[pbs-devel] [PATCH proxmox-backup 2/2] fix #5982: garbage collection: check atime updates are honored

[Thomas Lamprecht]

Am 17.02.25 um 16:36 schrieb Fabian Grünbichler:
> On February 17, 2025 2:12 pm, Christian Ebner wrote:
>> Check if the filesystem the chunk store is located on actually
>> updates the atime when performing the marking of the chunks in
>> phase 1 of the garbage collection. Since it is not enough to check if
>> a single/first chunks atime is updated, since the filesystem can be
>> mounted via the `relatime` option, find the first chunk which is'
>> outside the relatime's 24 hour cutoff window and check the update on
>> that chunk only.
> 
> given that our touching should punch through relatime (and does so on
> all filesystems we tested so far), couldn't we just
> 
> - stat the first chunk
> - touch the first chunk
> - check if timestamps have been updated
> - print a warning about the filesystem being potentially broken, and
> - if the option is enabled, suggest the user report the details to us
> - only continue if the option is explicitly disabled
> 
> that way we should get a real world survey of broken file systems that
> could inform our decision to drop the 24h window (or keep it).. if we
> introduce an option (defaulting to yes for now) conditionalizing the 24h
> window, we could even tell users with semi-broken storages (see below)
> to explicitly set that option in case we later switch the default,
> although I am not sure whether such storages exist for real.

+1; one (additional) option _might_  be to trigger suck a check on
datastore creation, e.g. create the all-zero chunk and then do that
test. As of now that probably would not win us much, but if we make
the 24h-wait opt-in then users would be warned early enough, or we
could even auto-set that option in such a case.