On Tue, Sep 05, 2017 at 05:01:10PM +0300, Marat Khalili wrote: > Dear experts, > > At first reaction to just switching autodefrag on was positive, but > mentions of re-duplication are very scary. Main use of BTRFS here is > backup snapshots, so re-duplication would be disastrous. > > In order to stick to concrete example, let there be two files, 4KB > and 4GB in size, referenced in read-only snapshots 100 times each, > and some 4KB of both files are rewritten each night and then another > snapshot is created (let's ignore snapshots deletion here). AFAIU > 8KB of additional space (+metadata) will be allocated each night > without autodefrag. With autodefrag will it be perhaps 4KB+128KB or > something much worse? I'm going for 132 KiB (4+128). Of course, if there's two 4 KiB writes close together, then there's less overhead, as they'll share the range. Hugo. -- Hugo Mills | Once is happenstance; twice is coincidence; three hugo@... carfax.org.uk | times is enemy action. http://carfax.org.uk/ | PGP: E2AB1DE4 |
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