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[70.62.41.24]) by smtp.gmail.com with ESMTPSA id t199sm13147606qke.36.2019.09.13.04.04.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 13 Sep 2019 04:04:30 -0700 (PDT) Subject: Re: Feature requests: online backup - defrag - change RAID level To: Zygo Blaxell , General Zed Cc: Chris Murphy , Btrfs BTRFS References: <20190911132053.Horde._wJd24LqxxXx9ujl2r5i7PQ@server53.web-hosting.com> <20190911173725.Horde.aRGy9hKzg3scN15icIxdbco@server53.web-hosting.com> <81f4870e-3ee9-7780-13aa-918d24ca10d8@gmail.com> <20190912151841.Horde.-wdqt-14W0sbNwBxzhWVB6B@server53.web-hosting.com> <20190912173440.Horde.WmxNqLlw7nsFNa-Ux9TTgbz@server53.web-hosting.com> <20190912185726.Horde.HMciH9Z16kV4fK10AfUeRA8@server53.web-hosting.com> <20190912235427.GE22121@hungrycats.org> From: "Austin S. Hemmelgarn" Message-ID: Date: Fri, 13 Sep 2019 07:04:28 -0400 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.0 MIME-Version: 1.0 In-Reply-To: <20190912235427.GE22121@hungrycats.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 2019-09-12 19:54, Zygo Blaxell wrote: > On Thu, Sep 12, 2019 at 06:57:26PM -0400, General Zed wrote: >> >> Quoting Chris Murphy : >> >>> On Thu, Sep 12, 2019 at 3:34 PM General Zed wrote: >>>> >>>> >>>> Quoting Chris Murphy : >>>> >>>>> On Thu, Sep 12, 2019 at 1:18 PM wrote: >>>>>> >>>>>> It is normal and common for defrag operation to use some disk space >>>>>> while it is running. I estimate that a reasonable limit would be to >>>>>> use up to 1% of total partition size. So, if a partition size is 100 >>>>>> GB, the defrag can use 1 GB. Lets call this "defrag operation space". >>>>> >>>>> The simplest case of a file with no shared extents, the minimum free >>>>> space should be set to the potential maximum rewrite of the file, i.e. >>>>> 100% of the file size. Since Btrfs is COW, the entire operation must >>>>> succeed or fail, no possibility of an ambiguous in between state, and >>>>> this does apply to defragment. >>>>> >>>>> So if you're defragging a 10GiB file, you need 10GiB minimum free >>>>> space to COW those extents to a new, mostly contiguous, set of exents, >>>> >>>> False. >>>> >>>> You can defragment just 1 GB of that file, and then just write out to >>>> disk (in new extents) an entire new version of b-trees. >>>> Of course, you don't really need to do all that, as usually only a >>>> small part of the b-trees need to be updated. >>> >>> The `-l` option allows the user to choose a maximum amount to >>> defragment. Setting up a default defragment behavior that has a >>> variable outcome is not idempotent and probably not a good idea. >> >> We are talking about a future, imagined defrag. It has no -l option yet, as >> we haven't discussed it yet. >> >>> As for kernel behavior, it presumably could defragment in portions, >>> but it would have to completely update all affected metadata after >>> each e.g. 1GiB section, translating into 10 separate rewrites of file >>> metadata, all affected nodes, all the way up the tree to the super. >>> There is no such thing as metadata overwrites in Btrfs. You're >>> familiar with the wandering trees problem? >> >> No, but it doesn't matter. >> >> At worst, it just has to completely write-out "all metadata", all the way up >> to the super. It needs to be done just once, because what's the point of >> writing it 10 times over? Then, the super is updated as the final commit. > > This is kind of a silly discussion. The biggest extent possible on > btrfs is 128MB, and the incremental gains of forcing 128MB extents to > be consecutive are negligible. If you're defragging a 10GB file, you're > just going to end up doing 80 separate defrag operations. Do you have a source for this claim of a 128MB max extent size? Because everything I've seen indicates the max extent size is a full data chunk (so 1GB for the common case, potentially up to about 5GB for really big filesystems) > > 128MB is big enough you're going to be seeking in the middle of reading > an extent anyway. Once you have the file arranged in 128MB contiguous > fragments (or even a tenth of that on medium-fast spinning drives), > the job is done. > >> On my comouter the ENTIRE METADATA is 1 GB. That would be very tolerable and >> doable. > > You must have a small filesystem...mine range from 16 to 156GB, a bit too > big to fit in RAM comfortably. > > Don't forget you have to write new checksum and free space tree pages. > In the worst case, you'll need about 1GB of new metadata pages for each > 128MB you defrag (though you get to delete 99.5% of them immediately > after). > >> But that is a very bad case, because usually not much metadata has to be >> updated or written out to disk. >> >> So, there is no problem. >> >>