From: Qu Wenruo <quwenruo.btrfs@gmx.com>
To: Atemu <atemu.main@gmail.com>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: BUG: btrfs send: Kernel's memory usage rises until OOM kernel panic after sending ~37GiB
Date: Sun, 27 Oct 2019 21:43:00 +0800 [thread overview]
Message-ID: <944da320-f5ff-9290-27a1-e39aed7d6d21@gmx.com> (raw)
In-Reply-To: <CAE4GHg=-K3JdhvQpTC8fPGBm1sVLDOUW+UkBCSZJwz27fkW90A@mail.gmail.com>
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On 2019/10/27 下午8:55, Atemu wrote:
>> This depends on how shared one file extent is.
>
> But shouldn't it catch that and cancel the btrfs send before it panics
> the kernel due to its memory usage?
Backref walk is quite tricky in btrfs, we don't really have a good way
to detect whether it's a good idea or not, until we crash...
But at least, we have some plan to fix it, hopefully sooner than later.
>
>> If one file extent is shared 10,000 times for one subvolume, and you
>> have 1000 snapshots of that subvolume, it will really go crazy.
>
>> But as I mentioned, snapshot is another catalyst for such problem.
>
> I only have two snapshots of the subvolume but some the extents might
> very well be shared many many times.
>
>> I can't say for 100% sure. We need more info on that.
>
> Sure.
>
>> That's trim (or discard), not hole punching.
>
> I didn't mean discarding the btrfs to the underlying storage, I meant
> mounting the filesystems in the image files sitting inside the btrfs
> through a loop device and running fstrim on them.
> The loop device should punch holes into the underlying image files
> when it receives a discard, right?
That's correctly, that will punch holes for *unused* space.
But still, all 0 extents are still considered used, thus won't really work.
Since deduperemover has already skipped all 0 extents, it should be a
big problem I guess?
Thanks,
Qu
>
>> Normally hole punching is done by ioctl fpunch(). Not sure if dupremove
>> does that too.
>
> Duperemove doesn't punch holes afaik it can only ignore the 0 pages
> and not dedup them.>
>> Extent tree dump can provide per-subvolume level view of how shared one
>> extent is.
>
>> It's really hard to determine, you could try the following command to
>> determine:
>> # btrfs ins dump-tree -t extent --bfs /dev/nvme/btrfs |\
>> grep "(.*_ITEM.*)" | awk '{print $4" "$5" "$6" size "$10}'
>>
>> Then which key is the most shown one and its size.
>>
>> If a key's objectid (the first value) shows up multiple times, it's a
>> kinda heavily shared extent.
>>
>> Then search that objectid in the full extent tree dump, to find out how
>> it's shared.
>
> Thanks, I'll try that out when I can unmount the btrfs.
>
>> You can see it's already complex...
>
> That's not an issue, I'm fluent in bash ;)
>
> - Atemu
>
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next prev parent reply other threads:[~2019-10-27 13:43 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-26 17:46 BUG: btrfs send: Kernel's memory usage rises until OOM kernel panic after sending ~37GiB Atemu
2019-10-27 0:50 ` Qu Wenruo
2019-10-27 10:33 ` Atemu
2019-10-27 11:34 ` Qu Wenruo
2019-10-27 12:55 ` Atemu
2019-10-27 13:43 ` Qu Wenruo [this message]
2019-10-27 15:19 ` Atemu
2019-10-27 15:19 ` Atemu
2019-10-27 23:16 ` Qu Wenruo
2019-10-28 12:26 ` Atemu
2019-10-28 11:30 ` Filipe Manana
2019-10-28 12:36 ` Qu Wenruo
2019-10-28 12:43 ` Filipe Manana
2019-10-28 14:58 ` Martin Raiber
2019-10-28 12:44 ` Atemu
2019-10-28 13:01 ` Filipe Manana
2019-10-28 13:44 ` Atemu
2019-10-31 13:55 ` Atemu
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