GeckoFTL: Scalable flash translation techniques for very large flash devices

The volume of metadata needed by a flash translation layer (FTL) is proportional to the storage capacity of a flash device. Ideally, this metadata should reside in the device's integrated RAM to enable fast access. However, as flash devices scale to terabytes, the necessary volume of metadata is exceeding the available integrated RAM. Moreover, recovery time after power failure, which is proportional to the size of the metadata, is becoming impractical. The simplest solution is to persist more metadata in flash. The problem is that updating metadata in flash increases the amount of internal IOs thereby harming performance and device lifetime. In this paper, we identify a key component of the metadata called the Page Validity Bitmap (PVB) as the bottleneck. PVB is used by the garbage-collectors of state-of-the-art FTLs to keep track of which physical pages in the device are invalid. PVB constitutes 95% of the FTL's RAM-resident metadata, and recovering PVB after power fails takes a significant proportion of the overall recovery time. To solve this problem, we propose a page-associative FTL called GeckoFTL, whose central innovation is replacing PVB with a new data structure called Logarithmic Gecko. Logarithmic Gecko is similar to an LSM-tree in that it first logs updates and later reorganizes them to ensure fast and scalable access time. Relative to the baseline of storing PVB in flash, Logarithmic Gecko enables cheaper updates at the cost of slightly more expensive garbagecollection queries. We show that this is a good trade-off because (1) updates are intrinsically more frequent than garbage-collection queries to page validity metadata, and (2) flash writes are more expensive than flash reads. We demonstrate analytically and empirically through simulation that GeckoFTL achieves a 95% reduction in space requirements and at least a 51% reduction in recovery time by storing page validity metadata in flash while keeping the contribution to internal IO overheads 98% lower than the baseline.