linux-kernel - Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD?

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Message-ID: <20131124010316.2ca6a8ce@alan.etchedpixels.co.uk>
Date:	Sun, 24 Nov 2013 01:03:16 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Pavel Machek <pavel@....cz>
Cc:	Ric Wheeler <ricwheeler@...il.com>, Howard Chu <hyc@...as.com>,
	"Theodore Ts'o" <tytso@....edu>, Chinmay V S <cvs268@...il.com>,
	Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD?

> Yes, and I'm arguing that is a bug (as in, < 0.01% people are using
> hdparm correctly).

Generally speaking if you are using hdparm for tuning it means we need to
fix something in the ATA layer so you don't have to !

> I guess it would be safer not to reattach drives after power
> fail... (also I wonder what this does to data integrity. Drive lost
> content of its writeback cache, but kernel continues... Journal will
> not prevent data corruption in this case).

For good or bad its very hard to tell if a drive randomly powers off or
we merely get a bus reset in the ATA case. In the SATA case we do at
least get the relevant events to handle it nicely as we *should* see a
DevExch event. We also can't tell a power fail event from a hot drive
swap, so we most definitely want to re-attach the drive, just so long as
we ensure that it comes back on a different device node.

Alan
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