Online enabling of page level checksums

On 1/22/17 5:13 AM, Magnus Hagander wrote:

If the system is interrupted before the background worker is done, it
starts over from the beginning. Previously touched blocks will be read
and verified, but not written (because their checksum is already
correct). This will take time, but not re-generate the WAL.

Another option would be to store a watermark of the largest block ID
that had been verified for each relation/fork. That would be used to
determine if checksums could be trusted, and more usefully would allow
you to start where you left off if the verification process got interrupted.

I think the actual functions and background worker could go in an
extension that's installed and loaded only by those who need it. But the
core functionality of being able to have "checksum in progress" would
have to be in the core codebase.

If it was in contrib I think that'd be fine.
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So as mentioned on IRC, I have a patch that I am working to rebase on HEAD with the following design. It is very similar to what you have proposed, so maybe we can use my development notes as a jumping-off point for discussion/refinement.

* Incremental Checksums

PostgreSQL users should have a way up upgrading their cluster to use data checksums without having to do a costly pg_dump/pg_restore; in particular, checksums should be able to be enabled/disabled at will, with the database enforcing the logic of whether the pages considered for a given database are valid.

Considered approaches for this are having additional flags to pg_upgrade to set up the new cluster to use checksums where they did not before (or optionally turning these off). This approach is a nice tool to have, but in order to be able to support this process in a manner which has the database online while the database is going throught the initial checksum process.

In order to support the idea of incremental checksums, this design adds the following things:

** pg_control:

Keep "data_checksum_version", but have it indicate *only* the algorithm version for checksums. i.e., it's no longer used for the data_checksum enabled/disabled state.

Add "data_checksum_state", an enum with multiple states: "disabled", "enabling", "enforcing" (perhaps "revalidating" too; something to indicate that we are reprocessing a database that purports to have been completely checksummed already)

An explanation of the states as well as the behavior of the checksums for each.

- disabled => not in a checksum cycle; no read validation, no checksums written. This is the current behavior for Postgres *without* checksums.

- enabling => in a checksum cycle; no read validation, write checksums. Any page that gets written to disk will be a valid checksum. This is required when transitioning a cluster which has never had checksums, as the page reads would normally fail since they are uninitialized.

- enforcing => not in a checksum cycle; read validation, write checksums. This is the current behavior of Postgres *with* checksums.

(caveat: I'm not certain the following state is needed (and the current version of this patch doesn't have it)):

- revalidating => in a checksum cycle; read validation, write checksums. The difference between this and "enabling" is that we care if page reads fail, since by definition they should have been validly checksummed, as we should verify this.

Add "data_checksum_cycle", a counter that gets incremented with every checksum cycle change. This is used as a flag to verify when new checksum actions take place, for instance if we wanted to upgrade/change the checksum algorithm, or if we just want to support periodic checksum validation.

This variable will be compared against new values in the system tables to keep track of which relations still need to be checksummed in the cluster.

** pg_database:

Add a field "datlastchecksum" which will be the last checksum cycle which has completed for all relations in that database.

** pg_class:

Add a field "rellastchecksum" which stores the last successful checksum cycle for each relation.

** The checksum bgworker:

When the enabling event is initiated, we will iterate over all databases, checking for all databases where the "datlastchecksum" field is < the current checksum cycle. For each of these, we will spawn a bgworker to connect to these dbs and iterate over pg_class looking for "rellastchecksum < data_checksum_cycle". If it finds none (i.e., every record has rellastchecksum == data_checksum_cycle) then it marks the containing database as up-to-date by updating "datlastchecksum = data_checksum_cycle". We can presumably skip over temporary and unlogged relations here.

For any relation that it finds in the database which is not checksummed, it starts an actual worker to handle the checksum process for this table. Since the state of the cluster is already either "enforcing" or "revalidating", any block writes will get checksums added automatically, so the only thing the bgworker needs to do is load each block in the relation and explicitly mark as dirty (unless that's not required for FlushBuffer() to do its thing). After every block in the relation is visited this way and checksummed, its pg_class record will have "rellastchecksum" updated.

(XXX: how to handle databases where connections are disabled, like "template1"?)

When all database have "datlastchecksum" == data_checksum_cycle, we initiate checksumming of any global cluster heap files. When the global cluster tables heap files have been checksummed, then we consider the checksum cycle complete, change pg_control's "data_checksum_state" to "enforcing" and consider things fully up-to-date.

