# Disk Caching Design [![Slack](https://slack.min.io/slack?type=svg)](https://slack.min.io)

This document explains some basic assumptions and design approach, limits of the disk caching feature. If you're looking to get started with disk cache, we suggest you go through the [getting started document](https://github.com/minio/minio/blob/master/docs/disk-caching/README.md) first.

## Command-line

```
minio server -h
...
...
  CACHE:
     MINIO_CACHE_DRIVES: List of mounted cache drives or directories delimited by ";"
     MINIO_CACHE_EXCLUDE: List of cache exclusion patterns delimited by ";"
     MINIO_CACHE_EXPIRY: Cache expiry duration in days
     MINIO_CACHE_MAXUSE: Maximum permitted usage of the cache in percentage (0-100).
...
...

  7. Start minio server with edge caching enabled on '/mnt/drive1', '/mnt/drive2' and '/mnt/export1 ... /mnt/export24',
     exclude all objects under 'mybucket', exclude all objects with '.pdf' as extension
     with expiry up to 40 days.
     $ export MINIO_CACHE_DRIVES="/mnt/drive1;/mnt/drive2;/mnt/export{1..24}"
     $ export MINIO_CACHE_EXCLUDE="mybucket/*;*.pdf"
     $ export MINIO_CACHE_EXPIRY=40
     $ export MINIO_CACHE_MAXUSE=80
     $ minio server /home/shared
```

## Assumptions
- Disk cache size defaults to 80% of your drive capacity.
- The cache drives are required to be a filesystem mount point with [`atime`](http://kerolasa.github.io/filetimes.html) support to be enabled on the drive. Alternatively writable directories with atime support can be specified in MINIO_CACHE_DRIVES
- Expiration of each cached entry takes user provided expiry as a hint, and defaults to 90 days if not provided.
- Garbage collection sweep of the expired cache entries happens whenever cache usage is > 80% of drive capacity, GC continues until sufficient disk space is reclaimed.
- An object is only cached when drive has sufficient disk space.

## Behavior
Disk caching caches objects for both **uploaded** and **downloaded** objects i.e

- Caches new objects for entries not found in cache while downloading. Otherwise serves from the cache.
- Caches all successfully uploaded objects. Replaces existing cached entry of the same object if needed.
- When an object is deleted, corresponding entry in cache if any is deleted as well.
- Cache continues to work for read-only operations such as GET, HEAD when backend is offline.
- Cache disallows write operations when backend is offline.

> NOTE: Expiration happens automatically based on the configured interval as explained above, frequently accessed objects stay alive in cache for a significantly longer time.

### Crash Recovery
Upon restart of minio server after a running minio process is killed or crashes, disk caching resumes automatically. The garbage collection cycle resumes and any previously cached entries are served from cache.

## Limits
- Bucket policies are not cached, so anonymous operations are not supported when backend is offline.
- Objects are distributed using deterministic hashing among the list of configured cache drives. If one or more drives go offline, or cache drive configuration is altered in any way, performance may degrade to a linear lookup time depending on the number of disks in cache.

