Message-ID: <4B82C642.8020409@cheapcomplexdevices.com>
Date: Mon, 22 Feb 2010 10:00:34 -0800
From: Ron Mayer <rm_pg@cheapcomplexdevices.com>
User-Agent: Thunderbird 2.0.0.23 (X11/20090817)
MIME-Version: 1.0
To: Bruce Momjian <bruce@momjian.us>
CC: Greg Smith <greg@2ndquadrant.com>,
	Ron Mayer <rm_pg@cheapcomplexdevices.com>,
	pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: SSD + RAID
References: <201002221437.o1MEbDJ13005@momjian.us>
In-Reply-To: <201002221437.o1MEbDJ13005@momjian.us>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

Bruce Momjian wrote:
> Greg Smith wrote:
>> .... If you have a regular SATA drive, it almost certainly 
>> supports proper cache flushing....
> 
> OK, but I have a few questions.  Is a write to the drive and a cache
> flush command the same?

I believe they're different as of ATAPI-6 from 2001.

> Which file systems implement both?

Seems ZFS and recent ext4 have thought these interactions out
thoroughly.   Find a slow ext4 that people complain about, and
that's the one doing it right :-).

Ext3 has some particularly odd annoyances where it flushes and waits
for certain writes (ones involving inode changes) but doesn't bother
to flush others (just data changes).   As far as I can tell, with
ext3 you need userspace utilities to make sure flushes occur when
you need them.    At one point I was tempted to try to put such
userspace hacks into postgres.

I know less about other file systems.  Apparently the NTFS guys
are aware of such stuff - but don't know what kinds of fsync equivalent
you'd need to make it happen.

Also worth noting - Linux's software raid stuff (MD and LVM)
need to handle this right as well - and last I checked (sometime
last year) the default setups didn't.

>  I thought a
> write to the drive was always assumed to flush it to the platters,
> assuming the drive's cache is set to write-through.

Apparently somewhere around here:
http://www.t10.org/t13/project/d1410r3a-ATA-ATAPI-6.pdf
they were separated in the IDE world.