From: Bob Williams <linux@barrowhillfarm.org.uk>
To: linux-btrfs@vger.kernel.org
Subject: Re: A question about subvolumes
Date: Fri, 04 Jul 2014 22:06:55 +0100 [thread overview]
Message-ID: <53B7176F.1020409@barrowhillfarm.org.uk> (raw)
In-Reply-To: <53B710C9.9020601@libero.it>
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On 04/07/14 21:38, Goffredo Baroncelli wrote:
> On 07/04/2014 04:38 PM, Bob Williams wrote:
>> I have a disc formatted as btrfs, on which is mounted /home.
>>
>> /home/bob is a regular directory.
>>
>> /home/bob/Documents is a btrfs subvolume
>>
>> /home is btrfs root
>>
>> If I do
>>
>> # mv /home/bob /home/bob_original # btrfs subvolume create
>> /home/bob # mv /home/bob_original/* /home/bob/ # rm
>> /home/bob_original
>>
>> will the original subvolume /home/bob/Documents survive this
>> operation, and will it now exist as a subvolume under the new
>> subvolume /home/bob?
>
> Yes.
>
> A subvolume is a way to partition a btrfs file-system. You can
> think a subvolume like a filesystem. For objects like files and
> directory a move command between *different* subvolumes is equal to
> a copy+remove. If fact is like you are moving data between
> different file-systems.
>
> Instead moving a subvolume in *its btrfs filesystem* is a cheap
> operation (is like moving a link); this is true even if you move a
> subvolume between different subvolumes.
>
> To increase the speed when you move files between subvolumes (of
> the *same* btrfs filesystem), you could do a "cp --reflink" + "rm"
> instead of a "mv"; eg
>
> # mv subvolume-A/* subvolume-B/
>
> is equal to
>
> # cp -rf subvolume-A/* subvolume-B/ # rm -rf subvolume-A/*
>
> but # cp --reflink -R subvolume-A/* subvolume-B/ # rm -rf
> subvolume-A/
>
> is faster because "cp --reflink" shared the data between source
> and destination. This means that you are doing a copy (and a
> delete) of metadata only.
>
>
Thank you, Goffredo. As the current /home/bob is not a subvolume, but
a regular linux directory/folder, will the "cp --reflink" still carry
the same speed advantage?
In other words, using your example above, will this work:
# cp --reflink -R normal_directory-A/* subvolume-B/
# rm -rf normal_directory-A/
- --
Bob Williams
System: Linux 3.11.10-17-desktop
Distro: openSUSE 13.1 (x86_64) with KDE Development Platform: 4.13.2
Uptime: 06:00am up 5 days 20:14, 3 users, load average: 0.00, 0.04, 0.08
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next prev parent reply other threads:[~2014-07-04 21:06 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-04 14:38 A question about subvolumes Bob Williams
2014-07-04 20:38 ` Goffredo Baroncelli
2014-07-04 21:06 ` Bob Williams [this message]
2014-07-05 6:27 ` Goffredo Baroncelli
2014-07-05 11:43 ` Bob Williams
2014-07-05 16:06 ` Austin S Hemmelgarn
2014-07-05 23:49 ` Chris Samuel
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