On Thu, Apr 26, 2018 at 8:33 AM, Andres Freund <andres@anarazel.de> wrote:
> On 2018-04-25 14:41:44 -0400, Robert Haas wrote:
>> On Mon, Apr 16, 2018 at 2:13 AM, Andrew Gierth
>> <andrew@tao11.riddles.org.uk> wrote:
>> > The code that detects sequential behavior can not distinguish between
>> > pread() and lseek+read, it looks only at the actual offset of the
>> > current request compared to the previous one for the same fp.
>> >
>> > Thomas> +1 for adopting pread()/pwrite() in PG12.
>> >
>> > ditto
>>
>> Likewise.
>
> +1 as well. Medium term I forsee usage of at least pwritev(), and
> possibly also preadv(). Being able to write out multiple buffers at once
> is pretty crucial if we ever want to do direct IO.
Also if we ever use threads and want to share file descriptors we'd
have to use it.
CC'ing Oskari Saarenmaa who proposed a patch for this a couple of years back[1].
Oskari, would you like to update your patch and post it for the
September commitfest? At first glance, it probably needs autoconf-fu
to check if pread()/pwrite() are supported and fallback code, so
someone should update the patch to do that or explain why it's not
needed based on standards we require. At least Windows apparently
needs special handling (ReadFile() and WriteFile() with an OVERLAPPED
object).
Practically speaking, are there any Unix-like systems outside museums
that don't have it? According to the man pages I looked at, this
stuff is from System V R4 (1988) and appeared in ancient BSD strains
too. Hmm, I suppose it's possible that pademelon and gaur don't: they
apparently run HP-UX 10.20 (1996) which Wikipedia tells me is derived
from System V R3! I can see that current HP-UX does have them... but
unfortunately their man pages don't have a HISTORY section.
FWIW these functions just showed up in the latest POSIX standard[2]
(issue 7, 2017/2018?), having moved from "XSI option" to "base".
[1] https://www.postgresql.org/message-id/flat/7fdcb664-4f8a-8626-75df-ffde85005829%40ohmu.fi
[2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/pread.html
--
Thomas Munro
http://www.enterprisedb.com
