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Comment by spudlyo | Hacker News Reader

I must be missing something. Why doesn't PostgreSQL use O_DIRECT when doing non-journaled writes to the data blocks? I know there are alignment restrictions on the userspace buffers used to do the reads and writes, but these don't seem insurmountable, and the payoff is huge. Is it because it's not POSIX?

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Using O_DIRECT well requires taking on a surprising amount of additional resource management responsibility. Also, the implementation is not portable and, combined with the additional responsibility previously mentioned, adds a ton of conditional compilation. The alignment requirements are not an issue at all; anyone using O_DIRECT is also going to be used properly aligned I/O buffers in any case.

O_DIRECT can have a lot of benefits. However, while O_DIRECT may be simple to use by itself, the other design and implementation requirements that are indirectly dragged along are not trivial by any means. Consequently, you should not be using O_DIRECT unless you can manage the substantial indirect overhead in implementation. It turns off a lot of OS features most developers take for granted.

Boo hoo. If that's what it takes to perform, then it should be done. Ingres had it in the 90's. A win for OLTP, sometimes a lose for DSS queries with scans.

You've essentially just waved your hands. Conditional complication is not an issue at all; anyone working on a large portable program is going to have large swaths of conditionally compiled code. Phrases like "resource management responsibility" and "substantial indirect overhead" are pretty vague, and aren't really helping me understand why O_DIRECT can't be used.

It turns off a lot of OS features most developers take for granted.

Yeah, like OS buffering. What else?

I use O_DIRECT almost exclusively in the software I write; I am familiar with the overhead. You can use O_DIRECT very easily without a lot of overhead if you wish but the performance will be much worse than not using it unless you competently reimplement many other parts of the OS functionality at the same time.

To use O_DIRECT effectively in Linux, at a minimum you need to write a buffer cache that is both fast and adaptive to workloads with proper cache replacement schedulers (no LRU crap), you need to write a complete I/O scheduler replacement (e.g. using io_submit and related interfaces) which is not portable at all, and you need to stop using almost all file system APIs (it interferes with the I/O scheduling bypass).

O_DIRECT is easy to use for very narrow cases. Taking advantage of it for anything slightly complicated in terms of file I/O usage and you bite off a lot more low-level implementation or the performance will actually be worse.

how can you change mount option online?

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