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 [thread overview]
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.
next prev parent reply other threads:[~2019-09-14 1:58 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 [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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190913215642.Horde.MvjbFry-r1RYjoGFfEha7aE@server53.web-hosting.com \
--to=general-zed@zedlx.com \
--cc=ce3g8jdj@umail.furryterror.org \
--cc=linux-btrfs@vger.kernel.org \
--cc=lists@colorremedies.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).