| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Noah Misch <noah(at)leadboat(dot)com>, Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Direct I/O |
| Date: | 2023-04-09 02:18:09 |
| Message-ID: | 20230409021809.jr2nqbjz5rm7dwvl@awork3.anarazel.de |
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| Lists: | pgsql-hackers |
Hi,
On 2023-04-09 13:55:33 +1200, Thomas Munro wrote:
> I think that particular thing might relate to modifications of the
> user buffer while a write is in progress (breaking btrfs's internal
> checksums). I don't think we should ever do that ourselves (not least
> because it'd break our own checksums). We lock the page during the
> write so no one can do that, and then we sleep in a synchronous
> syscall.
Oh, but we actually *do* modify pages while IO is going on. I wonder if you
hit the jack pot here. The content lock doesn't prevent hint bit
writes. That's why we copy the page to temporary memory when computing
checksums.
I think we should modify the test to enable checksums - if the problem goes
away, then it's likely to be related to modifying pages while an O_DIRECT
write is ongoing...
Greetings,
Andres Freund
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