What's the benefit of btrfs-scrub when you only have one drive?

ravas · September 3, 2019, 12:48am

btrfs scrub is used to scrub a btrfs filesystem, which will read all data and metadata blocks from all devices and verify checksums. Automatically repair corrupted blocks if there’s a correct copy available.Note**:** Scrub is not a filesystem checker (fsck) and does not verify nor repair structural damage in the filesystem.

https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-scrub

Scrub operation reads all data and metadata from the devices and verifies the checksums. It’s not mandatory, but may point out problems with faulty hardware early as it touches data that might not be in use and bitrot.

If there’s a redundancy of data/metadata, ie. the DUP or RAID1/5/6 profiles, scrub is able to repair the data automatically if there’s a good copy available.

github.com

kdave/btrfsmaintenance/blob/master/README.md

Btrfs maintenance toolbox
=========================

Table of contents:
* [Quick start](#quick-start)
* [Distro integration](#distro-integration)
* [Tuning periodic snapshotting](#tuning-periodic-snapshotting)

This is a set of scripts supplementing the btrfs filesystem and aims to automate
a few maintenance tasks. This means the *scrub*, *balance*, *trim* or
*defragmentation*.

Each of the tasks can be turned on/off and configured independently. The
default config values were selected to fit the default installation profile
with btrfs on the root filesystem.

Overall tuning of the default values should give a good balance between effects
of the tasks and low impact of other work on the system. If this does not fit
your needs, please adjust the settings.

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I understand the benefit if you have RAID data duplication, but I’m curious about the benefits when you don’t have that.

The quote above indicates it might “point out problems with faulty hardware”. Has anyone ever encountered that? What do you do in that situation – reformat or start shopping for a new drive?

I’m also wondering about the benefit of it being done automatically by btrfsmaintenance on a single drive. How am I going to be informed if it does find “problems with faulty hardware”?

gogalthorp · September 3, 2019, 3:54am

Makes installing Linux harder maybe??? Other then that I don’t know of an advantage

arvidjaar · September 3, 2019, 6:17am

Single drive can still have DUP profile and this is default for metadata. Also periodical scrub gives you early warning of data corruption. It is always better to be notified in advance and have time to decide how to handle it than to get EIO when actually working with data.

ravas · September 3, 2019, 8:01am

Thanks – I missed that; although, it looks like it’s not the default for a SSD.

A single device filesystem will default to DUP, unless a SSD is detected. Then it will default to single.

https://btrfs.wiki.kernel.org/index.php/Manpage/mkfs.btrfs

If it tells me there is some corrupt data, is the first step to check if an old snapshot is clean?

arvidjaar · September 3, 2019, 7:25pm

[QUOTE=ravas;2913200]looks like it’s not the default for a SSD.
Indeed, you are right.

If it tells me there is some corrupt data, is the first step to check if an old snapshot is clean?

I would say “yes”.