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Subject: Re: number of subvolumes
To: Linux fs Btrfs <linux-btrfs@vger.kernel.org>
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From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
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Date: Thu, 31 Aug 2017 07:18:23 -0400
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On 2017-08-31 02:49, Ulli Horlacher wrote:
> On Thu 2017-08-24 (18:45), Peter Grandi wrote:
> 
>> As usual with Btrfs, there are corner cases to avoid: 'defrag'
>> should be done before 'balance'
> 
> Good hint. So far I did it the other way: balance before defrag.
> I will switch.
For reference, the reason to do things this way is that defragmenting a 
filesystem may result in undoing some of the work balance did.
> 
> 
>> and with compression switched off
> 
> I have filesystems with compress mount option:
> 
> framstag@fex:~: grep /local /etc/fstab
> LABEL=local /local btrfs  defaults,compress,user_subvol_rm_allowed 0 2
> 
> and a weekly cronjob, which does a defrag and balance.
> I cannot disable compression.
> Any hint here?
Having compression enabled causes no issues with defray and balance. 
There appears to be a prevalent belief however that defrag is pointless 
if you're using compression, probably because some versions of 
`filefrag` don't report compressed extents properly (they list each 128k 
compressed unit as one extent, which is wrong).
> 
> 
>> I prefer dump-and-reload.
> 
> What do you mean by this?
I believe he means to copy everything off the filesystem, recreate it, 
and copy everything back in.  That will actually get you much closer to 
an optimal layout than a defrag=balance cycle, but it also takes a long 
time, requires extra space, and the layout will usually become 
sub-optimal almost immediately when you start writing to the filesystem.