How hyMap minimizes write amplification and maximizes your storage systems life
hyMap® is our flash management firmware that implements enhanced Flash Translation Layer (FTL) mapping which significantly improves random write performances, reliability, and endurance.
First, a brief reminder of some of the basics of flash memory architecture relevant to this discussion. Flash memory is organized in a hierarchy of blocks, pages and then memory cells. Pages are typically between 8 KB and 16 KB, for MLC memory. Blocks can be up to 8 MB.
Data is written a page at a time. However, only whole blocks can be erased. This means that the flash controller must use smart algorithms for tracking the optimal methods of allocating blocks when pages have to be erased or rewritten.
Logical to physical address mapping
Like all flash storage systems, the controller also has to manage the mapping from the logical addresses used by the host operating system to physical addresses in the flash memory.
One simple method that has been used for a long time is block-based mapping, where there is a one-to-one mapping between addresses within logical and physical blocks. In other words, logical page 1 will always be mapped to physical page 1 in a block.
A more sophisticated method is to use page-based mapping, where logical pages are mapped to physical pages. This provides more flexibility because logical page 1 can now be mapped to any page within any block.
Sub-page mapping means that the address mapping is done at a level smaller than a page. This provides greater flexibility in the use of the memory, but is more complicated because writes still have to be done at the level of an entire page. hyMap® uses sub-page based mapping.
Write amplification and endurance
An important parameter of flash storage is the Write Amplification Factor (WAF). This is a measurement of how many more writes are made to the flash memory than actually performed by the host. The extra writes may be needed for functions such as erasing blocks before data is written or just a mismatch in the size of data written and the page size. For example, a sequential write of 4 KB of data to a flash memory with 16 KB pages will result in a WAF of 4 (because the entire page has to be written).
The address mapping used can make a big difference to the WAF. In the case of block-based mapping write amplification can be enormous, especially for small random writes. If a block has to be erased to accommodate a single sector write, this can result in a WAF of 2000 because the entire previous contents of the block have to be copied to a new block (with the addition of the new data).
On the other hand, with sub-page mapping, even if a block has to be erased because no unused block is available, only the new sector of data has to be written to the new block. The old sector of data is marked as invalid and the other data in that block remains where it was. This means that the worst-case WAF is reduced to 16.
The overall WAF obtained in practice depends on many factors, including the pattern of use. hyMap® can be compiled for different sub-page granularity and tuned to the expected usage model. So, in real-world applications with a mix of payload sizes, we would expect sub-page mapping to give an improvement of more than 30x.
The WAF is a significant factor affecting the endurance of SSDs and NAND flash based storage systems. By reducing the number of extra writes, sub-page mapping can significantly increase the lifetime of a drive.
Conclusions
hyMap® is constantly being optimized to aid the design of innovative storage systems. Optimizing the mapping strategy to the applications usage patterns results in a higher performance, lower WAF and increased endurance. This means you have lower costs, increased capacity and more competitive products.
If you want to better understand the life expectancy of your storage solution, we have a ‘Lifetime Estimation Tool’ that can give you an insight into the WAF and other aspects of your storage design. Get in touch with us today, if you want access to this tool!