>On Thu 2017-08-31 (09:05), Ulli Horlacher wrote:
>> When I do a
>> btrfs filesystem defragment -r /directory
>> does it defragment really all files in this directory tree, even if it
>> contains subvolumes?
>> The man page does not mention subvolumes on this topic.
>
>No answer so far :-(
>
>But I found another problem in the man-page:
>
> Defragmenting with Linux kernel versions < 3.9 or >= 3.14-rc2 as well as
> with Linux stable kernel versions >= 3.10.31, >= 3.12.12 or >= 3.13.4
> will break up the ref-links of COW data (for example files copied with
> cp --reflink, snapshots or de-duplicated data). This may cause
> considerable increase of space usage depending on the broken up
> ref-links.
>
>I am running Ubuntu 16.04 with Linux kernel 4.10 and I have several
>snapshots.
>Therefore, I better should avoid calling "btrfs filesystem defragment -r"?
>
>What is the defragmenting best practice?
>Avoid it completly?
My question is the same as the OP in this thread, so I came here to
read the answers before asking. However, it turns out that I still
need to ask something. Should I ask here or start a new thread? (I'll
assume here, since the topic is the same.)
Based on the answers here, it sounds like I should not run defrag at
all. However, I have a performance problem I need to solve, so if I
don't defrag, I need to do something else.
Here's my scenario. Some months ago I built an over-the-top powerful
desktop computer / workstation and I was looking forward to really
fantastic performance improvements over my 6 year old Ubuntu machine.
I installed Arch Linux on BTRFS on the new computer (on an SSD). To my
shock, it was no faster than my old machine. I focused a lot on
Firefox performance because I use Firefox a lot and that was one of
the applications in which I was most looking forward to better
performance.
I tried everything I could think of and everything recommended to me
in various forums (except switching to Windows) and the performance
remained very disappointing.
Then today I read the following:
Gotchas - btrfs Wiki
https://btrfs.wiki.kernel.org/index.php/Gotchas
Fragmentation: Files with a lot of random writes can become
heavily fragmented (10000+ extents) causing excessive multi-second
spikes of CPU load on systems with an SSD or large amount a RAM. On
desktops this primarily affects application databases (including
Firefox). Workarounds include manually defragmenting your home
directory using btrfs fi defragment. Auto-defragment (mount option
autodefrag) should solve this problem.
Upon reading that I am wondering if fragmentation in the Firefox
profile is part of my issue. That's one thing I never tested
previously. (BTW, this system has 256 GB of RAM and 20 cores.)
Furthermore, on the same BTRFS Wiki page, it mentions the performance
penalties of many snapshots. I am keeping 30 to 50 snapshots of the
volume that contains the Firefox profile.
Would these two things be enough to turn top-of-the-line hardware into
a mediocre-preforming desktop system? (The system performs fine on
benchmarks -- it's real life usage, particularly with Firefox where it
is disappointing.)
After reading the info here, I am wondering if I should make a new
subvolume just for my Firefox profile(s) and not use COW and/or not
keep snapshots on it and mount it with the autodefrag option.
As part of this strategy, I could send snapshots to another disk using
btrfs send-receive. That way I would have the benefits of snapshots
(which are important to me), but by not keeping any snapshots on the
live subvolume I could avoid the performance problems.
What would you guys do in this situation?
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