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

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

Am 18.02.25 um 13:39 schrieb Christian Ebner:
> On 2/18/25 12:53, Thomas Lamprecht wrote:
>> +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.
> 
> Only checking the atime update check on datastore creation is not enough 
> I think, as the backing filesystem might get remounted with changed 
> mount parameters? Or do you mean to *also* check on datastore creation 
> already to early on detect issues? Although, in my testing even with 
> `noatime` the atime update seems to be honored by the way the garbage 
> collection performs the time updates (further details see below).

yes, I meant doing that additionally to checking on GC.

> Anyways, creating the all-zero chunk and use that for the check sounds 
> like a good optimization to me, as that allows to avoid conditional 
> checking in the phase 1 of garbage collection. However, at the cost of 
> having to make sure that it is never cleaned up by phase 2...

I saw your second reply already but even without that in mind it would
be IMO fine to only use the all-zero chunk for the on-create check, as
I would not see it as a big problem if it then gets pruned during
GC, if the latter would use an actually existing chunk. But no hard
feelings here at all either way.

> Regarding the 24 hour waiting period, as mentioned above I noted that 
> atime updates are honored even if I set the `noatime` for an ext4 or 
> `atime=off` on zfs.
> Seems like the utimensat() bypasses this directly, as it calls into 
> vfs_utimes() [0], which sets this to be an explicit time update, 
> followed by the notify_change() [1], which then calls the setattr() of 
> the corresponding filesystem [2] via the given inode.
> This seems to bypass the atime_needs_update() [3], only called by 
> touch_atime(). The atime_needs_update() also checks the 
> relatime_needs_update() [4].
> 
> Although not conclusive (yet).
> 

Yeah, that would support making this opt-in. FWIW, we could maybe "sell"
this as sort of feature by not just transforming it into a boolean
"24h-wait period <true|false>" option but rather a more generic
"wait-period <X hours>" option that defaults to 0 hours (or maybe a
few minutes if we want to support minute granularity). Not sure if there
are enough (real world) use cases to warrant this, so mostly mentioned
for the sake of completeness.


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