Messages in this thread | | | Date | Tue, 10 Sep 2002 09:03:57 -0700 | From | Patrick Mansfield <> | Subject | Re: [RFC] Multi-path IO in 2.5/2.6 ? |
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On Tue, Sep 10, 2002 at 03:02:01PM +0200, Lars Marowsky-Bree wrote: > On 2002-09-09T11:40:26,
> With multipathing, you want the lower level to hand you the error > _immediately_ if there is some way it could be related to a path failure and > no automatic retries should take place - so you can immediately mark the path > as faulty and go to another. > > However, on a "access beyond end of device" or a clear read error, failing a > path is a rather stupid idea, but instead the error should go up immediately. > > This will need to be sorted regardless of the layer it is implemented in. > > How far has this been already addressed in 2.5 ? >
The current scsi multi-path code handles the above cases. There is a scsi_path_decide_disposition() that fails paths independent of the result of the IO. It is similiar to the current scsi_decide_disposition, except it also fails the path. So for a check condition of media error, the IO might be marked as SUCCESS (meaning completed with an error), but the path would not be modified (there are more details than this).
> For user-space reprobing of failed paths, it may be vital to expose the > physical paths too. Then "reprobing" could be as simple as > > dd if=/dev/physical/path of=/dev/null count=1 && enable_path_again >
Yes, that is a good idea, I was thinking that this should be done via sg, and modify sg to allow path selection - so no matter what, sg could be used to probe a path. I have no plans to expose a user level device for each path, but a device model "file" could be exposed for the state of each path, and its state controlled via driverfs.
> I dislike reprobing in kernel space, in particular using live requests as > the LVM1 patch by IBM does it. > > > Sincerely, > Lars Marowsky-Brée <lmb@suse.de>
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