DESCRIPTION: This PR removes the task dependencies between shard moves
for which the shards belong to different colocation groups. This change
results in scheduling multiple tasks in the RUNNABLE state. Therefore it
is possible that the background task monitor can run them concurrently.
Previously, all the shard moves planned in a rebalance operation took
dependency on each other sequentially.
For instance, given the following table and shards
colocation group 1 colocation group 2
table1 table2 table3 table4 table 5
shard11 shard21 shard31 shard41 shard51
shard12 shard22 shard32 shard42 shard52
if the rebalancer planner returned the below set of moves
` {move(shard11), move(shard12), move(shard41), move(shard42)}`
background rebalancer scheduled them such that they depend on each other
sequentially.
```
{move(reftables) if there is any, none}
|
move( shard11)
|
move(shard12)
| {move(shard41)<--- move(shard12)} This is an artificial dependency
move(shard41)
|
move(shard42)
```
This results in artificial dependencies between otherwise independent
moves.
Considering that the shards in different colocation groups can be moved
concurrently, this PR changes the dependency relationship between the
moves as follows:
```
{move(reftables) if there is any, none} {move(reftables) if there is any, none}
| |
move(shard11) move(shard41)
| |
move(shard12) move(shard42)
```
---------
Co-authored-by: Jelte Fennema <jelte.fennema@microsoft.com>
DESCRIPTION: Drop `SHARD_STATE_TO_DELETE` and use the cleanup records
instead
Drops the shard state that is used to mark shards as orphaned. Now we
insert cleanup records into `pg_dist_cleanup` so "orphaned" shards will
be dropped either by maintenance daemon or internal cleanup calls. With
this PR, we make the "cleanup orphaned shards" functions to be no-op, as
they would not be needed anymore.
This PR includes some naming changes about placement functions. We don't
need functions that filter orphaned shards, as there will be no orphaned
shards anymore.
We will also be introducing a small script with this PR, for users with
orphaned shards. We'll basically delete the orphaned shard entries from
`pg_dist_placement` and insert cleanup records into `pg_dist_cleanup`
for each one of them, during Citus upgrade.
We also have a lot of flakiness fixes in this PR.
Co-authored-by: Jelte Fennema <github-tech@jeltef.nl>
DESCRIPTION: Add a rebalancer that uses background tasks for its
execution
Based on the baclground jobs and tasks introduced in #6296 we implement
a new rebalancer on top of the primitives of background execution. This
allows the user to initiate a rebalance and let Citus execute the long
running steps in the background until completion.
Users can invoke the new background rebalancer with `SELECT
citus_rebalance_start();`. It will output information on its job id and
how to track progress. Also it returns its job id for automation
purposes. If you simply want to wait till the rebalance is done you can
use `SELECT citus_rebalance_wait();`
A running rebalance can be canelled/stopped with `SELECT
citus_rebalance_stop();`.
**Intro**
This adds support to Citus to change the CPU priority values of
backends. This is created with two main usecases in mind:
1. Users might want to run the logical replication part of the shard moves
or shard splits at a higher speed than they would do by themselves.
This might cause some small loss of DB performance for their regular
queries, but this is often worth it. During high load it's very possible
that the logical replication WAL sender is not able to keep up with the
WAL that is generated. This is especially a big problem when the
machine is close to running out of disk when doing a rebalance.
2. Users might have certain long running queries that they don't impact
their regular workload too much.
**Be very careful!!!**
Using CPU priorities to control scheduling can be helpful in some cases
to control which processes are getting more CPU time than others.
However, due to an issue called "[priority inversion][1]" it's possible that
using CPU priorities together with the many locks that are used within
Postgres cause the exact opposite behavior of what you intended. This
is why this PR only allows the PG superuser to change the CPU priority
of its own processes. Currently it's not recommended to set `citus.cpu_priority`
directly. Currently the only recommended interface for users is the setting
called `citus.cpu_priority_for_logical_replication_senders`. This setting
controls CPU priority for a very limited set of processes (the logical
replication senders). So, the dangers of priority inversion are also limited
with when using it for this usecase.
**Background**
Before reading the rest it's important to understand some basic
background regarding process CPU priorities, because they are a bit
counter intuitive. A lower priority value, means that the process will
be scheduled more and whatever it's doing will thus complete faster. The
default priority for processes is 0. Valid values are from -20 to 19
inclusive. On Linux a larger difference between values of two processes
will result in a bigger difference in percentage of scheduling.
