[mattb314]

Do you think you could point me towards a source for "mixing reads & writes with a SSD lowers performance"? This seems reasonable, but I've never actually heard it before, and a cursory googling didn't turn much up. I have heard before that small, random writes on SSDs are much more expensive than sequential writes because random writes can produce write amplification and increase fragmentation. It was my impression that SSDs stand to benefit significantly from log structured filesystems despite the mixed read/write load, but I don't follow it that closely and could be wrong.

[noahdesu]

https://www.usenix.org/conference/atc14/technical-sessions/p...

[mattb314]

Thanks! This talk is pretty cool.

In case anyone else is interested, in addition to the above video, this [1] paper goes into some of the details, including this explanation:

"Reads and writes on SSDs can interfere with each other for many reasons. (1) Both operations share many critical resources, such as the ECC engines and the lock-protected mapping table, etc. Parallel jobs accessing such resources need to be serialized. (2) Both writes and reads can generate background operations internally, such as readahead and asynchronous write-back [6]. (3) Mingled reads and writes can foil certain internal optimizations. For example, flash memory chips often provide a cache mode [2] to pipeline a sequence of reads or writes. A flash memory plane has two registers, data register and cache register. When handling a sequence of reads or writes, data can be transferred between the cache register and the controller, while concurrently moving another page between the flash medium and the data register. However, such pipelined operations must be performed in one direction, so mingling reads and writes would interrupt the pipelining."

Also relevant to the "seek" conversation below: it turns out many SSDs have read-ahead caches built in, so sequential reads are much faster than random reads after an initial warm-up, just like hard disks (the difference, however is closer to ~5x than to 100x difference you see in HDDs).

[1] http://bit.csc.lsu.edu/~fchen/publications/papers/hpca11.pdf