Re: Changing the state of data checksums in a running cluster

Hi Daniel,

Thanks for rebasing the patch and submitting it again!

On 7/3/24 08:41, Daniel Gustafsson wrote:
> After some off-list discussion about the desirability of this feature, where
> several hackers opined that it's something that we should have, I've decided to
> rebase this patch and submit it one more time. There are several (long)
> threads covering the history of this patch [0][1], related work stemming from
> this [2] as well as earlier attempts and discussions [3][4]. Below I try to
> respond to a summary of points raised in those threads.
>
> The mechanics of the patch hasn't changed since the last posted version, it has
> mainly been polished slightly. A high-level overview of the processing is:
> It's using a launcher/worker model where the launcher will spawn a worker per
> database which will traverse all pages and dirty them in order to calculate and
> set the checksum on them. During this inprogress state all backends calculated
> and write checksums but don't verify them on read. Once all pages have been
> checksummed the state of the cluster will switch over to "on" synchronized
> across all backends with a procsignalbarrier. At this point checksums are
> verified and processing is equal to checksums having been enabled initdb. When
> a user disables checksums the cluster enters a state where all backends still
> write checksums until all backends have acknowledged that they have stopped
> verifying checksums (again using a procsignalbarrier). At this point the
> cluster switches to "off" and checksums are neither written nor verified. In
> case the cluster is restarted, voluntarily or via a crash, processing will have
> to be restarted (more on that further down).
>
> The user facing controls for this are two SQL level functions, for enabling and
> disabling. The existing data_checksums GUC remains but is expanded with more
> possible states (with on/off retained).
>
>
> Complaints against earlier versions
> ===================================
> Seasoned hackers might remember that this patch has been on -hackers before.
> There has been a lot of review, and AFAICT all specific comments have been
> addressed. There are however a few larger more generic complaints:
>
> * Restartability - the initial version of the patch did not support stateful
> restarts, a shutdown performed (or crash) before checksums were enabled would
> result in a need to start over from the beginning. This was deemed the safe
> orchestration method. The lack of this feature was seen as serious drawback,
> so it was added. Subsequent review instead found the patch to be too
> complicated with a too large featureset. I thihk there is merit to both of
> these arguments: being able to restart is a great feature; and being able to
> reason about the correctness of a smaller patch is also great. As of this
> submission I have removed the ability to restart to keep the scope of the patch
> small (which is where the previous version was, which received no review after
> the removal). The way I prefer to frame this is to first add scaffolding and
> infrastructure (this patch) and leave refinements and add-on features
> (restartability, but also others like parallel workers, optimizing rare cases,
> etc) for follow-up patches.
>

I 100% support this approach.

Sure, I'd like to have a restartable tool, but clearly that didn't go
particularly well, and we still have nothing to enable checksums online.
That doesn't seem to benefit anyone - to me it seems reasonable to get
the non-restartable tool in, and then maybe later someone can improve
this to make it restartable. Thanks to the earlier work we know it's
doable, even if it was too complex.

This way it's at least possible to enable checksums online with some
additional care (e.g. to make sure no one restarts the cluster etc.).
I'd bet for vast majority of systems this will work just fine. Huge
systems with some occasional / forced restarts may not be able to make
this work - but then again, that's no worse than now.

> * Complexity - it was brought up that this is a very complex patch for a niche
> feature, and there is a lot of truth to that. It is inherently complex to
> change a pg_control level state of a running cluster. There might be ways to
> make the current patch less complex, while not sacrificing stability, and if so
> that would be great. A lot of of the complexity came from being able to
> restart processing, and that's not removed for this version, but it's clearly
> not close to a one-line-diff even without it.
>

I'd push back on this a little bit - the patch looks like this:

50 files changed, 2691 insertions(+), 48 deletions(-)

and if we ignore the docs / perl tests, then the two parts that stand
out are

src/backend/access/transam/xlog.c | 455 +++++-
src/backend/postmaster/datachecksumsworker.c | 1353 +++++++++++++++++

I don't think the worker code is exceptionally complex. Yes, it's not
trivial, but a lot of the 1353 inserts is comments (which is good) or
generic infrastructure to start the worker etc.

> Other complaints were addressed, in part by the invention of procsignalbarriers
> which makes this synchronization possible. In re-reading the threads I might
> have missed something which is still left open, and if so I do apologize for
> that.
>
>
> Open TODO items:
> ================
> * Immediate checkpoints - the code is currently using CHECKPOINT_IMMEDIATE in
> order to be able to run the tests in a timely manner on it. This is overly
> aggressive and dialling it back while still being able to run fast tests is a
> TODO. Not sure what the best option is there.
>

Why not to add a parameter to pg_enable_data_checksums(), specifying
whether to do immediate checkpoint or wait for the next one? AFAIK
that's what we do in pg_backup_start, for example.

> * Monitoring - an insightful off-list reviewer asked how the current progress
> of the operation is monitored. So far I've been using pg_stat_activity but I
> don't disagree that it's not a very sharp tool for this. Maybe we need a
> specific function or view or something? There clearly needs to be a way for a
> user to query state and progress of a transition.
>

Yeah, I think a view like pg_stat_progress_checksums would work.

> * Throttling - right now the patch uses the vacuum access strategy, with the
> same cost options as vacuum, in order to implement throttling. This is in part
> due to the patch starting out modelled around autovacuum as a worker, but it
> may not be the right match for throttling checksums.
>

IMHO it's reasonable to reuse the vacuum throttling. Even if it's not
perfect, it does not seem great to invent something new and end up with
two different ways to throttle stuff.

> * Naming - the in-between states when data checksums are enabled or disabled
> are called inprogress-on and inprogress-off. The reason for this is simply
> that early on there were only three states: inprogress, on and off, and the
> process of disabling wasn't labeled with a state. When this transition state
> was added it seemed like a good idea to tack the end-goal onto the transition.
> These state names make the code easily greppable but might not be the most
> obvious choices for anything user facing. Is "Enabling" and "Disabling" better
> terms to use (across the board or just user facing) or should we stick to the
> current?
>

I think the naming is fine. In the worst case we can rename that later,
seems more like a detail.

> There are ways in which this processing can be optimized to achieve better
> performance, but in order to keep goalposts in sight and patchsize down they
> are left as future work.
>

+1

regards

--
Tomas Vondra
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