linux-kernel - Re: Out-of-order writing by disk drives

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Message-Id: <E1Lv6Ut-0008Rm-Dq@a4.complang.tuwien.ac.at>
Date:	Sat, 18 Apr 2009 11:06:03 +0200 (CEST)
From:	"Anton Ertl" <anton@...s.complang.tuwien.ac.at>
To:	folkert@...heusden.com (Folkert van Heusden)
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Out-of-order writing by disk drives

Folkert van Heusden wrote:
> 
> > I have released a new version of hdtest, a program that tests whether
> > hard disks write out-of-order relative to the order that the writes
> > were passed to them from the OS.  You find the program at
> > http://www.complang.tuwien.ac.at/anton/hdtest/
> 
> Not sure if it matters but it seems open-iscsi (both target and
> initiator are linux systems) works fine with respect to the write
> barriers: while running hdtest on an iscsi device I suddenly stopped the
> traffic flowing using an iptables DROP-rule. Then of course I stopped
> the iscsi initiator, removed the rules, restarted the initator and ran
> hdcheck: all above the line have the correct magic.

hdtest does not use barriers (if it did, my results would hopefully be
different; BTW, how would I use device barriers from a user program?).
But it does write to the device opened with O_SYNC.  So I expect the
kernel to pass the request synchronously to the device (due to
O_SYNC), but the device has no particular reason (like barriers) to
write the stuff in-order.  So I would expect your disconnection not to
result in out-of-order writing just like I would not expect
disconnecting the USB or SATA connection to have that effect when
using a setup like I did (but I have not tried that).

In short, your experiment tells nothing about barriers over iSCSI,
because barriers are not used (AFAIK).

- anton
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