** Function API:

Interface to the functionality will be via the following Utility functions:

- pg_enable_checksums(void) => turn checksums on for a cluster. Will error if the state is anything but "disabled". If this is the first time this cluster has run this, this will initialize ControlFile->data_checksum_version to the preferred built-in algorithm (since there's only one currently, we just set it to 1). This increments the ControlFile->data_checksum_cycle variable, then sets the state to "enabling", which means that the next time the bgworker checks if there is anything to do it will see that state, scan all the databases' "datlastchecksum" fields, and start kicking off the bgworker processes to handle the checksumming of the actual relation files.

- pg_disable_checksums(void) => turn checksums off for a cluster. Sets the state to "disabled", which means bg_worker will not do anything.

- pg_request_checksum_cycle(void) => if checksums are "enabled", increment the data_checksum_cycle counter and set the state to "enabling". (Alterantely, if we use the "revalidate" state here we could ensure that existing checksums are validated on read to alert us of any blocks with problems. This could also be made to be "smart" i.e., interrupt existing running checksum cycle to kick off another one (not sure of the use case), effectively call pg_enable_checksums() if the cluster has not been explictly enabled before, etc; depends on how pedantic we want to be.

** Design notes/implications:

When the system is in one of the modes which write checksums (currently everything but "disabled") any new relations/databases will have their "rellastchecksum"/"datlastchecksum" counters prepopulated with the current value of "data_checksum_counter", as we know that any space used for these relations will be checksummed, and hence valid. By pre-setting this, we remove the need for the checksum bgworker to explicitly visit these new relations and force checksums which will already be valid.

With checksums on, we know any full-heap-modifying operations will be properly checksummed, we may be able to pre-set rellastchecksum for other operations such as ALTER TABLEs which trigger a full rewrite *without* having to explicitly have the checksum bgworker run on this. I suspect there are a number of other places which may lend themselves to optimization like this to avoid having to process relations explicitly. (Say, if we somehow were able to force a checksum operation on any full SeqScan and update the state after the fact, we'd avoid paying this penalty another time.)

** pg_upgrade:

With this additional complexity, we need to consider pg_upgrade, both now and in future versions. For one thing, we need to transfer settings from pg_control, plus make sure that pg_upgrade accepts deviances in any of the data_checksum-related settings in pg_control.

4 scenarios to consider if/what to allow:

*** non-checksummed -> non-checksummed

exactly as it stands now

*** checksummed -> non-checksummed

pretty trivial; since the system tables will be non-checksummed, just equivalent to resetting the checksum_cycle and pg_control fields; user data files will be copied or linked into place with the checksums, but since it is disbled they will be ignored.

*** non-checksummed -> checksummed

For the major version this patch makes it into, this will likely be the primary use case; add an --enable-checksums option to `pg_upgrade` to initially set the new cluster state to the checksums_enabling state, pre-init the system databases with the correct state and checksum cycle flag, or have checksums be the *default* option. Either way, this needs to be set in the initial system initialization.

*** checksummed -> checksummed

The potentially tricky case (but likely to be more common going forward as incremental checksums are supported).

Since we may have had checksum cycles in process in the old cluster or otherwise had the checksum counter we need to do the following:

- need to propagate data_checksum_state, data_checksum_cycle, and data_checksum_version. If we wanted to support a different CRC algorithm, we could pre-set the data_checksum_version to a different version here, increment data_checksum_cycle, and set data_checksum_state to either "enabling" or "revalidating", depending on the original state from the old cluster. (i.e., if we were in the middle of an initial checksum cycle (state == "enabling").

- new cluster's system tables may need to have the "rellastchecksum" and "datlastchecksum" settings saved from the previous system, if that's easy, to avoid a fresh checksum run if there is no need.

** Handling checksums on a standby:

How to handle checksums on a standby is a bit trickier since checksums are inherently a local cluster state and not WAL logged but we are storing state in the system tables for each database we need to make sure that the replicas reflect truthful state for the checksums for the cluster.

In order to manage this discrepency, we WAL log a few additional pieces of information; specifically:

- new events to capture/propogate any of the pg_control fields, such as: checksum version data, checksum cycle increases, enabling/disabling actions

- checksum background worker block ranges. (XXX: we could decide that we're okay with relations being all or nothing here, rendering this point moot.)

Some notes on the block ranges: This would effectively be a series of records containing (datid, relid, start block, end block) for explicit checksum ranges, generated by the checksum bgworker as it checksums individual relations. This could be broken up into a series of blocks so rather than having the granularity be by relation we could have these records get generated periodicaly (say in groups of 10K blocks or whatever, number to be determined) to allow standby checksum recalculation to be incremental so as not to delay replay unnecessarily as checksums are being created.

