Solid state drive technology for ProLiant servers

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times greater than that of MLC flash. SLC NAND’s higher performance and better reliability are
preferred for designing solid state drives that meet the requirements of unconstrained workload
environments. But there will also be MLC-based SSDs for use in read intensive application
environments with limited write requirements.
NAND architecture
NAND flash memory arrays are organized into pages and blocks. A page is the smallest unit. Page
size can vary between different NAND implementations, but they are typically 2KB, 4KB, or 8KB.
Pages are organized into blocks, with each block typically consisting of 64 pages. We call these
units NAND blocks to differentiate them from the 512-byte logical block of the SATA/SAS interface.
SLC NAND also can be implemented in a two-plane architecture that divides the device into two
physical planes, consisting of the odd and even blocks. Two-plane flash improves NAND
performance by allowing two pages read or programmed concurrently. It also allows concurrent
erasing of two blocks. Figure 1 shows a 4GB SLC NAND architecture consisting of 2K pages with 64
pages per block. NAND architecture continues to evolve at a rapid pace, with 8K pages becoming
common and four-plane designs on the horizon.
Figure 1. Organization of NAND memory
1 Block 1 Block
2048 blocks
per plane
4096 blocks
per device
1 page = (2K + 64 bytes ECC)
= 2,112 bytes
1 block = (2k + 64 bytes) x 64 pages
= (128K +4K) bytes
1 plane = 2048 blocks
= 2,112 MB
1 device = 2,112 MB x 2 planes
= 4,224 MB
Plane of
even-numbered blocks
(0,2,4,…,4094)
Plane of
odd-numbered blocks
(1,3,5,…,4095)
2,112 bytes 2,112 bytes
NAND flash has a specific protocol for writing and retrieving information. The smallest unit that can
be read or written is a page. Unlike disk drives, pages that contain existing data cannot be directly
overwritten with new data. They are first erased. NAND memory can only be erased in entire NAND
blocks, which typically consist of either 64 or 128 pages. One of the more important tasks for any
storage device built using NAND flash is effectively managing this asymmetry of the size of writes
versus erases. Table 2 provides a list of these basic NAND operations and their execution times.