Re: a question about Direct I/O and double buffering

On Apr 5, 2007, at 3:33 PM, david(at)lang(dot)hm wrote:

> On Thu, 5 Apr 2007, Xiaoning Ding wrote:
>
>>>
>>> To the best of my knowledge, Postgres itself does not have a
>>> direct IO
>>> option (although it would be a good addition). So, in order to
>>> use direct
>>> IO with postgres you'll need to consult your filesystem docs for
>>> how to
>>> set the forcedirectio mount option. I believe it can be set
>>> dynamically,
>>> but if you want it to be permanent you'll to add it to your
>>> fstab/vfstab
>>> file.
>>
>> I use Linux. It supports direct I/O on a per-file basis only. To
>> bypass OS buffer cache,
>> files should be opened with O_DIRECT option. I afraid that I have
>> to modify PG.
>
> as someone who has been reading the linux-kernel mailing list for
> 10 years, let me comment on this a bit.
>
> linux does have a direct i/o option,

Yes, I know applications can request direct i/o with the O_DIRECT
flag to open(), but can this be set to be forced for all applications
or for individual applications from "outside" the application (not
that I've ever heard of something like the second)?

> but it has significant limits on when and how you cna use it
> (buffers must be 512byte aligned and multiples of 512 bytes, things
> like that).

That's a standard limit imposed by the sector size of hard drives,
and is present in all direct i/o implementations, not just Linux.

> Also, in many cases testing has shon that there is a fairly
> significant performance hit for this, not a perfomance gain.

Those performance hits have been noticed for high i/o transaction
databases? The idea here is that these kinds of database manage
their own caches and having a separate filesystem cache in virtual
memory that works with system memory page sizes is an unneeded level
of indirection. Yes, you should expect other "normal" utilities will
suffer a performance hit as if you are trying to cp a 500 byte file
you'll still have to work with 8K writes and reads whereas with the
filesystem cache you can just write/read part of a page in memory and
let the cache decide when it needs to write and read from disk. If
there are other caveats to direct i/o on Linux I'd love to hear them.

erik jones <erik(at)myemma(dot)com>
software developer
615-296-0838
emma(r)