Re: On the issue of direct I/O and csum warnings

[Martin Raiber <martin@urbackup.org>]

On 23.07.2021 16:55 Jonas Aaberg wrote:
> Hi,
>
> I use btrfs on dm-crypt. About two months ago, I started to get:
>
> --
> BTRFS warning (device dm-0): csum failed root 257 ino 1068852 off
> 25690112 csum 0xa27faf9a expected csum 0x4c266278 mirror 1 BTRFS error
> (device dm-0): bdev /dev/mapper/disk0 errs: wr 0, rd 0, flush 0,
> corrupt 349, gen 0
> --
>
> kind of warning/errors on my laptop. I went a bought a new NVME disk
> because I'm rather found of my data, eventhough most is backup-ed up.
>
> A week later, I started to get the same kind of warning/error message
> on my new NVME. After half a day of memtest86, resulted in no memory
> errors found, I gave up on my otherwise stable laptop and started to
> use an old laptop that I've been to lazy to sell instead while looking
> out for a decent pre-owned newer laptop.
>
> Now I'm just about to install and move over to a newly bought laptop,
> when today my old laptop started to show the same warning/errors.
> My old laptop does not share a single part with the laptop which I
> previous got the "checksum failure" warnings on. Therefore I have a hard
> time to believe that I've gotten the same hardware failure twice.
>
> Then I found:
> <https://btrfs.wiki.kernel.org/index.php/Gotchas> and "Direct I/O and
> CRCs".
>
> Which I believe is what I've ran into. One of the affect files is
> a log file from syncthing on both computers.

I wouldn't be certain about the conclusion that it is the direct I/O csum issue. Are you sure syncthing is writing to logs via direct I/O? That would be bad e.g. because it disables btrfs compression and log files compress really well. So I'd say report additional information like kernel version (and if it is a vanilla kernel), how your btrfs is setup (metadata RAID1), etc.

> I have just one humble request, please do something about this
> checksum error message. Just add printk with a link to:
> <https://btrfs.wiki.kernel.org/index.php/Gotchas> and the issue of
> "Direct I/O and CRCs".
The problem is nothing can be done without impacting performance and direct I/O is used for performance. IMO it should be disabled by default (i.e. it just pretends to do direct I/O like ZFSOnLinux) and be able to be enabled via mount option.
>
> Maybe update the wiki with:
> `find <mountpoint> -inum <ino-number-from-warning-message>`
> would be a helpful as well.

btrfs inspect-internal inode-resolve <ino-number-from-warning-message> <fs>

is faster.