LMS scan performance

This thread motivated me to do some performance tests for full scans.

Setup: Linux 3.11, ext4 file system, up-to-date LMS 7.8, existing DBs (library.db 130MB, artwork.db 900MB), ~70k audio tracks, 85% mp3 / 15% flac, ~600GB

Scenario 1: music and DB on the same disk

- scan takes 02:38:33
- wiping the existing library DB took ~5 minutes, audio file discovery starts at 19:46
- nearly all of the time, it's IO bound. The only period the CPU is relevant is during the playlist scan (starts at 21:32) and SmartMix track export (starts at 21:35), both of which have lots of DB queries
- even during artwork pre-caching (starts at 21:43), IO remains the bottleneck (one custom pre-caching specification)

Scenario 2: music and DB on different disks

- scan takes 01:16:12
- again, nearly all of the time, it's IO bound


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