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2.7 KiB
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57 lines
2.7 KiB
Plaintext
---
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title = "Memory and Swap usage explained"
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github_url = "https://github.com/orgs/supabase/discussions/21460"
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date_created = "2024-02-22T19:01:27+00:00"
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topics = [ "database", "platform" ]
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keywords = [ "memory", "swap", "ram", "disk", "IO" ]
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database_id = "43b62743-252c-4069-a719-0c0737839fd9"
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---
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## Understanding swap and memory usage
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- Swap is part of Linux's tiered memory system
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- Serves as a backup memory space when RAM is limited.
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- System prioritizes keeping actively used data in RAM
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- Data is temporarily cached in memory for quick access
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- Swap is used for less frequently accessed or non-critical data
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- Default swappiness value of 60 indicates the system's preference to fill up swap for performance benefits
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## Compute instance configuration
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- Default swappiness: 60 (out of 100)
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- Determines how aggressively the system moves data from RAM to swap
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- Default value in Linux distributions
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- Changing the swappiness value significantly impacts system behavior, as it is a non-linear value. A small adjustment of 10 can result in a very different system behavior
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- Swap provision: 1024MB on every compute instance
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## Issues with high swap usage
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- When swap reaches its limits, the system faces decisions:
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1. Evict cached memory from swap, which affects disk performance
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2. Evict cached memory from the system's memory cache, leading to increased disk accesses
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3. Access the disk directly without caching, resulting in more frequent disk reads
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- Swap usage reaching limits indicates running out of free memory
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## Disk balance and AWS limits
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- Each project has a baseline 'balance replenishment rate'
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- The replenishment rate is higher on greater AWS instances
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- Disk IO usage below replenishment rate keeps balance at 99%
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- Disk IO usage above replenishment rate decreases balance
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- Balance increases when IO usage is below replenishment rate
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- AWS does not export relevant metrics for tracking IO usage
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## Monitoring recommendations
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- Monitor memory usage:
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- If usage is over 85% and swap usage is over 90% for an extended period, optimize database access or add more resources
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- Monitor CPU IOWait usage and disk IO metrics:
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- Exported metrics here: **https://github.com/supabase/grafana-agent-fly-example/blob/main/metrics.md**
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- Key metrics to track excessive disk IO usage include:
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- `node_disk_reads_completed_total`: Tracks the total number of completed disk reads.
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- `node_disk_io_time_seconds_total`: Measures the total time spent on disk IO operations.
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- `node_disk_io_now`: Indicates the current disk IO operations.
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- Differentiate system and data disks using the 'device' label
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- System disk: `{device="nvme0n1"}` (contains swap)
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- Data disk: `{device="nvme1n1"}` (dedicated to Postgres data)
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