From: Thomas Lamprecht <t.lamprecht@proxmox.com>
To: "Christian Ebner" <c.ebner@proxmox.com>,
"Proxmox Backup Server development discussion"
<pbs-devel@lists.proxmox.com>,
"Fabian Grünbichler" <f.gruenbichler@proxmox.com>
Subject: Re: [pbs-devel] [PATCH proxmox-backup 2/2] fix #5982: garbage collection: check atime updates are honored
Date: Tue, 18 Feb 2025 14:31:44 +0100 [thread overview]
Message-ID: <0a345432-cb61-43d3-af9c-6561f8798574@proxmox.com> (raw)
In-Reply-To: <0e1975cc-d111-4556-bb26-340604939a55@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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next prev parent reply other threads:[~2025-02-18 13:32 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-02-17 13:12 [pbs-devel] [PATCH proxmox proxmox-backup 0/2] fix #5892: check atime update is honored Christian Ebner
2025-02-17 13:12 ` [pbs-devel] [PATCH proxmox 1/2] pbs api types: Add check garbage collection atime updates flag Christian Ebner
2025-02-17 13:12 ` [pbs-devel] [PATCH proxmox-backup 2/2] fix #5982: garbage collection: check atime updates are honored Christian Ebner
2025-02-17 15:36 ` Fabian Grünbichler
2025-02-17 15:57 ` Christian Ebner
2025-02-18 11:53 ` Thomas Lamprecht
2025-02-18 12:39 ` Christian Ebner
2025-02-18 13:17 ` Christian Ebner
2025-02-18 13:31 ` Thomas Lamprecht [this message]
2025-02-18 13:38 ` Christian Ebner
2025-02-19 16:54 ` [pbs-devel] [PATCH proxmox proxmox-backup 0/2] fix #5892: check atime update is honored Christian Ebner
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