[dm-devel] reducing path test delays

[Lars Marowsky-Bree]

On 2005-04-04T15:48:36, "goggin, edward" <egoggin at emc.com> wrote:

> In the interest of reducing delay time when testing either good or
> failed paths, could we (1) prevent SCSI mid-layer initiated i/o
> retry of these requests and (2) insert the test path i/os at the
> head of the path's block i/o queue?  Doing either of these will
> require some kernel enhancement since there is no way to
> either request fail fast capability or insertion at queue head for
> a SCSI CDB issued from user space.
> 
> Currently the scsi_cmd_ioctl->sg_io->blk_execute_rq code
> path for a multipath test i/o does not set the REQ_FAILFAST
> bit in the flags field of the request structure and unconditionally
> inserts the request at the tail of the request queue for the scsi
> device representing the scsi path being tested.
> 
> Possibly the desire for each of these two attributes could be
> passed as a flag bit setting in the flag field of the sg_io_hdr
> structure passed into the kernel as part of the test path ioctl(2).

Jens: Indeed, being able to inflict the equivalent of BIO_RW_FAILFAST on
a raw request from user-space would be desireable.

Do you have any objections or suggestions how this could be done best?



Mit freundlichen Grüßen,
    Lars Marowsky-Brée <lmb at suse.de>

-- 
High Availability & Clustering
SUSE Labs, Research and Development
SUSE LINUX Products GmbH - A Novell Business