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SSDs
(Solid State Drives) (Experimental Examinations (Do reads and writes…
SSDs
(Solid State Drives)
CHEN, Feng; KOUFATY, David A.; ZHANG, Xiaodong. Understanding intrinsic characteristics and system implications of flash memory based solid state drives. In: ACM SIGMETRICS Performance Evaluation Review. ACM, 2009. p. 181-192.
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Flash Memory
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Package
One or two dies
multiple planes
- K's (2048)of blocks
- 1 or 2 page sizepage size registers
(as an I/O buffer).
block with 64-128 pages
- page has 2KB or 4KB data part
- metadata area (e.g. 128 bytes) for storing
Error Correcting Code (ECC)
3 majors operations:
read, write, and erase
Requirement: a block must be erased before being programmed (written). An erase operation can take as long as 3.5ms and must be conducted in block granularity (erase block).
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SSD Internals
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In a reading, data is first read from flash memory to the register on the plane
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SSDs: SSD-L, SSD-M, and SSD-H
(Low-end, Middle-class, High-end)
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4 workloads
- Random Read
- Random Write
- Sequential Read
- Sequential Write.
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- SSDs is better, or comparable, to HDDs
- Higherend SSDs show better performance,
especially for Random Write
- Experimental Examinations
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Writes on SSD
Low-end SSDs strong correlated with access patterns. High-end SSDs with disk-cache are nearly insensitive to access patterns.
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Internal Fragmentation
SLC better than MLC. Rarely happen in practice. Slow down writes into SSD to create more opportunity to cluster consecutive small writes.
Comparing with HDD
SSD not necessarily win in all categories. SSDs better in Random workloads. HDD with simplified management and repeatable performance. SSDs with performance uncertainties and dynamics.
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