Re: Improve WALRead() to suck data directly from WAL buffers when possible

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

To: Andres Freund <andres@anarazel.de>

Cc: Jeff Davis <pgsql@j-davis.com>, Dilip Kumar <dilipbalaut@gmail.com>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, pgsql-hackers@lists.postgresql.org

Date: 2023-10-20T16:49:32Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Add XLogCtl->logInsertResult
- f3ff7bf83bce 17.0 cited
Add assert to WALReadFromBuffers().
- 9ecbf54075a9 17.0 landed
Read WAL directly from WAL buffers.
- 91f2cae7a4e6 17.0 landed
Additional write barrier in AdvanceXLInsertBuffer().
- 766571be1659 17.0 landed
Use 64-bit atomics for xlblocks array elements.
- c3a8e2a7cb16 17.0 landed
Don't trust unvalidated xl_tot_len.
- bae868caf222 17.0 cited

Attachments

v11-0001-Allow-WAL-reading-from-WAL-buffers.patch (application/x-patch) patch v11-0001
v11-0002-Add-test-module-for-verifying-WAL-read-from-WAL-.patch (application/x-patch) patch v11-0002

On Thu, Oct 12, 2023 at 4:13 AM Andres Freund <andres@anarazel.de> wrote:
>
> On 2023-10-03 16:05:32 -0700, Jeff Davis wrote:
> > On Sat, 2023-01-14 at 12:34 -0800, Andres Freund wrote:
> > > One benefit would be that it'd make it more realistic to use direct
> > > IO for WAL
> > > - for which I have seen significant performance benefits. But when we
> > > afterwards have to re-read it from disk to replicate, it's less
> > > clearly a win.
> >
> > Does this patch still look like a good fit for your (or someone else's)
> > plans for direct IO here? If so, would committing this soon make it
> > easier to make progress on that, or should we wait until it's actually
> > needed?
>
> I think it'd be quite useful to have. Even with the code as of 16, I see
> better performance in some workloads with debug_io_direct=wal,
> wal_sync_method=open_datasync compared to any other configuration. Except of
> course that it makes walsenders more problematic, as they suddenly require
> read IO. Thus having support for walsenders to send directly from wal buffers
> would be beneficial, even without further AIO infrastructure.

I'm attaching the v11 patch set with the following changes:
- Improved input validation in the function that reads WAL from WAL
buffers in 0001 patch.
- Improved test module's code in 0002 patch.
- Modernized meson build file in 0002 patch.
- Added commit messages for both the patches.
- Ran pgindent on both the patches.

Any thoughts are welcome.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com