Re: Using async discard by default with SSDs?

[David Sterba <dsterba@suse.cz>]

On Wed, Jul 27, 2022 at 10:50:01AM -0400, Chris Murphy wrote:
> On Tue, Jul 26, 2022, at 6:10 PM, Chris Murphy wrote:
> > On Tue, Jul 26, 2022, at 5:36 PM, David Sterba wrote:
> >> What is quickly here?
> >
> > Seconds, less than a minute, all the blocks pointed to by backup roots 
> > are empty (zeros).
> >
> >  The default timeout is set to 2 minutes and that's
> >> what I've seen when testing that on a fresh filesystem. 
> >
> > Ok I'll retest with 5.19 series. The last time I tested time to drive 
> > gc backup rootswas circa 5.8.
> 
> With 5.19, backup roots remain available for a surprisingly long time,
> definitely more than 2 minutes. It's not an exhaustive test, just a
> dozen samples over a half hour, but I never once saw zeros returned.
> Two out of those dozen, I saw the block already overwritten with a
> leaf of a newer generation and a completely different owner (e.g. csum
> tree written at the block address for the backup tree root)  - which
> would make that backup root useless.
> 
> What is a likely target kernel version to make discard=async the
> default? The merge window for 5.20 closes August 14. Is 5.21 a
> practical target?

The changes for the next merge window are supposed to be done a week or
two before it opens, but as this is a simple change I think I can
squeeze it in.