Storage hierarchy
Cache, memory -> hard disks, SSD, Tape, Optical Disk
(读写速度，成本)
Access time
Time taken before drive is ready to transfer data
（物理设备（硬盘，内存..）在进行数据的转换前需要索引到目标位置所消耗的时间）
一般来说，
内存：纳秒级
SSD：微秒级
HDD：毫秒级
Storage device information
-
Characters of storage device：
- Capacity （bytes）
- Cost（price per byte of storage）
- Bandwidth (number of bytes that can be transferred per second; read bandwidth is not equal to write bandwidth)
- Latency(waiting time for response/delivery of data)
Basic function/operation：CRUD
Time to complete an operation depends on both bandwidth and latency
CompletionTime = Latency + Size/Bandwidth
Influence factor：
Technology（HDD or SSD）；Operation type,（read or write）；number of operations in the workload； Access pattern（sequential or random）-
Access pattern：
- Sequential: data to be accessed are located next to each other or sequentially on the device
- Random: data located randomly on the storage device
Hard Disk Drive
- One or more spinning magnetic platters
- Typically two surfaces per platter
- Disk arm positions over the radial position (tracks) where data are stored
- It swings across tracks (but do not extend/shrink)
- Data is read/written by a read/write head as platter spins
Hard disk head movement while copying files between two folders：https://www.youtube.com/watch?v=BlB49F6ExkQ
Physical characteristics：
2.5‘’ in laptops， 3.5‘’ common in desktops
rotational speed： 4,800/5,400/7,200,10,000 RPM (rotations per minute)
platter number: 5~7
current capicity: 10 TB (Western Digital)Disk organization: platter -> tracks -> sectors
Each platter consists of a number of tracks;
Each track is divided into N fixed size sectors (sector size: 4KB)
CHS (cylinder-head-sector)
Early way to address a sector （Logical Block Addressing) is more common now）
example:
# cylinders: 256
# heads: 16 (i.e., 8 platters, 2 heads/platter)
# sectors/track: 64
sector size = 4KB
capacity of the drive:
2^8 * 2^6 * 2^2* 2^10 * 2^4 = 2^30 = 1GB
overall：capacity = C * H * S * sector size
According to CHS, data can be located before transferring, then data can be transferred
T = Tseek + Trotation + Ttransfer
Tseek : Time to get the disk head on right track
Trotation :Time to wait for the right sector to rotate under the head
Ttransfer: Time to actually transfer the data
-
rotational latency: waiting for the right sector to rotate under the head
On average: about 1⁄2 of time of a full rotation
rotation.png
example:
Assume 10,000 RPM (rotations per minute)
60000 ms/ 10000 rotations = 6ms / rotation
-
seek times (For multiple tracks): waiting for the head to the right track
On average seek time is about 1/3 max seek time
seek the track.png
3.transfer time (related to transmission bandwidth)
Assume that data will be transferred: 512KB, 128 MB/sec transmission bandwidth
Transfer time: 512KB/128MB * 1000ms = 4ms
- Actual bandwidth
Actual bandwidth = amount data/ autual time
actually time = Tseek + Trotation + Ttransfer
Sector vs. Block
- Block is the smallest unit of the file system
- Sector is the smallest unit of the hard disk
- Block has 1 or more sectors
Sequential vs. Random
Sequential operation:
- May assume all sectors involved are on the same track
-- need to seek to the right track or rotate to the first sector
-- But no rotation/seeking needed afterward
Random operation: May assume all sectors are on different tracks and sectors
example: 7ms avg seek, 10,000 RPM 50 MB/sec transfer rate 4KB/block
Sequential access of 10 MB:
– Completion time = 7ms + 60*1000/10000/2 ms + 10/50 *1000 ms = 210ms
– Actual bandwidth = 10MB/210ms = 47.62 MB/s
Random access of 10 MB
– block numbers: 10*1000/4 = 2500 (assume 1 block = 1 sector)
– Completion time = 2500 * (7 + 3 + 4/50) = 25.2s
– Actual bandwidth = 10MB / 25.2s = 0.397 MB/s
Solid State Drive
- All electronic, made from flash memory
- Limited lifetime, can only write a limited number of times.
- Significantly better latency: no seek or rotational delay
- Much better performance on random (however, write has much higher latency than read )
structures of SSD
- SSD contains a number of flash memory chips
chip -> dies -> planes -> blocks -> pages (rows) -> cells
• Typically, a chip may have 1, 2, or 4 dies
• A die may have1or 2 planes
• A plane has a number of blocks
• A block has a number of pages
* A page has a number of cells
- Page is the smallest unit of data transfer between SSD and main memory
How data is stored in SSD
- Cells are made of floating-gate transistors : By applying high positive/negative voltage to control gate, electrons can be attracted to or repelled from floating gate
- State = 1, if no electrons in the floating gate
- State = 0, if there are electrons (negative charges)
– Electrons stuck there even when power is off
– So state is retained
- Data in SSD are represented by the '101010...' formats, that is the state of the eletrons
Read Operations
- Electrons on the floating gate affect the threshold voltage for the floating gate transistor to conduct
-
Higher voltage needed when gate has electrons
Read operation.png
Steps:
• Apply Vint (intermediate voltage)
• If the current is detected, gate has no electrons=> bit = 1
• If no current, gate must have electrons => bit = 0
- Page is the smallest unit that can be read (about more details, I choose to give up.)
Write and erase
Write: 1 => 0
– Apply high positive voltage (>> voltage for read) to the control gate
– Attract electrons from channel to floating gate (through quantum tunneling)
– Page is the smallest unit for writeErase: 0 => 1 (make electrons empty)
– Need to apply much higher negative voltage to the control gate
– Get rid of electrons from floating gate
– May stress surrounding cells(dangerous to do on individual pages)
– Block is the smallest unit for erase
P/E cycle (1->0->1->0...)
P: program/write;
E: erase
- what is P/E cycle?
Data are written to cells (P): cell value from 1 -> 0 – Then erased (E): 0 -> 1 -
why P/E cycle?
Every write & erase damages oxide layer surrounding the floating-gate to some extent
P/E cycle.png
latency: read < write < erase
MLC (Multi-level cell)
floating gate can hold a number of electrons to represent different states
-
SLC vs. MLC
– Less complex
– Faster
– More reliable
– Less storage
– More costly
MLC example.png
an example about the write page of SSD
