Some numbers for back-of-the-envelope usage calculations

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Some quick numbers:

  • Read sequentially from main memory: 4 GB/s (1Mb needs 0.25 ms)
  • Read sequentially from SSD: 1 GB/s (1Mb needs 1 ms)
  • Read sequentially from 1 Gbps Ethernet: 100 MB/s (1Mb needs 10 ms)
  • Read sequentially from disk: 30 MB/s (1Mb needs 30 ms)

main memory 4x > SSD
SSD 10x > 1 Gbps Ethernet
1 Gbps Ethernet 3 > disk

more statistics:

  • 6-7 world-wide round trips per second
  • 2,000 round trips per second within a data center
  • 1 RPS = 2.5 million requests per month
  • 40 RPS = 100 million requests per month
  • 400 RPS = 1 billion requests per month

Here is a detailed table:

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Latency Comparison Numbers
----------------------------------------------------------------------------------------
L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns                      14x L1 cache
Mutex lock/unlock                          100   ns
Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy            10,000   ns       10 us
Send 1 KB bytes over 1 Gbps network     10,000   ns       10 us
Read 4 KB randomly from SSD*           150,000   ns      150 us          ~1GB/sec SSD
Read 1 MB sequentially from memory     250,000   ns      250 us
Round trip within same datacenter      500,000   ns      500 us
Read 1 MB sequentially from SSD*     1,000,000   ns    1,000 us    1 ms  ~1GB/sec SSD, 4X memory
Disk seek                           10,000,000   ns   10,000 us   10 ms  20x datacenter roundtrip
Read 1 MB sequentially from 1 Gbps  10,000,000   ns   10,000 us   10 ms  40x memory, 10X SSD
Read 1 MB sequentially from disk    30,000,000   ns   30,000 us   30 ms 120x memory, 30X SSD
Send packet CA->Netherlands->CA    150,000,000   ns  150,000 us  150 ms

Notes
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1 ns = 10^-9 seconds
1 us = 10^-6 seconds = 1,000 ns
1 ms = 10^-3 seconds = 1,000 us = 1,000,000 ns

Powers of two table

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Power           Exact Value         Approx Value        Bytes
---------------------------------------------------------------
7                             128
8                             256
10                           1024   1 thousand           1 KB
16                         65,536                       64 KB
20                      1,048,576   1 million            1 MB
30                  1,073,741,824   1 billion            1 GB
32                  4,294,967,296                        4 GB
40              1,099,511,627,776   1 trillion           1 TB

Example Calculations

For example, if we are designing a financial transaction system, we need to calculate the data size of transactions:

Size per transaction:

  • user_id - 8 bytes
  • created_at - 5 bytes
  • seller - 32 bytes
  • amount - 5 bytes
    Total: ~50 bytes/transaction

If the system has 5 billion transactions each month, the new transaction data size will be:
250 GB/month = 50 bytes * 5 billion /month, which will be
9 TB of new transaction content in 3 years

Assume most are new transactions instead of updates to existing ones, then we have:
2,000 transactions per second on average
200 read requests per second on average

References

https://github.com/donnemartin/system-design-primer