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System Design Basic 3 - Estimation

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Estimation

Power of 2

  1. A byte is a sequence of 8 bits.

  2. An ASCII character uses only 1 byte(8 bits) memory.

  3. Data volume

    Power Approximate value Full name Short name
    10 1 Thousand 1 Kilobyte 1 KB
    20 1 Million 1 Megabyte 1 MB
    30 1 Billion 1 Gigabyte 1 GB
    40 1 Trillion 1 Terabyte 1 TB
    50 1 Quadrillion 1 Petabyte 1 PB

Latency numbers every programmer should know

  1. Some normal operation cost in 2010. So may be slow now.
Operation name Time
L1 cache reference 0.5 ns
Branch mispredict 5 ns
L2 cache reference 7 ns
Mutex lock/unlock 100 ns
Main memory reference 100 ns
Compress 1K bytes with Zippy 10,000 ns = 10 µs
Send 2K bytes over 1 Gbps network 20,000 ns = 20 µs
Read 1 MB sequentially from memory 250,000 ns = 250 µs
Round trip within the same datacenter 500,000 ns = 500 µs
Disk seek 10,000,000 ns = 10 ms
Read 1 MB sequentially from the network 10,000,000 ns = 10 ms
Read 1 MB sequentially from disk 30,000,000 ns = 30 ms
Send packet CA (California) ->Netherlands->CA 150,000,000 ns = 150 ms
  1. ns = nanosecond, µs = microsecond, ms = millisecond

1 ns = 10^-9 seconds

1 µs= 10^-6 seconds = 1,000 ns

1 ms = 10^-3 seconds = 1,000 µs = 1,000,000 ns

  • Memory is fast but the disk is slow
  • Avoid disk seeks
  • Compress data before sending them through network

  • It takes time to send data between different data centers.

  • Visualization

    https://colin-scott.github.io/personal_website/research/interactive_latency.html

    image-20230308153034830

Availability numbers

  1. SLA: an agreement between service provider and customers, defines the server uptime you should deliver.

  2. The more nines, the better

    Availability % Downtime per day Downtime per week Downtime per month Downtime per year
    99% 14.40 minutes 1.68 hours 7.31 hours 3.65 days
    99.99% 8.64 seconds 1.01 minutes 4.38 minutes 52.60 minutes
    99.999% 864.00 6.05 seconds 26.30 seconds 5.26 minutes
    99.9999% 86.40 milliseconds 604.80 2.63 seconds 31.56 seconds

Example：Estimate Twitter QPS and storage requirements

  1. Assumptions:
    • 300 million monthly active users.
    • 50% of users use Twitter daily.
    • Users post 2 tweets per day on average.
    • 10% of tweets contain media.
    • Data is stored for 5 years.
  2. Estimations:
    • Query per seconds QPS estimate:
      • DAU(daily active user): 300 million * 50% = 150 million
      • Tweets QPS: 150 million * 2 tweets / 24hours/3600s = \~3500
      • PeekQPS = 2 * QPS = \~7000
    • Storage estimate:
      • Average tweet size:
        • Tweet_id 64 bytes
        • Text 140 bytes
      • Media 1MB
        • 150 million * 2 *10% * 1MB = 30TB
        • 5-year media storage: 30 TB * 365 *5 = \~55PB

Tips

  1. Rounding and approximation
  2. Write down your assumption
  3. Label your units
  4. QPS, peak QPS, storage, cache, number of servers, etc. are commonly asked.
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