[systemd-devel] Slow startup of systemd-journal on BTRFS

[Goffredo Baroncelli]

Hi all,

I would like to share a my experience about a slowness of systemd when used on BTRFS.

My boot time was very high (about ~50 seconds); most of time it was due to NetworkManager which took about 30-40 seconds to start (this data came from "systemd-analyze plot").

I make several attempts to address this issue. Also I noticed that sometime this problem disappeared; but I was never able to understand why.

However this link

	https://bugzilla.redhat.com/show_bug.cgi?id=1006386

suggested me that the problem could be due to a bad interaction between systemd and btrfs. NetworkManager was innocent. 
It seems that systemd-journal create a very hight fragmented files when it stores its log. And BTRFS it is know to behave slowly when a file is highly fragmented.
This had caused a slow startup of systemd-journal, which in turn had blocked the services which depend by the loggin system.

In fact after I de-fragmented the files under /var/log/journal [*], my boot time decreased of about 20second (from 50s to 30s).

Unfortunately I don't have any data to show. The next time I will try to collect more information. But I am quite sure that when the log are highly fragmented systemd-journal becomes very slow on BTRFS.

I don't know if the problem is more on the systemd side or btrfs side. What I know is that both the projects likely will be important in the near futures, and both must work well together.

I know that I can "chattr +C" to avoid COW for some files; but I don't want to lost also the checksum protection. 

If someone is able to suggest me how FRAGMENT the log file, I can try to collect more scientific data.


BR
G.Baroncelli

[*] 
# btrfs fi defrag /var/log/journal/*/*



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