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Date:      Mon, 10 Mar 2003 07:54:53 -0800
From:      Terry Lambert <tlambert2@mindspring.com>
To:        Nuno Teixeira <nunotex@aeiou.pt>
Cc:        freebsd-current@freebsd.org
Subject:   Re: softupdates && write cache && ata tags topic
Message-ID:  <3E6CB54D.A8A7F7F0@mindspring.com>
References:  <20030308151549.GA622@gw.tex.bogus>

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Nuno Teixeira wrote:
> I understand the basic concept of the folowing techs: softupdates, disk
> write cache and ata tags.
> 
> My question is:
> 
>         It is safe to use softupdates + write cache + ata tags (IBM disk)?
> 
> I read someware that it not safe to use softupdate + write cache
> (without ata tags) and if it is not safe why FreeBSD 5.0 ships with them
> enabled?

See the discussion of 3 weeks ago with one of the engineers from
Maxtor, on this list.

The short answer is that the drive does not honor tags on a commit
to stable storage, before returning the request as satisfied, and
that's because on a write, ATA doesn't support disconnect with tagged
commands.

As to why write caching is on be default, there's a lot of going
back and forth on that, and it's flipped state maybe 4-5 times,
so far.  The short answer is that ATA drives are pig-slow with
write caching disabled, and users find that unacceptable.

-- Terry

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