From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
To: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>,
General Zed <general-zed@zedlx.com>
Cc: Chris Murphy <lists@colorremedies.com>,
Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: Feature requests: online backup - defrag - change RAID level
Date: Fri, 13 Sep 2019 07:04:28 -0400 [thread overview]
Message-ID: <fdec5a56-8337-4cfb-4d07-425e8785102d@gmail.com> (raw)
In-Reply-To: <20190912235427.GE22121@hungrycats.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 <lists@colorremedies.com>:
>>
>>> On Thu, Sep 12, 2019 at 3:34 PM General Zed <general-zed@zedlx.com> wrote:
>>>>
>>>>
>>>> Quoting Chris Murphy <lists@colorremedies.com>:
>>>>
>>>>> On Thu, Sep 12, 2019 at 1:18 PM <webmaster@zedlx.com> 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.
>>
>>
next prev parent reply other threads:[~2019-09-13 11:04 UTC|newest]
Thread overview: 111+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-09 2:55 Feature requests: online backup - defrag - change RAID level zedlryqc
2019-09-09 3:51 ` Qu Wenruo
2019-09-09 11:25 ` zedlryqc
2019-09-09 12:18 ` Qu Wenruo
2019-09-09 12:28 ` Qu Wenruo
2019-09-09 17:11 ` webmaster
2019-09-10 17:39 ` Andrei Borzenkov
2019-09-10 22:41 ` webmaster
2019-09-09 15:29 ` Graham Cobb
2019-09-09 17:24 ` Remi Gauvin
2019-09-09 19:26 ` webmaster
2019-09-10 19:22 ` Austin S. Hemmelgarn
2019-09-10 23:32 ` webmaster
2019-09-11 12:02 ` Austin S. Hemmelgarn
2019-09-11 16:26 ` Zygo Blaxell
2019-09-11 17:20 ` webmaster
2019-09-11 18:19 ` Austin S. Hemmelgarn
2019-09-11 20:01 ` webmaster
2019-09-11 21:42 ` Zygo Blaxell
2019-09-13 1:33 ` General Zed
2019-09-11 21:37 ` webmaster
2019-09-12 11:31 ` Austin S. Hemmelgarn
2019-09-12 19:18 ` webmaster
2019-09-12 19:44 ` Chris Murphy
2019-09-12 21:34 ` General Zed
2019-09-12 22:28 ` Chris Murphy
2019-09-12 22:57 ` General Zed
2019-09-12 23:54 ` Zygo Blaxell
2019-09-13 0:26 ` General Zed
2019-09-13 3:12 ` Zygo Blaxell
2019-09-13 5:05 ` General Zed
2019-09-14 0:56 ` Zygo Blaxell
2019-09-14 1:50 ` General Zed
2019-09-14 4:42 ` Zygo Blaxell
2019-09-14 4:53 ` Zygo Blaxell
2019-09-15 17:54 ` General Zed
2019-09-16 22:51 ` Zygo Blaxell
2019-09-17 1:03 ` General Zed
2019-09-17 1:34 ` General Zed
2019-09-17 1:44 ` Chris Murphy
2019-09-17 4:55 ` Zygo Blaxell
2019-09-17 4:19 ` Zygo Blaxell
2019-09-17 3:10 ` General Zed
2019-09-17 4:05 ` General Zed
2019-09-14 1:56 ` General Zed
2019-09-13 5:22 ` General Zed
2019-09-13 6:16 ` General Zed
2019-09-13 6:58 ` General Zed
2019-09-13 9:25 ` General Zed
2019-09-13 17:02 ` General Zed
2019-09-14 0:59 ` Zygo Blaxell
2019-09-14 1:28 ` General Zed
2019-09-14 4:28 ` Zygo Blaxell
2019-09-15 18:05 ` General Zed
2019-09-16 23:05 ` Zygo Blaxell
2019-09-13 7:51 ` General Zed
2019-09-13 11:04 ` Austin S. Hemmelgarn [this message]
2019-09-13 20:43 ` Zygo Blaxell
2019-09-14 0:20 ` General Zed
2019-09-14 18:29 ` Chris Murphy
2019-09-14 23:39 ` Zygo Blaxell
2019-09-13 11:09 ` Austin S. Hemmelgarn
2019-09-13 17:20 ` General Zed
2019-09-13 18:20 ` General Zed
2019-09-12 19:54 ` Austin S. Hemmelgarn
2019-09-12 22:21 ` General Zed
2019-09-13 11:53 ` Austin S. Hemmelgarn
2019-09-13 16:54 ` General Zed
2019-09-13 18:29 ` Austin S. Hemmelgarn
2019-09-13 19:40 ` General Zed
2019-09-14 15:10 ` Jukka Larja
2019-09-12 22:47 ` General Zed
2019-09-11 21:37 ` Zygo Blaxell
2019-09-11 23:21 ` webmaster
2019-09-12 0:10 ` Remi Gauvin
2019-09-12 3:05 ` webmaster
2019-09-12 3:30 ` Remi Gauvin
2019-09-12 3:33 ` Remi Gauvin
2019-09-12 5:19 ` Zygo Blaxell
2019-09-12 21:23 ` General Zed
2019-09-14 4:12 ` Zygo Blaxell
2019-09-16 11:42 ` General Zed
2019-09-17 0:49 ` Zygo Blaxell
2019-09-17 2:30 ` General Zed
2019-09-17 5:30 ` Zygo Blaxell
2019-09-17 10:07 ` General Zed
2019-09-17 23:40 ` Zygo Blaxell
2019-09-18 4:37 ` General Zed
2019-09-18 18:00 ` Zygo Blaxell
2019-09-10 23:58 ` webmaster
2019-09-09 23:24 ` Qu Wenruo
2019-09-09 23:25 ` webmaster
2019-09-09 16:38 ` webmaster
2019-09-09 23:44 ` Qu Wenruo
2019-09-10 0:00 ` Chris Murphy
2019-09-10 0:51 ` Qu Wenruo
2019-09-10 0:06 ` webmaster
2019-09-10 0:48 ` Qu Wenruo
2019-09-10 1:24 ` webmaster
2019-09-10 1:48 ` Qu Wenruo
2019-09-10 3:32 ` webmaster
2019-09-10 14:14 ` Nikolay Borisov
2019-09-10 22:35 ` webmaster
2019-09-11 6:40 ` Nikolay Borisov
2019-09-10 22:48 ` webmaster
2019-09-10 23:14 ` webmaster
2019-09-11 0:26 ` webmaster
2019-09-11 0:36 ` webmaster
2019-09-11 1:00 ` webmaster
2019-09-10 11:12 ` Austin S. Hemmelgarn
2019-09-09 3:12 webmaster
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