From: Hugo Mills <hugo@carfax.org.uk>
To: Marat Khalili <mkh@rqc.ru>
Cc: linux-btrfs@vger.kernel.org, Henk Slager <eye1tm@gmail.com>,
Duncan <1i5t5.duncan@cox.net>,
"Austin S. Hemmelgarn" <ahferroin7@gmail.com>,
A L <crimsoncottage@gmail.com>
Subject: Re: Is autodefrag recommended? -- re-duplication???
Date: Tue, 5 Sep 2017 14:39:14 +0000 [thread overview]
Message-ID: <20170905143914.GB23980@carfax.org.uk> (raw)
In-Reply-To: <82e453bf-ff11-a911-c432-3e0173b223b2@rqc.ru>
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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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prev parent reply other threads:[~2017-09-05 14:39 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-04 9:31 Is autodefrag recommended? Marat Khalili
2017-09-04 10:23 ` Henk Slager
2017-09-04 10:34 ` Duncan
2017-09-04 11:09 ` Henk Slager
2017-09-04 22:27 ` Duncan
2017-09-04 10:54 ` Hugo Mills
2017-09-05 11:45 ` Austin S. Hemmelgarn
2017-09-05 12:49 ` Henk Slager
2017-09-05 13:00 ` Austin S. Hemmelgarn
2017-09-05 12:36 ` A L
2017-09-05 14:01 ` Is autodefrag recommended? -- re-duplication??? Marat Khalili
2017-09-05 14:39 ` Hugo Mills [this message]
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