Re: Tracking of page changes for backup purposes. PTRACK [POC]

Hello hackers,

> I have two concerns about this:
> 1. Does this affect the performance of the database when PTRACK is not enabled?
> 2. What is the cost of having PTRACK enabled?

I played with this patch a bit and did a simple benchmark on my laptop.

Configuration:

```
make distclean && ./configure prefix=/some/path/ && make -s -j4
```

postgresql.conf:

```
max_prepared_transactions = 100
wal_level = logical
wal_keep_segments = 128
max_connections = 100
wal_log_hints = on
max_wal_senders = 8
wal_keep_segments = 64
listen_addresses = '*'
hot_standby = on
log_statement = all
max_locks_per_transaction = 256
shared_buffers = 1GB
```

The benchmark:

```
pgbench -i && pgbench -j 4 -c 4 -T 300 -P 10
```

Here are the results.

10.1, ptrack_enable=false

transaction type: <builtin: TPC-B (sort of)>
scaling factor: 1
query mode: simple
number of clients: 4
number of threads: 4
duration: 300 s
number of transactions actually processed: 28772
latency average = 41.705 ms
latency stddev = 18.274 ms
tps = 95.895605 (including connections establishing)
tps = 95.906434 (excluding connections establishing)

10.1, ptrack_enable=true

scaling factor: 1
query mode: simple
number of clients: 4
number of threads: 4
duration: 300 s
number of transactions actually processed: 28607
latency average = 41.951 ms
latency stddev = 18.454 ms
tps = 95.344363 (including connections establishing)
tps = 95.345290 (excluding connections establishing)

10.1, without ptrack

transaction type: <builtin: TPC-B (sort of)>
scaling factor: 1
query mode: simple
number of clients: 4
number of threads: 4
duration: 300 s
number of transactions actually processed: 28622
latency average = 41.928 ms
latency stddev = 18.238 ms
tps = 95.396155 (including connections establishing)
tps = 95.397406 (excluding connections establishing)

At first glance PTRACK doesn't seem to affect the overall performance
significantly.

There are a few minor issues with the patch. There is a missing '/'
symbol in the comment before ptrack_get_and_clear procedure:

```
* Get ptrack file as bytea and clear it */
bytea *
ptrack_get_and_clear(Oid tablespace_oid, Oid table_oid)
{
```

Also I believe the patch should include some changes of
postgresql.conf.sample.

I suggest to add this patch to the closest commitfest. Otherwise it can
be lost.

--
Best regards,
Aleksander Alekseev