Re: Recovery performance of DROP DATABASE with many tablespaces

On Tue, Jul 10, 2018 at 3:04 PM Michael Paquier <michael@paquier.xyz> wrote:
>
> On Thu, Jul 05, 2018 at 01:42:20AM +0900, Fujii Masao wrote:
> > TBH, I have no numbers measured by the test.
> > One question about your test is; how did you measure the *recovery time*
> > of DROP DATABASE? Since it's *recovery* performance, basically it's not easy
> > to measure that.
>
> It would be simple to measure the time it takes to replay this single
> DROP DATABASE record by putting two gettimeofday() calls or such things
> and then take the time difference.  There are many methods that you
> could use here, and I suppose that with a shared buffer setting of a
> couple of GBs of shared buffers you would see a measurable difference
> with a dozen of tablespaces or so.  You could also take a base backup
> after creating all the tablespaces, connect the standby and then drop
> the database on the primary to see the actual time it takes.  Your patch
> looks logically correct to me because DropDatabaseBuffers is a
> *bottleneck* with large shared_buffers, and it would be nice to see
> numbers.

Thanks for the comment!

I measured how long it takes to replay DROP DATABASE with 1000 tablespaces,
in master and patched version. shared_buffers was set to 16GB.

[master]
It took 8 seconds to replay DROP DATABASE with 1000 tablespaces,
as follows. In this case, 16GB shared_buffers was fully scanned 1000 times.

    2019-10-02 16:50:14 JST LOG:  redo starts at 0/2000028
    2019-10-02 16:50:22 JST LOG:  redo done at 0/300A298

[patched]
It took less than 1 second to replay DROP DATABASE with 1000 tablespaces,
as follows. In this case, 16GB shared_buffers was scanned only one time.

    2019-10-02 16:47:03 JST LOG:  redo starts at 0/2000028
    2019-10-02 16:47:03 JST LOG:  redo done at 0/3001588

Regards,

-- 
Fujii Masao