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Scrub detects and (when using the DUP or RAID1/10/5/6 profiles)
corrects errors introduced into the filesystem by failures in the
underlying disks.  You can run scrub as much or as little as you want,
but the longer you go between scrubs, the longer errors can accumulate
undetected, and the greater the risk that you'll have uncorrected errors
on multiple disks when one of your disks fails and needs to be replaced.
In that event, you will lose data or even the entire filesystem.

The ideal frequency for scrubs depends in part on how important your data
is.  If it's very important that you detect storage failures immediately,
you can run scrub once a day.  If the data is very unimportant--e.g. you
have good backups, and you don't care about extended downtime to restore
them--then you might not need to run scrub at all.

I run alternating SMART long self-tests and btrfs scrubs every 15 days
(i.e. SMART long self-tests every 30 days, btrfs scrub 15 days after
every SMART long self-test).

Note that after a power failure or unclean shutdown, you should run
scrub as soon as possible after rebooting, regardless of the normal
maintenance schedule.  This is especially important for RAID5/6 profiles
to regenerate parity blocks that may have been damaged by the parity
raid write hole issue.  A post-power-failure scrub can also detect some
drive firmware bugs.

Pay attention to the output of scrub, especially the per-device statistics
(btrfs scrub status -d and btrfs dev stat).  Errors reported here will
indicate which disks should be replaced in RAID arrays.

2.  Watch the amount of "unallocated" space on the filesystem.  If the
"unallocated" space (shown in 'btrfs fi usage') drops below 1GB, you are
at risk of running out of metadata space.  If you run out of metadata
space, btrfs becomes read-only and it can be difficult to recover.

To free some unused data space (convert it from "data" to "unallocated"):

	btrfs balance start -dlimit=5 /path/to/fs

This usually doesn't need to be done more than once per day, but it
depends on how busy your filesystem is.  If you have hundreds of GB of
unallocated space then you won't need to do this at all.

Never balance metadata (i.e. the -m option of btrfs balance) unless you
are converting to a different RAID profile (e.g. -mconvert=raid1,soft).
If there is sufficient metadata space allocated, then the filesystem
can be filled with data without any problems.  Balancing metadata can
reduce the amount of space allocated for metadata, then the filesystem
will be at risk of going read-only if it fills up.


Normally there will be some overallocation of metadata (roughly 3:2
allocated:used ratio).  Leave it alone--if the filesystem allocated
metadata space in the past, the filesystem may need it again in the
future.

Scrub and balancing are the main requirements.  Filesystems can operate
with just those two maintenance actions for years (outlasting all of
their original hardware), and recover from multiple disk failures (one
at a time) along the way.

(extract from a post of Zygo Blaxell on linux-btrfs@vger.kernel.org 3 july 2010 6:37AM)