Since the block range WAL records will be replayed before any of the pg_class/pg_database catalog records are replay, we'll be guaranteed to have the checksums calculated on the standby by the time it appears valid due to system state.

We may also be able to use the WAL records to speed up the processing of existing heap files if they are interrupted for some reason, this remains to be seen.

** Testing changes:

We need to add separate initdb checksum regression test which are outside of the normal pg_regress framework.

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On 23 January 2017 at 16:32, David Christensen <david@endpoint.com> wrote:

** Handling checksums on a standby:

How to handle checksums on a standby is a bit trickier since checksums are inherently a local cluster state and not WAL logged but we are storing state in the system tables for each database we need to make sure that the replicas reflect truthful state for the checksums for the cluster.

Not WAL logged? If the objective of this feature is robustness, it
will need to be WAL logged.

Relation options aren't accessed by the startup process during
recovery, so that part won't work in recovery. Perhaps disable
checksumming until the everything is enabled rather than relation by
relation.

If y'all serious about this then you're pretty late in the cycle for
such a huge/critical patch. So lets think of ways of reducing the
complexity...

Seems most sensible to add the "enable checksums for table" function
into VACUUM because then it will happen automatically via autovacuum,
or you could force the issue using something like
vacuumdb --jobs 4 --enable-checksums
That gives you vacuum_delay and a background worker for no effort

Hope that helps

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On Jan 23, 2017, at 10:53 AM, Simon Riggs <simon@2ndquadrant.com> wrote:

On 23 January 2017 at 16:32, David Christensen <david@endpoint.com> wrote:

** Handling checksums on a standby:

How to handle checksums on a standby is a bit trickier since checksums are inherently a local cluster state and not WAL logged but we are storing state in the system tables for each database we need to make sure that the replicas reflect truthful state for the checksums for the cluster.

Not WAL logged? If the objective of this feature is robustness, it
will need to be WAL logged.

Relation options aren't accessed by the startup process during
recovery, so that part won't work in recovery. Perhaps disable
checksumming until the everything is enabled rather than relation by
relation.

If y'all serious about this then you're pretty late in the cycle for
such a huge/critical patch. So lets think of ways of reducing the
complexity...

Seems most sensible to add the "enable checksums for table" function
into VACUUM because then it will happen automatically via autovacuum,
or you could force the issue using something like
vacuumdb --jobs 4 --enable-checksums
That gives you vacuum_delay and a background worker for no effort

I’m not sure that this will work as-is, unless we’re forcing VACUUM to ignore the visibility map. I had originally considered having this sit on top of VACUUm though, we just need a way to iterate over all relations and read every page.

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On Jan 22, 2017 11:13 AM, "Magnus Hagander" <magnus@hagander.net> wrote:

Yes, this means the entire db will end up in the transaction log since
everything is rewritten. That's not great, but for a lot of people that
will be a trade they're willing to make since it's a one-time thing. Yes,
this background process might take days or weeks - that's OK as long as it
happens online.

I'm not sure that's actually necessary. You could just log a wal record
saying "checksum this block" and if it gets replayed then recalculate the
checksum on that block again. This record could be exempt from the usual
rules for having a fpw.

There's no danger of torn pages from the checksum alone. The danger would
be if some other operation does dirty that page then your need to know that
the page is in this weird in between state where it's dirty but not yet had
a fpw written.

I'm not sure whether it's worth the infrastructure to have such a state
just for this or not. On the other hand it sounds like something that would
be useful.

On Jan 23, 2017, at 10:59 AM, David Christensen <david@endpoint.com> wrote:

On Jan 23, 2017, at 10:53 AM, Simon Riggs <simon@2ndquadrant.com> wrote:

On 23 January 2017 at 16:32, David Christensen <david@endpoint.com> wrote:

** Handling checksums on a standby:

How to handle checksums on a standby is a bit trickier since checksums are inherently a local cluster state and not WAL logged but we are storing state in the system tables for each database we need to make sure that the replicas reflect truthful state for the checksums for the cluster.

Not WAL logged? If the objective of this feature is robustness, it
will need to be WAL logged.

Relation options aren't accessed by the startup process during
recovery, so that part won't work in recovery. Perhaps disable
checksumming until the everything is enabled rather than relation by
relation.

