From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-bk0-f45.google.com ([209.85.214.45]:48123 "EHLO mail-bk0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758918Ab3DAQQv (ORCPT ); Mon, 1 Apr 2013 12:16:51 -0400 Received: by mail-bk0-f45.google.com with SMTP id j10so976474bkw.32 for ; Mon, 01 Apr 2013 09:16:50 -0700 (PDT) Message-ID: <5159B2F0.9060704@gmail.com> Date: Mon, 01 Apr 2013 19:16:48 +0300 From: Konstantinos Skarlatos MIME-Version: 1.0 To: Josef Bacik CC: linux-btrfs@vger.kernel.org Subject: Re: [RFC] Online dedup for Btrfs References: <20130401125034.GG1876@localhost.localdomain> <20130401153859.GJ1876@localhost.localdomain> In-Reply-To: <20130401153859.GJ1876@localhost.localdomain> Content-Type: text/plain; charset=UTF-8; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 1/4/2013 6:38 μμ, Josef Bacik wrote: > On Mon, Apr 01, 2013 at 08:50:34AM -0400, Josef Bacik wrote: >> Hello, >> >> I was bored this weekend so I hacked up online dedup for Btrfs. It's working >> quite well so I think it can be more widely tested. There are two ways to use >> it >> >> 1) Compatible mode - this is a bit slower but will handle being used by older >> kernels. We use the csum tree to find duplicate blocks. Since it is relatively >> easy to have crc32c collisions this also involves reading the block from disk >> and doing a memcmp with the block we want to write to verify it has the same >> data. This is way slow but hey, no incompat flag! >> >> 2) Incompatible mode - so this is the way you probably want to use it if you >> don't care about being able to go back to older kernels. You select your >> hashing function (at the momement I only support sha1 but there is room in the >> format to have different functions). This creates a btree indexed by the hash >> and the bytenr. Then we lookup the hash and just link the extent in if it >> matches the hash. You can use -o paranoid-dedup if you are paranoid about hash >> collisions and this will force it to do the memcmp() dance to make sure that the >> extent we are deduping really matches the extent. >> >> So performance wise obviously the compat mode sucks. It's about 50% slower on >> disk and about 20% slower on my Fusion card. We get pretty good space savings, >> about 10% in my horrible test (just copy a git tree onto the fs), but IMHO not >> worth the performance hit. >> >> The incompat mode is a bit better, only 15% drop on disk and about 10% on my >> fusion card. Closer to the crc numbers if we have -o paranoid-dedup. The space >> savings is better since it uses the original extent sizes, we get about 15% >> space savings. Please feel free to pull and try it, you can get it here >> >> git://git.kernel.org/pub/scm/linux/kernel/git/josef/btrfs-next.git dedup >> >> Thanks! >> > It's been pointed out to me that this is probably too serious, so just FYI it's > April 1st where I am. Thanks, Well I believed it too, and was writing an email with questions etc. I almost sent it, but then I saw git was downloading hundreds and hundreds of MB of data :) Well done anyway! > > Josef > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html