James Courtier-Dutton wrote:
Hi,
Thank you for your suggestion.
It does not help at all.
btrfs balance's behaviour seems to be unchanged by ionice.
It still takes 100% while working and starves all other processes of
disk access.
I can I get btrfs balance to work in the background, without adversely
affecting other applications?
Are you using block multiqueue or perhaps a scheduler other than CFQ?!
I may be wrong on this , but from memory I think the only scheduler that
plays nicely (from a practical point of view) with ionice is CFQ
check the output of:
cat /sys/block/sda/queue/scheduler
Of course you need to replace sda with the appropriate block device and
the selected scheduler will be shown in [brackets].
If that does not help you can maybe reduce the time your system is
slowed down by using the usage balance filter and increasing the value
bit by bit....
https://btrfs.wiki.kernel.org/index.php/Balance_Filters
On 23 December 2017 at 11:56, Alberto Bursi <alberto.bu...@outlook.it> wrote:
On 12/23/2017 12:19 PM, James Courtier-Dutton wrote:
Hi,
During a btrfs balance, the process hogs all CPU.
Or, to be exact, any other program that wishes to use the SSD during a
btrfs balance is blocked for long periods. Long periods being more
than 5 seconds.
Is there any way to multiplex SSD access while btrfs balance is
operating, so that other applications can still access the SSD with
relatively low latency?
My guess is that btrfs is doing a transaction with a large number of
SSD blocks at a time, and thus blocking other applications.
This makes for atrocious user interactivity as well as applications
failing because they cannot access the disk in a relatively low latent
manner.
For, example, this is causing a High Definition network CCTV
application to fail.
What I would really like, is for some way to limit SSD bandwidths to
applications.
For example the CCTV app always gets the bandwidth it needs, and all
other applications can still access the SSD, but are rate limited.
This would fix my particular problem.
We have rate limiting for network applications, why not disk access also?
Kind Regards
James
On most I/O intensive programs in Linux you can use "ionice" tool to
change the disk access priority of a process. [1]
This allows me to run I/O intensive background scripts in servers
without the users noticing slowdowns or lagging, of course this means
the process doing heavy I/O will run more slowly or get outright paused
if higher-priority processes need a lot of access to the disk.
It works on btrfs balance too, see (commandline example) [2].
If you don't start the process with ionice as in [2], you can always
change the priority later if you get the get the process ID. I use iotop
[3], which also supports commandline arguments to integrate its output
in scripts.
For btrfs scrub it seems to be possible to specify the ionice options
directly, while btrfs balance does not seem to have them (would be nice
to add them imho). [4]
For the sake of completeness, there is also "nice" tool for CPU usage
priority (also used in my scripts on servers to keep the scripts from
hogging the CPU for what is just a background process, and seen in [2]
commandline too). [5]
1. http://man7.org/linux/man-pages/man1/ionice.1.html
2.
https://unix.stackexchange.com/questions/390480/nice-and-ionice-which-one-should-come-first
3. http://man7.org/linux/man-pages/man8/iotop.8.html
4. https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-scrub
5. http://man7.org/linux/man-pages/man1/nice.1.html
-Alberto
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