From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
To: Rich Freeman <r-btrfs@gw.thefreemanclan.net>,
Duncan <1i5t5.duncan@cox.net>
Cc: Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: Why is dedup inline, not delayed (as opposed to offline)? Explain like I'm five pls.
Date: Mon, 18 Jan 2016 07:23:17 -0500 [thread overview]
Message-ID: <569CD935.4030708@gmail.com> (raw)
In-Reply-To: <CAGfcS_mepN3bcgSo6qgjVAyROoRpX3G05dAm=-g-eOhSkv+GZA@mail.gmail.com>
On 2016-01-16 13:07, Rich Freeman wrote:
> On Sat, Jan 16, 2016 at 9:10 AM, Duncan <1i5t5.duncan@cox.net> wrote:
>> Al posted on Sat, 16 Jan 2016 12:27:16 +0000 as excerpted:
>>
>>> Is there any urgency for dedup? What's wrong with storing the hash on
>>> disk with the block and having a separate process dedup the written data
>>> over time;
>>
>> There's actually uses for both inline and out-of-line[1] aka delayed
>> dedup. Btrfs already has a number of independent products doing various
>> forms of out-of-line dedup, so what's missing and being developed now is
>> the inline dedup option, which being directly in the write processing,
>> must be handled by btrfs itself -- it can't be primarily done by third
>> parties with just a few kernel calls, like out-of-line dedup can.
>
> Does the out-of-line dedup option actually utilize stored hashes, or
> is it forced to re-read all the data to compute hashes? If it is
> collecting checksums/etc is this done efficiently?
AFAIK, duperemove has the option to store block hashes in a database to
save them between runs (I'm pretty sure that it invalidates hashes if
the file containing the block changed, but I'm not certain).
>
> I think he is actually suggesting a hybrid approach where a bit of
> effort is done during operations to greatly streamline out-of-line
> deduplication. I'm not sure how close we are to that already, or if
> any room for improvement remains.
There isn't any implementation I know of that does this. In theory, it
would be pretty easy if we could somehow get block checksums from BTRFS
in userspace.
next prev parent reply other threads:[~2016-01-18 12:24 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-16 12:27 Why is dedup inline, not delayed (as opposed to offline)? Explain like I'm five pls Al
2016-01-16 14:10 ` Duncan
2016-01-16 18:07 ` Rich Freeman
2016-01-18 12:23 ` Austin S. Hemmelgarn [this message]
2016-01-23 22:22 ` Mark Fasheh
2016-01-20 14:49 ` Al
2016-01-20 14:43 ` Al
2016-01-21 8:23 ` Qu Wenruo
2016-01-21 14:53 ` Al
2016-01-21 17:23 ` Chris Murphy
2016-01-22 11:33 ` Al
2016-01-23 2:44 ` Chris Murphy
2016-02-02 2:55 ` Qu Wenruo
2016-01-18 1:36 ` Qu Wenruo
2016-01-18 3:10 ` Duncan
2016-01-18 3:16 ` Qu Wenruo
2016-01-18 3:51 ` Duncan
2016-01-18 12:48 ` Austin S. Hemmelgarn
2016-01-19 8:30 ` Duncan
2016-01-19 9:14 ` Duncan
2016-01-19 12:28 ` Austin S. Hemmelgarn
2016-01-19 15:40 ` Duncan
2016-01-20 8:32 ` Brendan Hide
2016-01-19 12:21 ` Austin S. Hemmelgarn
2016-01-20 15:12 ` Al
2016-01-20 18:21 ` Duncan
2016-01-20 14:53 ` Al
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