On 2018-07-17 13:54, Martin Steigerwald wrote:
Nikolay Borisov - 17.07.18, 10:16:
On 17.07.2018 11:02, Martin Steigerwald wrote:
Nikolay Borisov - 17.07.18, 09:20:
On 16.07.2018 23:58, Wolf wrote:
Greetings,
I would like to ask what what is healthy amount of free space to
keep on each device for btrfs to be happy?
This is how my disk array currently looks like
[root@dennas ~]# btrfs fi usage /raid
Overall:
Device size: 29.11TiB
Device allocated: 21.26TiB
Device unallocated: 7.85TiB
Device missing: 0.00B
Used: 21.18TiB
Free (estimated): 3.96TiB (min: 3.96TiB)
Data ratio: 2.00
Metadata ratio: 2.00
Global reserve: 512.00MiB (used: 0.00B)
[…]
Btrfs does quite good job of evenly using space on all devices.
No,
how low can I let that go? In other words, with how much space
free/unallocated remaining space should I consider adding new
disk?
Btrfs will start running into problems when you run out of
unallocated space. So the best advice will be monitor your device
unallocated, once it gets really low - like 2-3 gb I will suggest
you run balance which will try to free up unallocated space by
rewriting data more compactly into sparsely populated block
groups. If after running balance you haven't really freed any
space then you should consider adding a new drive and running
balance to even out the spread of data/metadata.
What are these issues exactly?
For example if you have plenty of data space but your metadata is full
then you will be getting ENOSPC.
Of that one I am aware.
This just did not happen so far.
I did not yet add it explicitly to the training slides, but I just make
myself a note to do that.
Anything else?
If you're doing a training presentation, it may be worth mentioning that
preallocation with fallocate() does not behave the same on BTRFS as it
does on other filesystems. For example, the following sequence of commands:
fallocate -l X ./tmp
dd if=/dev/zero of=./tmp bs=1 count=X
Will always work on ext4, XFS, and most other filesystems, for any value
of X between zero and just below the total amount of free space on the
filesystem. On BTRFS though, it will reliably fail with ENOSPC for
values of X that are greater than _half_ of the total amount of free
space on the filesystem (actually, greater than just short of half). In
essence, preallocating space does not prevent COW semantics for the
first write unless the file is marked NOCOW.
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