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From: Evangelos Foutras <evangelos@foutras.com>
To: Filipe Manana <fdmanana@kernel.org>, Ian Johnson <ian@ianjohnson.dev>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: Possible readdir regression with BTRFS
Date: Sat, 9 Sep 2023 14:52:19 +0300	[thread overview]
Message-ID: <00ed09b9-d60c-4605-b3b6-f4e79bf92fca@foutras.com> (raw)
In-Reply-To: <ZPweR/773V2lmf0I@debian0.Home>

Hi Filipe,

Please be aware that this bug might not be as harmless as it seems. I'm 
not sure if the fix you're preparing would also fix an issue we saw at 
Arch Linux but I thought I'd mention it here.

We have a package repository server with 4x10 TB drives in RAID10 (btrfs 
only, no mdadm). On multiple mirrors syncing from it we have seen rsync 
occasionally delete ~4 small (<10 MB) files that get frequently updated 
by renaming temporary files into them. This only happened with 6.4.12 
and went away after going back to 6.4.10 (the former had the commit Ian 
mentioned).

Unfortunately I don't have a reproducer for this. I can only describe 
what our repo-add script does and how rsync behaves during problematic 
syncs.

Our repo-add script frequently adds packages to the extra repo by doing:

   ln -f extra.db.tar.gz extra.db.tar.gz.old
   mv .tmp.extra.db.tar.gz extra.db.tar.gz

And the same for extra.files.tar.gz:

   ln -f extra.files.tar.gz extra.files.tar.gz.old
   mv .tmp.extra.files.tar.gz extra.files.tar.gz

While the server was running Linux 6.4.12, rsync on some mirrors would 
occasionally (3-4 times in the day) delete these files:

   deleting extra/os/x86_64/extra.files.tar.gz.old
   deleting extra/os/x86_64/extra.files.tar.gz
   deleting extra/os/x86_64/extra.db.tar.gz.old
   deleting extra/os/x86_64/extra.db.tar.gz

Since renames are atomic, I would expect this scenario to never happen.

Again, sorry for not being able to provide a proper reproducer like Ian; 
there is probably some timing interaction with how rsync does directory 
scanning and repo-add updating the directory entry during this time.

[1] 
https://gitlab.archlinux.org/pacman/pacman/-/blob/v6.0.2/scripts/repo-add.sh.in#L473

  reply	other threads:[~2023-09-09 11:52 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-09-09  1:07 Possible readdir regression with BTRFS Ian Johnson
2023-09-09  7:27 ` Filipe Manana
2023-09-09 11:52   ` Evangelos Foutras [this message]
2023-09-09 12:18     ` Filipe Manana
2023-09-11 11:56       ` Filipe Manana
2023-09-09 12:16   ` Filipe Manana
2023-09-09 19:27     ` Ian Johnson

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