Re: [HACKERS] Improve the concurency of vacuum full table and select statement on the same relation

[Jinyu]

The lock upgrade for vacuum full table tends to cause deadlock with other lock 
upgrade transaction which is from AccessShareLock to lockmode > 
AccessShareLock.  Tom Lane's concern is that it will cause vacuum full failed 
after do a lot of work.
But If we can always let other transaction failed to break deadlock instead of 
vacuum full table,  how about this lock upgrade solution?
For example: we can enlarge the 'DeadlockTimeout' for vacuum full table 
transaction to avoid deadlock check.


Jinyu Zhang
regards







At 2015-10-16 23:04:51, "Robert Haas"  wrote:
>On Thu, Oct 15, 2015 at 8:28 PM, Jim Nasby  wrote:
>> It's just how the authors of pg_repack decided to handle it. It seems pretty
>> reasonable, since you probably don't want an errant DDL statement to cause
>> the rollback of hours or days of pg_repack work.
>>
>> Ultimately, I don't think you'll find many people interested in working on
>> this, because the whole goal is to never need VACUUM FULL or pg_repack. If
>> you're clustering just for the sake of clustering, that has it's own set of
>> difficulties that should be addressed.
>
>I think the topic of online table reorganization is a pretty important
>one, actually.  That is a need that we have had for a long time,
>creates serious operational problems for users, and it's also a need
>that is not going away.  I think the chances of eliminating that need
>completely, even if we rearchitected or heap storage, are nil.
>
>I think the bigger issue is that it's a very hard problem to solve.
>pg_repack is one approach, but I've heard more than one person say
>that, as C-3PO said about the asteroid, it may not be entirely stable.
>
>-- 
>Robert Haas
>EnterpriseDB: http://www.enterprisedb.com
>The Enterprise PostgreSQL Company

Re: [HACKERS] Improve the concurency of vacuum full table and select statement on the same relation

[Jim Nasby]

On 10/16/15 10:04 AM, Robert Haas wrote:

On Thu, Oct 15, 2015 at 8:28 PM, Jim Nasby  wrote:

It's just how the authors of pg_repack decided to handle it. It seems pretty
reasonable, since you probably don't want an errant DDL statement to cause
the rollback of hours or days of pg_repack work.

Ultimately, I don't think you'll find many people interested in working on
this, because the whole goal is to never need VACUUM FULL or pg_repack. If
you're clustering just for the sake of clustering, that has it's own set of
difficulties that should be addressed.


I think the topic of online table reorganization is a pretty important
one, actually.  That is a need that we have had for a long time,
creates serious operational problems for users, and it's also a need
that is not going away.  I think the chances of eliminating that need
completely, even if we rearchitected or heap storage, are nil.


Yeah, which is why I made the comment about CLUSTER.


I think the bigger issue is that it's a very hard problem to solve.


ISTM nothing can be done here until there's some way to influence how 
pages get pulled from the FSM (IE: pull from one of the first X pages 
with free space). Maybe having some way to expose that would be enough 
of a start.



pg_repack is one approach, but I've heard more than one person say
that, as C-3PO said about the asteroid, it may not be entirely stable.


I looked at it recently, and it seems to be under active development. 
But I agree it'd be better if we could handle this internally.

--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com


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Re: [HACKERS] Improve the concurency of vacuum full table and select statement on the same relation

[Robert Haas]

On Thu, Oct 15, 2015 at 8:28 PM, Jim Nasby  wrote:
> It's just how the authors of pg_repack decided to handle it. It seems pretty
> reasonable, since you probably don't want an errant DDL statement to cause
> the rollback of hours or days of pg_repack work.
>
> Ultimately, I don't think you'll find many people interested in working on
> this, because the whole goal is to never need VACUUM FULL or pg_repack. If
> you're clustering just for the sake of clustering, that has it's own set of
> difficulties that should be addressed.

I think the topic of online table reorganization is a pretty important
one, actually.  That is a need that we have had for a long time,
creates serious operational problems for users, and it's also a need
that is not going away.  I think the chances of eliminating that need
completely, even if we rearchitected or heap storage, are nil.

I think the bigger issue is that it's a very hard problem to solve.
pg_repack is one approach, but I've heard more than one person say
that, as C-3PO said about the asteroid, it may not be entirely stable.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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Re: [HACKERS] Improve the concurency of vacuum full table and select statement on the same relation

[Jim Nasby]

On 10/13/15 10:18 AM, Jinyu wrote:

At 2015-10-12 23:46:12, "Jim Nasby"  wrote:

On 10/11/15 6:55 AM, Jinyu wrote:

Are there other solutions to improve the concurency of vacuum
full/cluster and select statement on the same relation?


ISTM that if we were going to put effort into this it makes more sense
to pull pg_repack into core. BTW, it's approach to this is to summarily
kill anything that attempts DDL on a table being repacked.


