Re: [HACKERS] Restricting maximum keep segments by repslots

Thank you for updating the patch.

On Tue, Jul 31, 2018 at 6:11 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> Hello.
>
> At Tue, 24 Jul 2018 16:47:41 +0900, Masahiko Sawada <sawada.mshk@gmail.com> wrote in
<CAD21AoD0rChq7wQE=_o95quopcQGjcVG9omwdH07nT5cm81hzg@mail.gmail.com>
>> On Mon, Jul 23, 2018 at 4:16 PM, Kyotaro HORIGUCHI
>> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>> > Hello.
>> >
>> > At Fri, 20 Jul 2018 10:13:58 +0900, Masahiko Sawada <sawada.mshk@gmail.com> wrote in
<CAD21AoDayePWwu4t=VPP5P1QFDSBvks1d8j76bXp5rbXoPbZcA@mail.gmail.com>
> ..
>> > Instead, I made the field be shown in flat "bytes" using bigint,
>> > which can be nicely shown using pg_size_pretty;
>>
>> Thank you for updating. I agree showing the remain in bytes.
>>
>> Here is review comments for v6 patch.
>>
>> @@ -967,9 +969,9 @@ postgres=# SELECT * FROM
>> pg_create_physical_replication_slot('node_a_slot');
>>   node_a_slot |
>>
>>  postgres=# SELECT * FROM pg_replication_slots;
>> -  slot_name  | slot_type | datoid | database | active | xmin |
>> restart_lsn | confirmed_flush_lsn
>> --------------+-----------+--------+----------+--------+------+-------------+---------------------
>> - node_a_slot | physical  |        |          | f      |      |             |
>> +  slot_name  | slot_type | datoid | database | active | xmin |
>> restart_lsn | confirmed_flush_lsn | wal_status | min_keep_lsn
>>
+-------------+-----------+--------+----------+--------+------+-------------+---------------------+------------+--------------
>> + node_a_slot | physical  |        |          | f      |      |
>>      |                     | unknown    | 0/1000000
>>
>> This funk should be updated.
>
> Perhaps you need a fresh database cluster.

I meant this was a doc update in 0004 patch but it's fixed in v7 patch.

While testing the v7 patch, I got the following result with
max_slot_wal_keep_size = 5GB and without wal_keep_segments setting.

=# select pg_current_wal_lsn(), slot_name, restart_lsn,
confirmed_flush_lsn, wal_status, remain, pg_size_pretty(remain) from
pg_replication_slots ;
 pg_current_wal_lsn | slot_name | restart_lsn | confirmed_flush_lsn |
wal_status |  remain  | pg_size_pretty
--------------------+-----------+-------------+---------------------+------------+----------+----------------
 2/A30000D8         | l1        | 1/AC000910  | 1/AC000948          |
streaming  | 16777000 | 16 MB
(1 row)

The actual distance between the slot limit and the slot 'l1' is about
1GB(5GB - (2/A30000D8 - 1/AC000910)) but the system view says the
remain is only 16MB. For the calculation of resetBytes in
GetOldestKeepSegment(), the current patch seems to calculate the
distance between the minSlotLSN and restartLSN when (curLSN -
max_slot_wal_keep_size) < minSlotLSN. However, I think that the actual
remained bytes until the slot lost the required WAL is (restartLSN -
(currLSN - max_slot_wal_keep_size)) in that case.

Also, 0004 patch needs to be rebased on the current HEAD.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center