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Current Sittuation

ATS currently supports SSDs, however it does not take the best advantage
of their special characteristics. In particular, SSDs could be used as
an addition level in the cache (between RAM and traditional platter-based
hard drives (HDDs)), and SSDs could benefit from reducing writes. Currently
we write all misses to disk but we could write only (for example) on
seeing the second miss to a particular URL.

Use Cases

  • large working set, few HDDs, low RAM reverse proxy
  • large working set, many HDDs, probably forward proxy, but perhaps shard CDN
  • no moving parts forward or reverse proxy, 0 HDDs

NOTE: situations with a small working set and adequate RAM fall will not benefit
performance-wise from an SSD, however because of the relatively limited number
of write cycles supported by SSDs currently, specialized SSD support might improve
longevity and reliability.

Features

  • support for re-writing warm content to an SSD if read from spinning media
    • this is uses the SSD as an additional level in the hierarchy between RAM and HDD
  • support for only writing the second miss in an SSD-only environment
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