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From: General Zed <general-zed@zedlx.com>
To: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
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 21:56:42 -0400
Message-ID: <20190913215642.Horde.MvjbFry-r1RYjoGFfEha7aE@server53.web-hosting.com> (raw)
In-Reply-To: <20190914005655.GH22121@hungrycats.org>


Quoting Zygo Blaxell <ce3g8jdj@umail.furryterror.org>:

> On Fri, Sep 13, 2019 at 01:05:52AM -0400, General Zed wrote:
>>
>> Quoting Zygo Blaxell <ce3g8jdj@umail.furryterror.org>:
>>
>> > On Thu, Sep 12, 2019 at 08:26:04PM -0400, General Zed wrote:
>> > >
>> > > Quoting Zygo Blaxell <ce3g8jdj@umail.furryterror.org>:
>> > >
>> > > > 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).
>> > >
>> > > Yes, here we are debating some worst-case scenaraio which is actually
>> > > imposible in practice due to various reasons.
>> >
>> > No, it's quite possible.  A log file written slowly on an active
>> > filesystem above a few TB will do that accidentally.  Every now and then
>> > I hit that case.  It can take several hours to do a logrotate on spinning
>> > arrays because of all the metadata fetches and updates associated with
>> > worst-case file delete.  Long enough to watch the delete happen, and
>> > even follow along in the source code.
>> >
>> > I guess if I did a proactive defrag every few hours, it might take less
>> > time to do the logrotate, but that would mean spreading out all the
>> > seeky IO load during the day instead of getting it all done at night.
>> > Logrotate does the same job as defrag in this case (replacing a file in
>> > thousands of fragments spread across the disk with a few large fragments
>> > close together), except logrotate gets better compression.
>> >
>> > To be more accurate, the example I gave above is the worst case you
>> > can expect from normal user workloads.  If I throw in some reflinks
>> > and snapshots, I can make it arbitrarily worse, until the entire disk
>> > is consumed by the metadata update of a single extent defrag.
>> >
>>
>> I can't believe I am considering this case.
>>
>> So, we have a 1TB log file "ultralog" split into 256 million 4 KB extents
>> randomly over the entire disk. We have 512 GB free RAM and 2% free disk
>> space. The file needs to be defragmented.
>>
>> In order to do that, defrag needs to be able to copy-move multiple extents
>> in one batch, and update the metadata.
>>
>> The metadata has a total of at least 256 million entries, each of some size,
>> but each one should hold at least a pointer to the extent (8 bytes) and a
>> checksum (8 bytes): In reality, it could be that there is a lot of other
>> data there per entry.
>
> It's about 48KB per 4K extent, plus a few hundred bytes on average for each
> reference.
>
>> The metadata is organized as a b-tree. Therefore, nearby nodes should
>> contain data of consecutive file extents.
>
> It's 48KB per item.  As you remove the original data extents, you will
> be touching a 16KB page in three trees for each extent that is removed:
> Free space tree, csum tree, and extent tree.  This happens after the
> merged extent is created.  It is part of the cleanup operation that
> gets rid of the original 4K extents.
>
> Because the file was written very slowly on a big filesystem, the extents
> are scattered pessimally all over the virtual address space, not packed
> close together.  If there are a few hundred extent allocations between
> each log extent, then they will all occupy separate metadata pages.
> When it is time to remove them, each of these pages must be updated.
> This can be hit in a number of places in btrfs, including overwrite
> and delete.
>
> There's also 60ish bytes per extent in any subvol trees the file
> actually appears in, but you do get locality in that one (the key is
> inode and offset, so nothing can get between them and space them apart).
> That's 12GB and change (you'll probably completely empty most of the
> updated subvol metadata pages, so we can expect maybe 5 pages to remain
> including root and interior nodes).  I haven't been unlucky enough to
> get a "natural" 12GB, but I got over 1GB a few times recently.
>
> Reflinks can be used to multiply that 12GB arbitrarily--you only get
> locality if the reflinks are consecutive in (inode, offset) space,
> so if the reflinks are scattered across subvols or files, they won't
> share pages.
>
>> The trick, in this case, is to select one part of "ultralog" which is
>> localized in the metadata, and defragment it. Repeating this step will
>> ultimately defragment the entire file.
>>
>> So, the defrag selects some part of metadata which is entirely a descendant
>> of some b-tree node not far from the bottom of b-tree. It selects it such
>> that the required update to the metadata is less than, let's say, 64 MB, and
>> simultaneously the affected "ultralog" file fragments total less han 512 MB
>> (therefore, less than 128 thousand metadata leaf entries, each pointing to a
>> 4 KB fragment). Then it finds all the file extents pointed to by that part
>> of metadata. They are consecutive (as file fragments), because we have
>> selected such part of metadata. Now the defrag can safely copy-move those
>> fragments to a new area and update the metadata.
>>
>> In order to quickly select that small part of metadata, the defrag needs a
>> metatdata cache that can hold somewhat more than 128 thousand localized
>> metadata leaf entries. That fits into 128 MB RAM definitely.
>>
>> Of course, there are many other small issues there, but this outlines the
>> general procedure.
>>
>> Problem solved?
>
> Problem missed completely.  The forward reference updates were the only
> easy part.
>
> My solution is to detect this is happening in real time, and merge the
> extents while they're still too few to be a problem.  Now you might be
> thinking "but doesn't that mean you'll merge the same data blocks over
> and over, wasting iops?" but really it's a perfectly reasonable trade
> considering the interest rates those unspent iops can collect on btrfs.
> If the target minimum extent size is 192K, you turn this 12GB problem into
> a 250MB one, and the 1GB problem that actually occurs becomes trivial.
>
> Another solution would be to get the allocator to reserve some space
> near growing files reserved for use by those files, so that the small
> fragments don't explode across the address space.  Then we'd get locality
> in all four btrees.  Other filesystems have heuristics all over their
> allocators to do things like this--btrfs seems to have a very minimal
> allocator that could stand much improvement.

Ok, a fine solution. Basically, you improve the autodefrag to detect  
this specific situation.

Another way to solve this issue is to run the on-demand defrag  
sufficiently often. You order the defrag to  defragment only that one  
specific file, or you order it to find and defrag only 0.1% of most  
fragmented files (and the pathological file should fall within those  
0.1%).

But, this is a very specific and rare case that we are talking about here.

So, that's it.




  parent reply index

Thread overview: 111+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-09  2:55 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 [this message]
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
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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