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Re: POSIX file updates

From: James Mansion <james(at)mansionfamily(dot)plus(dot)com>
To: Mark Mielke <mark(at)mark(dot)mielke(dot)cc>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: POSIX file updates
Date: 2008-03-31 20:41:29
Message-ID: 47F14C79.2080806@mansionfamily.plus.com (view raw or flat)
Thread:
Lists: pgsql-performance
Mark Mielke wrote:
> Is there anything in POSIX that seems to suggest this? :-) (i.e. why 
> are you going under the assumption that the answer is yes - did you 
> read something?)
>
It was something somewhere on the Sun web site, relating to tuning Solaris
filesystems.  Or databases. Or ZFS. :-(

Needless to say I can't find a search string that finds it now. I 
remember being surprised
though, since I wasn't aware of it either.
> I don't believe POSIX has any restriction such as you describe - or if 
> it does, and I don't know about it, then most UNIX file systems (if 
> not most file systems on any platform) are not POSIX compliant.
That, I can believe.

>
> Linux itself, even without NCQ, might choose to reorder the writes. If 
> you use ext2, the pressure to push pages out is based upon last used 
> time rather than last write time. It can choose to push out pages at 
> any time, and it's only every 5 seconds or so the the system task 
> (bdflush?) tries to force out all dirty file system pages. NCQ 
> exaggerates the situation, but I believe the issue pre-exists NCQ or 
> the SCSI equivalent of years past.
Indeed there do seem to be issues with Linux and fsync. Its one of 
things I'm trying to get a
handle on as well - the relationship between fsync and flushes of 
controller and/or disk caches.
>
> The rest of your email relies on the premise that POSIX enforces such 
> a thing, or that systems are POSIX compliant. :-)
>
True. I'm hoping someone (Jignesh?) will be prompted to remember.

It may have been something in a blog related to ZFS vs other 
filesystems, but so far I'm coming
up empty in google. doesn't feel like something I imagined though.

James


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