On 12 Jan 2018, at 13:24, Austin S. Hemmelgarn wrote:
OK, I've gotten a lot of good feedback on this, and the general
consensus seems to be:
* If we're going to recommend regular balance, we should explain how
it actually helps things.
* We should mention the performance interactions with qgroups, as well
as warning people off of running other things like scrubs or defrag
concurrently.
* The filters should be reasonably tame in terms of chunk selection.
* BTRFS should ideally get smarter about this kind of thing so the
user doesn't have to be.
To that end, I propose the following text for the FAQ:
Q: Do I need to run a balance regularly?
A: While not strictly necessary for normal operations, running a
filtered balance regularly can help prevent your filesystem from
ending up with ENOSPC issues. The following command run daily on each
BTRFS volume should be more than sufficient for most users:
`btrfs balance start -dusage=25 -dlimit=2..10 -musage=25
-mlimit=2..10`
If you are running a kernel older than version 4.4 and can't upgrade,
the following should be used instead:
`btrfs balance start -dusage=25 -musage=25`
Both of these commands will effectively compact partially full chunks
on the filesystem so that new chunks have more space to be allocated.
For more information on what the commands actually mean, check out
`man btrfs-balance`
When run regularly, both of these should complete extremely fast on
most BTRFS volumes. Note that these may run significantly slower on
volumes which have quotas enabled. Additionally, it's best to make
sure other things aren't putting a lot of load on the filesystem while
running a balance, so try to make sure this doesn't run at the same
time as a scrub or defrag.
A full, unfiltered balance (one without any options passed in) is
completely unnecessary for normal usage of a filesystem.
Hi Austin,
From the discussion we've had I have the impression there might be
another way to answer this FAQ that is as valid as this one: monitor
usage. You may never need to balance, or hardly ever.
Your suggestion for regular balance has clear advantages: It's
set-and-forget. And to apply the advice the user doesn't need to
understand the allocator, interpret usage stats, or figure out filters.
On the other hand, not needing to run balance is quite appealing. So is
avoiding yet another cron/timer job if I don't really need it.
Question about your proposed text, "When run regularly, both of these
should complete extremely fast..." Does that imply that it might not be
fast if, say, you've never run balance before on a filesystem with a lot
of unused space in allocated chunks?
Tom
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