Title: Solid%20State%20Drive
1Solid State Drive
2(No Transcript)
3NAND Flash Memory
- Main storage component of Solid State Drive (SSD)
- USB Drive, cell phone, touch pad
4Solid State Drive (nowadays)
5Solid State Drive (SSD) architecture
6Comparison
7Comparison
Attribute SSD HDD
Random access time 0.1 ms 5-10 ms
Bandwidth 100-500 MB/s 100 MB/s sequential
Price/GB 0.9-2 0.1
Size Up to 2TB, 250GB common 4TB
Power consumption 5 watts Up to 20 watts
Read/write symmetry No Yes
Noise No Yes (spin, rotate)
8Characteristics of SSD
- High reliability (no moving parts)
- Small form factor
- Less noise
- Read/Write latency w.r.t. HDD
- No seek latency
- Out-of-place update
- Invalidate old version, and write new version
somewhere else - Invalidated version becomes garbage
- HDD, in-place update
- Garbage collection (GC)
- Erase at the unit of block
- LBN to PBN mapping (FTL)
- Emulate a block device
- Erase cycle limit (endurance)
- Read latency 25 us
- Write latency 0.25 ms
- Erase latency 2.5 ms
9- Advantages of NAND SSD
- Fast random read (25 us)
- Energy efficiency
- High reliability (no moving parts)
- Small form factor
- Less noise
- Widely deployed in high-end laptops
- Macbook air, ThinkPad X series, touch pad
- Increasingly deployed in enterprise environment
either as a secondary cache or main storage
10- Disadvantages of SSD
- Garbage collection (GC) problem of SSD
- Stemmed from the out-of-place update
characteristics - Update requests invalidate old version of pages
and then write new version of these pages to a
new place - Copy valid data to somewhere else (increasing
number of IOs) - Garbage collection is periodically started to
erase victim blocks and copy valid pages to the
free blocks (slow erase 10xW,100xR) - Blocks in the SSD have a limited number of erase
cycles - 100,000 for Single Level Chip (SLC), 5,000-10,000
for Multiple Level Chip (MLC), can be as low as
3,000 - May be quickly worn out in enterprise environment
- Performance is very unpredictable
- Due to unpredictable triggering of the
time-consuming GC process
11Drive read performance
12Flash Translation Layer (FTL)
- Page-mapping
- Block-mapping
- Log-block-mapping
13Flash Translation Layer (FTL)
- Page-mapping
- Mapping any Logical Block Address (LBA) to any
Physical Page Address (LPA) - Require huge RAM to store the mapping entries
- 512GB SSD requires 4GB RAM to store the mapping
table - Each mapping entry is 16 Byte, each page size is
2 KB
14Flash Translation Layer (FTL)
- Block-mapping
- Map logical address to fixed offset of the
physical address by taking module - LBNN, N is the number of pages in each block
- LBN with the same offset may conflict, high
overhead
15Flash Translation Layer (FTL)
16Issues about deployment
- Mixing read and write degrades the performance
- Bandwidth drops as more data is being written
- Garbage collection
- The performance becomes worse in enterprise
environment - Bandwidth can drop to 20 of the bandwidth of the
datasheets - Endurance
- Wear out in 23 days in enterprise environment
- MLC can only wear
17Mixed writes and reads
18Improve write performance
- OS write caching
- Flash specific file system
- Drive Write Caching
- Multiple concurrent erase blocks
- Expensive 200400 /GB
19- Flash File System
- ZFS, has be optimized
- JFFS2