Messages in this thread | | | Subject | Re: Out-of-order writing by disk drives | Date | Sat, 18 Apr 2009 11:06:03 +0200 (CEST) | From | "Anton Ertl" <> |
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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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