linux-btrfs.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Josef Bacik <jbacik@fb.com>
To: Mitch Harder <mitch.harder@sabayonlinux.org>,
	Brendan Hide <brendan@swiftspirit.co.za>
Cc: Martin <m_btrfs@ml1.co.uk>, linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: What to do about snapshot-aware defrag
Date: Mon, 2 Jun 2014 09:19:21 -0400	[thread overview]
Message-ID: <538C79D9.5040305@fb.com> (raw)
In-Reply-To: <CAKcLGm_Ou57upZUvUJT0RH3trVAH3DgWJp2k38q9P6vAkgjwKQ@mail.gmail.com>

On 06/01/2014 11:07 PM, Mitch Harder wrote:
> On Sat, May 31, 2014 at 6:51 PM, Brendan Hide <brendan@swiftspirit.co.za> wrote:
>> On 2014/05/31 12:00 AM, Martin wrote:
>>>
>>> OK... I'll jump in...
>>>
>>> On 30/05/14 21:43, Josef Bacik wrote:
>>>>
>>>> [snip]
>>>>
>>>> Option 1: Only relink inodes that haven't changed since the snapshot was
>>>> taken.
>>>>
>>>> Pros:
>>>> -Faster
>>>> -Simpler
>>>> -Less duplicated code, uses existing functions for tricky operations so
>>>> less likely to introduce weird bugs.
>>>>
>>>> Cons:
>>>> -Could possibly lost some of the snapshot-awareness of the defrag.  If
>>>> you just touch a file we would not do the relinking and you'd end up
>>>> with twice the space usage.
>>>
>>> [...]
>>>
>>>
>>> Obvious way to go for fast KISS.
>>
>>
>> I second this - KISS is better.
>>
>> Would in-band dedupe resolve the issue with losing the "snapshot-awareness
>> of the defrag"? I figure that if someone absolutely wants everything deduped
>> efficiently they'd put in the necessary resources (memory/dedicated SSD/etc)
>> to have in-band dedupe work well.
>>
>>> One question:
>>>
>>> Will option one mean that we always need to mount with noatime or
>>> read-only to allow snapshot defragging to do anything?
>>
>>
>
> When snapshot-aware defrag first came out, I was convinced it was a
> "must-have" capability for nearly everybody using btrfs.  But, the
> more I look at my work load and common practices with btrfs, the more
> I am wondering just how often snapshot-aware defrag was actually doing
> something for me.
>
> I use a lot of snapshots.  But for the most part, once I touch a file
> in my current subvolume, the whole file needs to be COW-ed from it's
> previous version.
>

The whole file doesn't need to be cow'ed, just the part that you touch. 
  So old snapshot-aware defrag probably would have saved you quite a bit 
of space.  Thanks,

Josef

  reply	other threads:[~2014-06-02 13:19 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-05-30 20:43 What to do about snapshot-aware defrag Josef Bacik
2014-05-30 22:00 ` Martin
2014-05-31 23:51   ` Brendan Hide
2014-06-01  1:52     ` Duncan
2014-06-02  3:07     ` Mitch Harder
2014-06-02 13:19       ` Josef Bacik [this message]
2014-06-02 13:22   ` Josef Bacik
2014-06-03 23:54     ` Martin
2014-06-04  9:19       ` Erkki Seppala
2014-06-04 13:15         ` Martin
2014-06-04 19:33           ` Chris Murphy

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=538C79D9.5040305@fb.com \
    --to=jbacik@fb.com \
    --cc=brendan@swiftspirit.co.za \
    --cc=linux-btrfs@vger.kernel.org \
    --cc=m_btrfs@ml1.co.uk \
    --cc=mitch.harder@sabayonlinux.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).