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From: Duncan <1i5t5.duncan@cox.net>
To: linux-btrfs@vger.kernel.org
Subject: Re: btrfs space used issue
Date: Sat, 3 Mar 2018 06:59:26 +0000 (UTC)	[thread overview]
Message-ID: <pan$bee0f$57d3b021$cd11cf8a$d3c1d93d@cox.net> (raw)
In-Reply-To: CAFmraXhRtUh-HPV4vpeE3-0B=vCeK40b2HfsOk_boSFu+EjQhg@mail.gmail.com

vinayak hegde posted on Thu, 01 Mar 2018 14:56:46 +0530 as excerpted:

> This will happen over and over again until we have completely
> overwritten the original extent, at which point your space usage will go
> back down to ~302g.We split big extents with cow, so unless you've got
> lots of space to spare or are going to use nodatacow you should probably
> not pre-allocate virt images

Indeed.  Preallocation with COW doesn't make the sense it does on an 
overwrite-in-place filesystem.  Either nocow it and take the penalties 
that brings[1], or configure your app not to preallocate in the first 
place[2].

---
[1] On btrfs, nocow implies no checksumming or transparent compression, 
either.  Also, the nocow attribute needs to be set on the empty file, 
with the easiest way to do that being to set it on the parent directory 
before file creation, so it's inherited by any newly created files/
subdirs within it.

[2] Many apps that preallocate by default have an option to turn 
preallocation off.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman


  parent reply	other threads:[~2018-03-03  7:01 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-27 13:09 btrfs space used issue vinayak hegde
2018-02-27 13:54 ` Austin S. Hemmelgarn
2018-02-28  6:01   ` vinayak hegde
2018-02-28 15:22     ` Andrei Borzenkov
2018-03-01  9:26       ` vinayak hegde
2018-03-01 10:18         ` Andrei Borzenkov
2018-03-01 12:25           ` Austin S. Hemmelgarn
2018-03-03  6:59         ` Duncan [this message]
2018-03-05 15:28           ` Christoph Hellwig
2018-03-05 16:17             ` Austin S. Hemmelgarn
2018-02-28 19:09 ` Duncan
2018-02-28 19:24   ` Austin S. Hemmelgarn
2018-02-28 19:54     ` Duncan
2018-02-28 20:15       ` Austin S. Hemmelgarn

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