure I fully understand the issues with garbage collection/trim on SSD's but I'll write it how I understand it. It may be correct or completely off base but I'm sure somebody smarter than me will confirm or deny this info. So with that, here we go...
SSD's can only write to an erased block. Since files on drives are really only deleted from the partition tables but the data remains in the physical location on the drive, those blocks aren't erased when you delete a file, on mechanical or SSDs.
Since an SSD can only write to an erased block, eventually, you will have used up all the blocks at some point of writing data to the drive. In order to write data to the drive after that, the SSD must find the block it is going to write data to, move the contents that isn't supposed to be deleted to the buffer, erase the block, rewrite the data that isn't supposed to be deleted back to the block and then write the new data to the erased portion, all of which takes time and causes a performance hit.
Garbage collection is a function in the firmware of SSD's that erases these blocks once the data is removed. Most will only do it once the drive detects all the blocks have been written to and the drive is idle. Some OS's have "trim" support which helps tell the SSD what blocks to erase and when to do it to help keep the drive performing well.
Mechanical drives can change a block on the fly regardless of what it's set to currently so they don't have performance problems as a result of writing to areas already written to or that aren't erased. So no, they don't have the risk of performance loss like the SSD's. However, keep in mind, many people are using SSD's for their cache drives with great success and there are recommended ones posted on the unRaid forums (as well as ones to stay away from).
That's how I understand it, some of this may even be accurate!!!!
SSD's can only write to an erased block. Since files on drives are really only deleted from the partition tables but the data remains in the physical location on the drive, those blocks aren't erased when you delete a file, on mechanical or SSDs.
Since an SSD can only write to an erased block, eventually, you will have used up all the blocks at some point of writing data to the drive. In order to write data to the drive after that, the SSD must find the block it is going to write data to, move the contents that isn't supposed to be deleted to the buffer, erase the block, rewrite the data that isn't supposed to be deleted back to the block and then write the new data to the erased portion, all of which takes time and causes a performance hit.
Garbage collection is a function in the firmware of SSD's that erases these blocks once the data is removed. Most will only do it once the drive detects all the blocks have been written to and the drive is idle. Some OS's have "trim" support which helps tell the SSD what blocks to erase and when to do it to help keep the drive performing well.
Mechanical drives can change a block on the fly regardless of what it's set to currently so they don't have performance problems as a result of writing to areas already written to or that aren't erased. So no, they don't have the risk of performance loss like the SSD's. However, keep in mind, many people are using SSD's for their cache drives with great success and there are recommended ones posted on the unRaid forums (as well as ones to stay away from).
That's how I understand it, some of this may even be accurate!!!!
