Обсуждение: Making WAL archiving faster — multi-file support and async ideas
Hi hackers,
We’ve been thinking about how to make WAL archiving faster.
This topic was previously discussed in [1], and we’ve taken a first step by implementing the attached patch, which adds support for archiving multiple WAL files in one go.
The idea is straightforward: instead of invoking the archive command or callback once per WAL file, we allow passing a batch of files. The patch introduces support for new placeholders:
%F– list of WAL file names%P– list of their full paths%N– number of files
Since PostgreSQL already reads multiple files into memory and caches them before archiving, this change avoids repeated fork() calls and reduces overhead in high-throughput setups.
Of course, there are trade-offs. After discussing with Andrey Borodin, we noted that if even one file in the batch fails to archive, we currently have to retry the whole batch. While it’s technically possible to return a list of successfully archived files, that would complicate the API and introduce messy edge cases.
So we’re also exploring a more flexible idea: an asynchronous archiver mode.
The idea is to have PostgreSQL write WAL file names (marked .ready) into a FIFO or pipe, and let an archive process or library asynchronously consume and archive them. It would send back confirmations (or failures) through another pipe, allowing PostgreSQL to retry failed files as needed. This could decouple archiving from the archiver loop and open the door to more efficient and parallel implementations.
We’d appreciate feedback on both directions:
Do you think the idea in the attached patch — batching WAL files for archiving — is viable? Is it something worth pursuing?
What do you think about the async archiver concept? Would it fit PostgreSQL’s architecture and operational expectations?
Thanks,
Stepan Neretin
Вложения
Hi, On Tue, Jul 29, 2025 at 3:56 PM Stepan Neretin <slpmcf@gmail.com> wrote: > > We’ve been thinking about how to make WAL archiving faster. > This topic was previously discussed in [1], and we’ve taken a first step by implementing the attached patch, which addssupport for archiving multiple WAL files in one go. > The idea is straightforward: instead of invoking the archive command or callback once per WAL file, we allow passing abatch of files. > Thanks for the patch! My first comments are related to code style and naming. I believe that fixing them will make the review process more convenient. 1) Please, try to make your diff with vanilla code as small as possible. If you believe that some formatting of the code "around" your solution can be made better, you can provide a different .patch file, containing such corrections. 2) You have added "archive_files_cb". Maybe it should be renamed to something like "archive_multiple_files_cb"? During review, I regularly messed up these callbacks and functions with the same naming (...file and ...files). 3) shell_archive.c : I think that we should place "shell_archive_file" code over "run_archive_command" code. Again, it will noticeably reduce diff. 4) pgarch.c : definition of ArchiveXlogArg and ArchiveFilesArg structures and ArchiveCallbackFn type definition should be placed in the head of the file. 5) "pgarch_ArchiverCopyLoopMulti" function : I guess that "ArchiveCallbacks->check_configured_cb != NULL" should be placed at the beginning of the while loop (by analogy with pgarch_ArchiverCopyLoop). 6) Please, run pgindent on your patch. -- Best regards, Daniil Davydov
As far as I know, tools like cp naturally process files in the order they are received. This ensures that files are processed correctly, and it seems easier and more reliable to build on this concept than to introduce a new parallel paradigm. Given that the archiver already uses a priority queue, this should not be difficult to implement.
Thanks for considering my feedback.
Best regards,
Alyona Vinter
Hi hackers,
We’ve been thinking about how to make WAL archiving faster.
This topic was previously discussed in [1], and we’ve taken a first step by implementing the attached patch, which adds support for archiving multiple WAL files in one go.
The idea is straightforward: instead of invoking the archive command or callback once per WAL file, we allow passing a batch of files. The patch introduces support for new placeholders:
%F– list of WAL file names
%P– list of their full paths
%N– number of filesSince PostgreSQL already reads multiple files into memory and caches them before archiving, this change avoids repeated
fork()calls and reduces overhead in high-throughput setups.Of course, there are trade-offs. After discussing with Andrey Borodin, we noted that if even one file in the batch fails to archive, we currently have to retry the whole batch. While it’s technically possible to return a list of successfully archived files, that would complicate the API and introduce messy edge cases.
So we’re also exploring a more flexible idea: an asynchronous archiver mode.
The idea is to have PostgreSQL write WAL file names (marked
.ready) into a FIFO or pipe, and let an archive process or library asynchronously consume and archive them. It would send back confirmations (or failures) through another pipe, allowing PostgreSQL to retry failed files as needed. This could decouple archiving from the archiver loop and open the door to more efficient and parallel implementations.We’d appreciate feedback on both directions:
Do you think the idea in the attached patch — batching WAL files for archiving — is viable? Is it something worth pursuing?
What do you think about the async archiver concept? Would it fit PostgreSQL’s architecture and operational expectations?
Thanks,
Stepan Neretin
... could attempt a restore using the incomplete archive. While we hope this would cause a clear error during recovery, there is a risk that partial application of non-sequential segments might lead to silent corruption or other unforeseen issues.