Re: [LSF/MM/BPF TOPIC] Buffered atomic writes

[Andres Freund <andres@anarazel.de>]

Hi,

On 2026-02-17 23:57:50 +0530, Ojaswin Mujoo wrote:
> From my mental model and very high level understanding of Postgres' WAL
> model [1] I am under the impression that for moving from full page
> writes to RWF_ATOMIC, we would need to ensure that the **disk** write IO
> of any data buffer should go in an untorn fashion.

Right.


> Now, coming to your example, IIUC here we can actually tolerate to do
> the 2nd write above non atomically because it is already a sort of full
> page write in the journal.
>
> So lets say if we do something like:
>
> 0. Buffer has some initial value on disk
> 1. Write new rows into buffer
> 2. Write the buffer as RWF_ATOMIC
> 3. Overwrite the complete buffer which will journal all the contents
> 4. Write the buffer as non RWF_ATOMIC
> 5. Crash
>
> I think it is still possible to satisfy my assumption of **disk** IO
> being untorn. Example, here we can have an RWF_ATOMIC implementation
> where the data on disk after crash could either be in initial state 0.
> or be the new value after 4. This is not strictly the old or new
> semantic but still ensures the data is consistent.

The way I understand Jan is that, unless we are careful with the write in 4),
the write for 0) could still be in progress, with the copy from userspace to
the pagecache from 4 happening in the middle of the DMA for the write from 0),
leading to a torn page on-disk, even though the disk actually behaved
correctly.


> My naive understanding says that as long as disk has consistent/untorn
> data, like above, we can recover via the journal.

Yes, if that were true, we could recover. But if my understanding of Jan's
concern is right, that'd not necessarily be guaranteed.

Greetings,

Andres Freund