**Handling the usecases**
Usecase 1 can be achieved by setting `citus.cpu_priority_for_logical_replication_senders`
to the priority value that you want it to have. It's necessary to set
this both on the workers and the coordinator. Example:
```
citus.cpu_priority_for_logical_replication_senders = -10
```
Usecase 2 can with this PR be achieved by running the following as
superuser. Note that this is only possible as superuser currently
due to the dangers mentioned in the "Be very carefull!!!" section.
And although this is possible it's **NOT** recommended:
```sql
ALTER USER background_job_user SET citus.cpu_priority = 5;
```
**OS configuration**
To actually make these settings work well it's important to run Postgres
with more a more permissive value for the 'nice' resource limit than
Linux will do by default. By default Linux will not allow a process to
set its priority lower than it currently is, even if it was lower when
the process originally started. This capability is necessary to reset
the CPU priority to its original value after a transaction finishes.
Depending on how you run Postgres this needs to be done in one of two
ways:
If you use systemd to start Postgres all you have to do is add a line
like this to the systemd service file:
```conf
LimitNice=+0 # the + is important, otherwise its interpreted incorrectly as 20
```
If that's not the case you'll have to configure `/etc/security/limits.conf`
like so, assuming that you are running Postgres as the `postgres` OS user:
```
postgres soft nice 0
postgres hard nice 0
```
Finally you'd have add the following line to `/etc/pam.d/common-session`
```
session required pam_limits.so
```
These settings would allow to change the priority back after setting it
to a higher value.
However, to actually allow you to set priorities even lower than the
default priority value you would need to change the values in the
config to something lower than 0. So for example:
```conf
LimitNice=-10
```
or
```
postgres soft nice -10
postgres hard nice -10
```
If you use WSL2 you'll likely have to do another thing. You have to
open a new shell, because when PAM is only used during login, and
WSL2 doesn't actually log you in. You can force a login like this:
```
sudo su $USER --shell /bin/bash
```
Source: https://stackoverflow.com/a/68322992/2570866
[1]: https://en.wikipedia.org/wiki/Priority_inversion
The new shard copy code that was created for shard splits has some
advantages over the old shard copy code. The old code was using
worker_append_table_to_shard, which wrote to disk twice. And it also
didn't use binary copy when that was possible. Both of these issues
were fixed in the new copy code. This PR starts using this new copy
logic also for shard moves, not just for shard splits.
On my local machine I created a single shard table like this.
```sql
set citus.shard_count = 1;
create table t(id bigint, a bigint);
select create_distributed_table('t', 'id');
INSERT into t(id, a) SELECT i, i from generate_series(1, 100000000) i;
```
I then turned `fsync` off to make sure I wasn't bottlenecked by disk.
Finally I moved this shard between nodes with `citus_move_shard_placement`
with `block_writes`.
Before this PR a move took ~127s, after this PR it took only ~38s. So for this
small test this resulted in spending ~70% less time.
And I also tried the same test for a table that contained large strings:
```sql
set citus.shard_count = 1;
create table t(id bigint, a bigint, content text);
select create_distributed_table('t', 'id');
INSERT into t(id, a, content) SELECT i, i, 'aunethautnehoautnheaotnuhetnohueoutnehotnuhetncouhaeohuaeochgrhgd.athbetndairgexdbuhaobulrhdbaetoausnetohuracehousncaoehuesousnaceohuenacouhancoexdaseohusnaetobuetnoduhasneouhaceohusnaoetcuhmsnaetohuacoeuhebtokteaoshetouhsanetouhaoug.lcuahesonuthaseauhcoerhuaoecuh.lg;rcydabsnetabuesabhenth' from generate_series(1, 20000000) i;
```
As of master branch, Citus does all the modifications to replicated tables
(e.g., reference tables and distributed tables with replication factor > 1),
via 2PC and avoids any shardstate=3. As a side-effect of those changes,
handling node failures for replicated tables change.
With this PR, when one (or multiple) node failures happen, the users would
see query errors on modifications. If the problem is intermitant, that's OK,
once the node failure(s) recover by themselves, the modification queries would
succeed. If the node failure(s) are permenant, the users should call
`SELECT citus_disable_node(...)` to disable the node. As soon as the node is
disabled, modification would start to succeed. However, now the old node gets
behind. It means that, when the node is up again, the placements should be
re-created on the node. First, use `SELECT citus_activate_node()`. Then, use
`SELECT replicate_table_shards(...)` to replicate the missing placements on
the re-activated node.
Ignore orphaned shards in more places
Only use active shard placements in RouterInsertTaskList
Use IncludingOrphanedPlacements in some more places
Fix comment
Add tests
Emel Şimşek
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Gokhan Gulbiz
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