Re: [PATCH 0/2] convert libsas to the libata new eh

[Brian King <brking@linux.vnet.ibm.com>]

On 01/23/2011 09:41 AM, James Bottomley wrote:
> The bad news is that this is a pretty horrific conversion.  Because of
> the amount of control the new eh wants (and the slew of monolithic
> assumptions it makes), the entire error handler routine has to be sliced
> apart and sewn back together to make it callable as part of an existing
> error handler (rather than trying to do everything on its own).
> 
> The even worse news is that unless you have an existing strategy
> handler, you can't make use of this.  That means that ipr is
> unconvertable in its current form.  Given the two really nasty options:
> give ipr its own error handler via its own transport class, or attach
> ipr to libsas and let it do the work, I'd choose the latter.

I've got a patch set that's been sitting collecting dust for quite some time
that implements yet another option. The basic idea is that we move away from
the idea of having a single scsi_host for the entire SAS adapter, but
rather have one for the SAS devices and one for each SATA rphy.

This allows us to use libata's EH pretty much as is, but does introduce
some issues to solve:

1. Adapter queue depth. Since we have multiple scsi_hosts for a single
   HBA, we lose the adapter queue depth tracking done by scsi core.
   Anything in block that can help us here?
2. Locking gets messy since we have a lock for each sata rphy as well
   as one for the sas scsi_host.

There were some other smaller issues as well. I liked the idea of moving
to multiple scsi_host's since that eliminates the issue of userspace polling an
empty CD drive causing EH to get invoked and quiescing all SAS I/O as well,
but it does result in potentially a lot of scsi_hosts getting created.

-Brian


-- 
Brian King
Linux on Power Virtualization
IBM Linux Technology Center