SSD

SSD

1. Advantages

   • higher bandwidth
   • lower latency access
     • Every block can be accessed in the same amount of time as any other
     • This alone provides a significant speed increase

2. Types of Flash Memory

   • NAND (general type)
     • uses non-volitile Floating Gate Transistors
       • creates cage and encourages electrons to migrate into or out of the cage using quantum tunneling effect
     • faster than other devices using NAND
       • how NAND chips are addressed
       • caching and computing shortcuts
     • Logical NOT A and B
   • NOR
     • Logical NOT A or B
     • Simpler in theory, but requires significant connectivity. Provides byte level addressing, which is unnecessary
     • Wires each row together and each column together so each individual bit position can be read
   • NAND (SSD Specific)
     • Connects columns of flash cells together in series, which eliminates a lot of connections
       • Eliminates ability to read and write each bit individually. Data is read and written a data page (row) at a time

3. Read/Write

   • can read/write from individual pages, but they cannot overwrite pages
     • Fresh SSD is full of 1's (no charge)
       • To avoid corruption and damage, SSDs are erased (reset to no charge) in entire blocks
         • This is done to limit damage, because clearing a cell requires higher voltages to reduce the chance of it affecting adjacent cells (tunneling inhibition)
           • Short version: Flash storage can only write to empty, freshly erased pages, where as magnetic storage can write wherever it wants and update data "in-place"
             • Because of this, SSDs get slower as they age
   • SSDs will natively defragment data
     • When new data can fit into a block that contains a stale page, the SSD will dump the entire block into cache, erase the block, then rearrange the page order while removing old pages to include new pages

4. Single Level Cell

   • Most reliable and least complex
   • Common for enterprise applications because of greater reliability
   • Each cell holds 1 bit
   • Typical write cycles?

5. Multi-Level cell

   • Store more than 1 bit per cell, Commonly 2
   • Increase in write cycles and complexity
   • Fraction of the price of a SLC drive, which makes them popular for consumer use
   • Typical Write Cycles?

6. Additional endurance problems

   • Current SSDs should last more than 5 years before any type of degredation sets in
   • Advances in process decrease the amount of residual charge cells can absorb

7. Erasure cycle

   • Done when necessary or when the SSD has free time
   • Each cycle, some charge is trapped in the dielectric layer of material that makes up the floating gate, increasing the amount of current required and time to erase
   • Because this effect becomes so great, cells eventually become worthless to store data on. The fact that SSDs stripe data (one of their tricks) across blocks compounds this problem

8. Affects of the controller

   • All controller handle management of pages and blocks
   • SSDs write to more than one flash chip at a time
     • Controller has multiple channels it can use to address different flash chips, writing in stripe like RAID
     • Manufacturers often use their own scheme for adding parity
   • Often use large amounts of DDR2 or DDR3 RAM as cache
     • Greatly decreases latency being able to tell the CPU the SSD is done writing
     • Increasing risk of corruption. Some SSDs have a capacitor to allow it to finish writing in case of power loss
   • Garbage collection
     • Process of eliminating "stale" blocks by storing good pages on a different block and erasing the first
     • TRIM is a command sent from the OS to let the SSD know what pages are taken up by deleted files and can be considered stale and cleaned
     • too frequently will shorten the life of the drive
     • Goal of Garbage Collection and Wear Leveling is to keep the write amplification of the drive in check, though both processes increase write amplification
   • Wear Leveling
     • Techniques used to keep all of the flash cells on the SSD at roughly the same use