Re: btrfs balance problems

[Nikolay Borisov <n.borisov.lkml@gmail.com>]

On 23.12.2017 13:19, 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?

So how are you running btrfs balance? Are you using any filters
whatsoever? The documentation
[https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-balance] has the
following warning:

Warning: running balance without filters will take a lot of time as it
basically rewrites the entire filesystem and needs to update all block
pointers.


> 
> Kind Regards
> 
> James
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