Re: Load distributed checkpoint

Thread added to TODO list:

* Reduce checkpoint performance degredation by forcing data to disk
more evenly

http://archives.postgresql.org/pgsql-hackers/2006-12/msg00337.php
http://archives.postgresql.org/pgsql-hackers/2007-01/msg00079.php

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ITAGAKI Takahiro wrote:
> This is a proposal for load distributed checkpoint.
> (It is presented on postgresql anniversary summit in last summer.)
>
>
> We offen encounters performance gap during checkpoint. The reason is write
> bursts. Storage devices are too overworked in checkpoint, so they can not
> supply usual transaction processing.
>
> Checkpoint consists of the following four steps, and the major performance
> problem is 2nd step. All dirty buffers are written without interval in it.
>
> 1. Query information (REDO pointer, next XID etc.)
> 2. Write dirty pages in buffer pool
> 3. Flush all modified files
> 4. Update control file
>
> I suggested to write pages with sleeping in 2nd step, using normal activity
> of the background writer. It is something like cost-based vacuum delay.
> Background writer has two pointers, 'ALL' and 'LRU', indicating where to
> write out in buffer pool. We can wait for the ALL clock-hand going around
> to guarantee all pages to be written.
>
> Here is pseudo-code for the proposed method. The internal loop is just the
> same as bgwriter's activity.
>
> PrepareCheckPoint(); -- do step 1
> Reset num_of_scanned_pages by ALL activity;
> do {
> BgBufferSync(); -- do a part of step 2
> sleep(bgwriter_delay);
> } while (num_of_scanned_pages < shared_buffers);
> CreateCheckPoint(); -- do step 3 and 4
>
>
> We may accelerate background writer to reduce works at checkpoint instead of
> the method, but it introduces another performance problem; Extra pressure
> is always put on the storage devices to keep the number of dirty pages low.
>
>
> I'm working about adjusting the progress of checkpoint to checkpoint timeout
> and wal segments limitation automatically to avoid overlap of two checkpoints.
> I'll post a patch sometime soon.
>
> Comments and suggestions welcome.
>
> Regards,
> ---
> ITAGAKI Takahiro
> NTT Open Source Software Center
>
>
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