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
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