In-drive SSD compression bumps up over-provisioning capacity, reduces write amplification and extends endurance – how long the SSD lasts under warranty before wearing out.
The utility of data compression is strengthened with QLC (quad level) NAND flash drives which made their market debut in May this year. They have 33 per cent greater capacity than today’s TLC triple cell flash but endurance and performance is worse. This is because reading and writing the extra bit per cell is slower and wears out the drive more quickly.
So says Seagate which notes its Nytro 1551 TLC SSD drive compresses the data it stores.
In a presentation at last month’s 2019 Flash Memory Summit, Seagate managing technologist Erich Haratsch said the Nytro 1551 SSD’s compression algorithm is lossless and runs inline at full data rate in the drive’s flash translation layer (FTL).
The impact on read and write latencies is unquantified but low. However, less data after compression means the SSD writes less and therefore write performance improves.
A traditional FTL writes data in chunks with equal physical sizes that fit into a flash page. After compression the data chunks are varying sizes and the FTL has to write these to the media.
With random write workloads, write amplification – an SSD’s extra writes due to its internal processes – increases as over-provisioning decreases.
For example, take a 12TB SSD, with 1TB over-provisioning – space set aside for use when flash cells wear out – and write 12TB of data to it. This data is compressed by 25 per cent, with only 9TB actually written, leaving 3TB of empty space. In effect the drive’s over-provisioning capacity has risen to 4TB.
Haratsch’s take on in-drive SSD compression can be downloaded from the FMS 2019 site.
For more reading Western Digital has a useful primer, Understanding SSD Endurance.
Also, check out our article, Samsung: Drive writes per day is not where SSD endurance is at.