If y'all serious about this then you're pretty late in the cycle for
such a huge/critical patch. So lets think of ways of reducing the
complexity...

Seems most sensible to add the "enable checksums for table" function
into VACUUM because then it will happen automatically via autovacuum,
or you could force the issue using something like
vacuumdb --jobs 4 --enable-checksums
That gives you vacuum_delay and a background worker for no effort

I’m not sure that this will work as-is, unless we’re forcing VACUUM to ignore the visibility map. I had originally considered having this sit on top of VACUUm though, we just need a way to iterate over all relations and read every page.

Another issue with this (that I think would still exist with the bgworker approach) is how to handle databases with datallowconn = 0. `vacuumdb`, at least, explicitly filters out these rows when iterating over databases to connect to, so while we could enable them for all databases, we can’t enable for the cluster without verifying that these disallowed dbs are already checksummed.
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On 1/23/17 1:11 PM, David Christensen wrote:

I’m not sure that this will work as-is, unless we’re forcing VACUUM to ignore the visibility map. I had originally considered having this sit on top of VACUUm though, we just need a way to iterate over all relations and read every page.

Another issue with this (that I think would still exist with the bgworker approach) is how to handle databases with datallowconn = 0. `vacuumdb`, at least, explicitly filters out these rows when iterating over databases to connect to, so while we could enable them for all databases, we can’t enable for the cluster without verifying that these disallowed dbs are already checksummed.

For a first pass, I think it's acceptable for autovac and vac to notice
if a relation needs checksums computed and ignore the VM in that case
(make sure it's ignoring all frozen bits too).

Likewise, for right now I think it's OK to force users that are enabling
this to manually connect to datallowcon=false and run vacuum.
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Jim Nasby <Jim.Nasby@BlueTreble.com> writes:

For a first pass, I think it's acceptable for autovac and vac to notice
if a relation needs checksums computed and ignore the VM in that case
(make sure it's ignoring all frozen bits too).

Likewise, for right now I think it's OK to force users that are enabling
this to manually connect to datallowcon=false and run vacuum.

I think it's a *complete* mistake to overload vacuum with this
responsibility. Build a separate tool with a separate scheduling policy.

As one reason why not: vacuum doesn't generally attempt to scan indexes
sequentially at all. Some of the index AMs might happen to do that, but
touching every page of an index is nowhere in vacuum's charter. Nor is
there a good reason for index AMs to be involved in the job of placing
checksums, but they'd all have to know about this if you insist on going
through vacuum.

regards, tom lane

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On 1/23/17 12:03 PM, Greg Stark wrote:

On Jan 22, 2017 11:13 AM, "Magnus Hagander" <magnus@hagander.net
<mailto:magnus@hagander.net>> wrote:

Yes, this means the entire db will end up in the transaction log
since everything is rewritten. That's not great, but for a lot of
people that will be a trade they're willing to make since it's a
one-time thing. Yes, this background process might take days or
weeks - that's OK as long as it happens online.

I'm not sure that's actually necessary. You could just log a wal record
saying "checksum this block" and if it gets replayed then recalculate
the checksum on that block again. This record could be exempt from the
usual rules for having a fpw.

There's no danger of torn pages from the checksum alone. The danger
would be if some other operation does dirty that page then your need to
know that the page is in this weird in between state where it's dirty
but not yet had a fpw written.

I'm not sure whether it's worth the infrastructure to have such a state
just for this or not. On the other hand it sounds like something that
would be useful.

I'm a bit concerned about how much fancy we're trying to put into a
first pass at this.

I think it's reasonable to require a fairly significant amount of effort
on the part of an admin to enable checksums. For that matter, I think
it'd be a significant improvement if there was NO way to enable
checksums, only disable them. That would at least allow us to enable
them by default in initdb and provide DBAs an easy way to disable them
if desired. That would get us a lot more data about whether checksums
help with corruption than we're going to get otherwise.

To put it another way, I think it's entirely reasonable *from a
technical standpoint* to enable by default in 10, with only the ability
to dynamically disable. Given the concerns that keep popping up about
dynamically enabling, I'm not at all sure that we could get that into 10.
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On 1/23/17 9:45 PM, Jim Nasby wrote:

To put it another way, I think it's entirely reasonable *from a
technical standpoint* to enable by default in 10, with only the ability
to dynamically disable. Given the concerns that keep popping up about
dynamically enabling, I'm not at all sure that we could get that into 10.

+1. I think this is a reasonable suggestion and realistic to complete
for inclusion in 10.

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