Re: freezes during snapshot creation/deletion -- to be expected? (Was: Re: btrfs based backup?)

[Chris Murphy <lists@colorremedies.com>]

On Wed, Nov 20, 2019 at 9:36 AM Christian Pernegger <pernegger@gmail.com> wrote:
>
> Hello,
>
> I've decided to go with a snapshot-based backup solution for our new
> Linux desktops -- thank you for the timely thread --, namely btrbk.
> A couple of subvolumes for different stuff, with hourly snapshots that
> regularly go to another machine. Brilliant in theory, less so in
> practice, because every time btrbk runs, the box'll freeze for a few
> seconds, as in, Firefox and LibreOffice, for instance, become entirely
> unresponsive, games hang and so on. (AFAICT, all it does is snapshot
> each subvolume and delete ones that are out of the retention period.)
>
> I'm aware that having many snapshots can impact performance of some
> operations, but I didn't think that "many" <= 200, "impact" = stop
> dead and "some operations" = light desktop use. These are decently
> specced, after all (Zen 2 8/12 core, 32 GB RAM, Samsung 970 Evo Plus).
> What I'm asking is, is this to be expected, does it just need tuning,
> is the hardware buggy, the kernel version (Ubuntu 18.04.3 HWE, their
> 5.0 series) a stinker, something else awry ...?


What are the mount options? And what's the workload immediate prior to
the snapshot? Or does it always happen no matter the workload?

I use Btrfs on a variety of hardware and storage devices, USB flash,
NVMe, hard drives, and a Samsung 940 EVO, and I can't say I experience
anything like a freeze or hang. If I'm doing something like updates
(dnf updates, RPM) and do a snapshot while the update is happening
(bit kooky because that snapshot represents an inbetween state of the
update, essentially useless except as an intentionally poking things
with a stick just to see what happens) I do see a user space "hang" as
a flush is required as part of the snapshot, and I see this flush
using top. But so far I only see it affect the snapshot command itself
(it's a delay rather than a hang). I don't see it affect GUI
responsiveness.

-- 
Chris Murphy