NVMe SSDs connected over PCI Express are widely used in performance critical systems, offering low latency and high throughput block storage accessed through Logical Block Addresses (LBAs). While NVMe provides an efficient host interface, the ultimate write performance is constrained by the internal programming characteristics of NAND flash memory.
In many FPGA-based, real-time, and hardware-accelerated systems, the NVMe host controller plays a critical role in determining how efficiently data is written to flash. When host write requests are not aligned to the SSD’s internal programming granularity, additional internal operations are triggered inside the SSD, increasing latency and reducing throughput.
The performance impact of unaligned NVMe writes, and the role of host side alignment control enabled by the iWave NVMe Host Controller IP Core, are therefore key considerations in achieving predictable and efficient NVMe storage access.
SSD Logical Addressing and Internal Write Granularity
Host systems access NVMe SSDs using Logical Block Addresses (LBAs). From the host’s perspective, the SSD appears as a linear array of logical blocks, each representing the minimum unit of data transfer supported by the interface, commonly referred to as a sector.
A read or write command always transfers an integer number of sectors starting at a specified LBA. Sector sizes are implementation-defined, with common values of 512 bytes or 4 KB.
Logical-to-Physical Translation Inside the SSD
Internally, the storage device contains a memory controller responsible for translating host visible LBAs into physical memory locations. In NAND flash-based SSDs, this translation is performed by a firmware layer commonly referred to as the Flash Translation Layer (FTL).
The FTL abstracts away the physical characteristics of flash memory, including wear levelling, bad block management, and garbage collection, while presenting a simple block addressable interface to the host.
ECC Pages and Flash Programming Units
Before host data can be programmed into flash memory, the SSD controller groups logical sectors together and adds error correction code (ECC) parity bits to ensure data reliability. This results in an ECC page (also known as a codeword).
An ECC page consists of:
- One or more host sectors
- Additional parity bits for error detection and correction
For example, an ECC page may combine form four 512-byte sectors (2 KB of host data) plus ECC overhead. The exact composition depends on the ECC strength and flash technology used
Minimum Addressable Unit
In most SSD architectures, the minimum independently programmable unit is larger than a single ECC page. Multiple ECC pages are grouped into a larger internal unit, referred to here as a fragment (or flash page).
A fragment represents the smallest unit of flash memory that can be programmed. Its size is determined by the number of ECC pages it contains. For example:
- Sector size: 512 bytes
- ECC page: 4 sectors (2 KB)
- Fragment: 2 ECC pages → 8 sectors (4 KB)
The minimum programmable(writeable) unit of an SSD is Fragment (or Page).
Definition of Aligned and Unaligned Writes
A write operation is considered aligned when:
- The starting LBA is a multiple of the number of sectors in a fragment
- The write size is an integer multiple of the fragment size (in sectors)
For example, if a fragment consists of eight sectors, then an aligned write must:
- Start at an LBA that is a multiple of eight
- Transfer 8, 16, 24, … sectors
Any write that does not satisfy both conditions is classified as an unaligned write. includes:
- Writes starting at a non-fragment boundary
- Writes smaller than a fragment
- Writes that partially overlap fragment boundaries
Below is an example of each type of unaligned writes
Read-Modify-Write Behaviour for Unaligned Writes
Aligned writes allow the SSD controller to directly assemble complete fragments and program them to flash with minimal overhead.
Unaligned writes, however, cannot be programmed directly. Because flash fragments cannot be partially updated, To handle such writes, the controller must first construct a complete fragment by combining new host data with existing data already stored in flash. This process requires a read-modify-write (RMW) sequence:
- Read
Existing flash data corresponding to the pre-pad and/or post-pad portions of the fragment is read from memory. - Modify
The preserved data is merged with the new host data, and ECC parity is recomputed for the entire fragment. - Write
The fully assembled fragment is programmed to a new physical flash location.
This additional internal activity is completely hidden from the host but directly increases latency, internal bandwidth consumption, and write amplification.
Alignment Control with iWave NVMe Host Controller IP Core
The iWave NVMe Host Controller IP Core provides transparent, LBA-based access to NVMe SSDs, allowing system designers to specify exactly which logical blocks are read or written. This access is exposed through control registers, enabling the user to program the Start LBA and Number of LBAs for each operation.
The iWave NVMe Host controller IP core is alignment aware and internally manages all write operations, whether or not the Start LBA and Number of LBAs are aligned with the SSD’s internal programming granularity. It automatically adjusts, segments, or coalesces requests as needed to ensure efficient, flash-friendly writes. As a result, write throughput is preserved regardless of user specified alignment.
Conclusion
Although NVMe provides a low latency, high-bandwidth interface, write performance is fundamentally constrained by NAND flash programming units. Unaligned host writes trigger read-modify-write operations, increasing internal data movement, bandwidth usage, and write amplification, which leads to higher latency, lower throughput, and faster flash wear.
Although SSDs use buffering and write coalescing to mitigate these effects, they cannot fully compensate for inefficient host I/O. In performance critical iWave NVMe Host Controller IP Core such as high-IOPS, real-time, or hardware-based deployments ensuring writes are aligned to the SSD’s logical blocks and internal programming boundaries is essential for predictable performance, improved endurance, and efficient flash utilization.
For more information, please visit www.iwave-global.com or reach out to us at mktg@iwave-global.com
