Week 6 - Solid State Drives SSDs Flashcards
What are SSDs
Solid State Drives contain no moving parts. They have no mechanical processes. They use integrated circuits to store information.
This means they are quicker and quieter but more expensive so the cababilites are not yet as large.
There are no moving parts to ‘speed’ is measured in data transfer time.
HDD: 50 to 120 MB per second
SSD: 200 to 500 MB per second
SSD Structure
2 important components CONTROLLER CHIP (x1) and MEMORY CHIPS (mulltiple)
The MEMORY chips are usually nand flash memory.
NAND FLASH MEMORY:
- NON VOLATILE (retains stored data even when powere is switched off).
- BLOCK BASED. Means it is organised into blocks each with multiple pages (page is the smallest unit that can be read or written to - varies in size). Writing and erasing operations are performedd at block level.
- has high density storage and fast speeds.
The CONTROLLER CHIP manages the data flow between the NAND flash memory and the host system.
It handles tasks such as WEAR LEVELLING , ERROR COTRRECTION, GARBAGE COLLECTION and optimizing data placement for performance. Has a Flash Translation Layer FTL. This is an abstrction layer that makes the SSD look like a HDD to the OS. It hides things like the wear levelling and garbage collection.
WEAR LEVELLING Uses algorithms to write and erase cycles evenly across the memory cells to increase life span by preventing certain cells from wearing out premeturely.
Limitations of NAND FLASH MEMORY
- unable to write over original data until the whole block is cleared*.
- It writes in pages but deletes in blocks. So for data to be overwritten the entire block must first be cleared.
- When data has previously been stored the GARBAGE COLLECTION process prepares the nand flash memory cells for new data.
So if there is allocated pages in the block that you need to clear, then you move these to a NEW BLOCK and delete the whole block that is now not needed. This is garbage collection.
- exception is when the data to be overwritten has all the same zero values, it can overwrite all the ones to zeros (can’t do it the other way around)
How is data stored
BLOCKS : - Physical memory blocks and virtual memory blocks.
Physical memory blocks have different sizes in bytes or kb.
Virtual memory blocks are made of one or more physical blocks. Have a LOGICAL BLOCK NUMBER (LBN).
PAGES: Physical pages and virtual pages. Physical pages are the smallest area data can be written to or read from.
Virtual pages consist of several physical pages and have Logical Page Numbers LPNs.
The PAGE STRUCTURE has DATA AREA and SPARE AREA.
The DATA AREA is a LOGICAL SECTOR 512 bytes in size.
Forensic Problems with SSDs
The drive controller often wipes unused info so we can’t find files in unallocated space. Also because the controller moves data around the physical drive it may not be stored in sequential sectors like HDD, so even if data is there is may be hard to find.