Please don't top-post, it leads to confusion.

>  >>it's approach to this is to summarily kill anything that attempts DDL
> on a table being repacked.
> Why? I am confused with it.  Could you please explain this?

It's just how the authors of pg_repack decided to handle it. It seems 
pretty reasonable, since you probably don't want an errant DDL statement 
to cause the rollback of hours or days of pg_repack work.


Ultimately, I don't think you'll find many people interested in working 
on this, because the whole goal is to never need VACUUM FULL or 
pg_repack. If you're clustering just for the sake of clustering, that 
has it's own set of difficulties that should be addressed.

--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com


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Re: [HACKERS] Improve the concurency of vacuum full table and select statement on the same relation

[Jinyu]

>>it's approach to this is to summarily kill anything that attempts DDL on a 
>>table being repacked.
Why? I am confused with it.  Could you please explain this?

Jinyu Zhang
thanks


At 2015-10-12 23:46:12, "Jim Nasby"  wrote:
>On 10/11/15 6:55 AM, Jinyu wrote:
>> Are there other solutions to improve the concurency of vacuum
>> full/cluster and select statement on the same relation?
>
>ISTM that if we were going to put effort into this it makes more sense 
>to pull pg_repack into core. BTW, it's approach to this is to summarily 
>kill anything that attempts DDL on a table being repacked.
>-- 
>Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
>Experts in Analytics, Data Architecture and PostgreSQL
>Data in Trouble? Get it in Treble! http://BlueTreble.com

Re: [HACKERS] Improve the concurency of vacuum full table and select statement on the same relation

[Jim Nasby]

On 10/11/15 6:55 AM, Jinyu wrote:

Are there other solutions to improve the concurency of vacuum
full/cluster and select statement on the same relation?


ISTM that if we were going to put effort into this it makes more sense 
to pull pg_repack into core. BTW, it's approach to this is to summarily 
kill anything that attempts DDL on a table being repacked.

--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com


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Re: [HACKERS] Improve the concurency of vacuum full table and select statement on the same relation

[Jinyu]

Get it now, thanks.
The lock upgrade(from ExclusiveLock to AccessExclusiveLock) may result in 
deadlocks iff one transaction first takes an AccessShareLock and then takes a 
lock(lockmode > AccessShareLock) on the same relation.

The single SQL statement can't take an AccessShareLock and then takes a 
lock(lockmode > AccessShareLock) on the same relation.
In fact, there is lock upgrade in transaction block which includes multiple 
query.like this transaction block "start transaction; select query; DML/DDL", 
it is from AccessShareLock to lockmode > AccessShareLock.

Now there may be deadlocks when run multiple transaction blocks even if no 
vacuum full. The some transaction will report error to break deadlock once 
deadlocks happen. So the vacuum full table may failed after doing lots of work 
in some time.

In some scenes, there are not explicit transaction block (no lock upgrade from 
AccessShareLock to lockmode > AccessShareLock), the deadlocks rarely happens. 
perhaps we can provide an option for vacuum full to let user choose whether 
"cluster/vacuum full" block select statement for very short time.

Are there other solutions to improve the concurency of vacuum full/cluster and 
select statement on the same relation?


Jinyu Zhang,
thanks


At 2015-10-10 23:34:41, "Tom Lane"  wrote:
>Jinyu  writes:
>> Proposal:  vacuum full table takes an ExclusiveLock on relation instead of 
>> AccessExclusiveLock at start. It can' block select statement before call 
>> function "finish_heap_swap". and select statement is safe because vacuum 
>> full table  copys tuples from old relation to new relation before calling 
>> function "finish_heap_swap". But it must take an AccessExclusiveLock on 
>> relation when call function "finish_heap_swap" in order to block select 
>> statement on the same relation.
>
>> This solution can improve the concurency. the following shows the reasons.
>
>What it's more likely to do is cause the vacuum full to fail altogether,
>after doing a lot of work.  Lock upgrade is a bad thing because it tends
>to result in deadlocks.
>
>   regards, tom lane

Re: [HACKERS] Improve the concurency of vacuum full table and select statement on the same relation

[Tom Lane]

Jinyu  writes:
> Proposal:  vacuum full table takes an ExclusiveLock on relation instead of 
> AccessExclusiveLock at start. It can' block select statement before call 
> function "finish_heap_swap". and select statement is safe because vacuum full 
> table  copys tuples from old relation to new relation before calling function 
> "finish_heap_swap". But it must take an AccessExclusiveLock on relation when 
> call function "finish_heap_swap" in order to block select statement on the 
> same relation.

> This solution can improve the concurency. the following shows the reasons.

What it's more likely to do is cause the vacuum full to fail altogether,
after doing a lot of work.  Lock upgrade is a bad thing because it tends
to result in deadlocks.

regards, tom lane


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