From: Paolo Bonzini <pbonzini@redhat.com>
To: Martin Wilck <mwilck@suse.com>, Hannes Reinecke <hare@suse.de>,
Christoph Hellwig <hch@lst.de>
Cc: Daniel Wagner <dwagner@suse.de>,
Mike Snitzer <snitzer@redhat.com>,
emilne@redhat.com,
"Martin K. Petersen" <martin.petersen@oracle.com>,
linux-block@vger.kernel.org, dm-devel@redhat.com,
linux-scsi@vger.kernel.org, nkoenig@redhat.com,
Bart Van Assche <Bart.VanAssche@sandisk.com>,
Alasdair G Kergon <agk@redhat.com>
Subject: Re: [dm-devel] [PATCH v5 3/3] dm mpath: add CONFIG_DM_MULTIPATH_SG_IO - failover for SG_IO
Date: Tue, 6 Jul 2021 12:13:07 +0200 [thread overview]
Message-ID: <e5fc1d54-8443-49f4-078e-1453c6a97ba7@redhat.com> (raw)
In-Reply-To: <a088aa5c8459c001403bda9384b38213f8232fc6.camel@suse.com>
On 05/07/21 15:48, Martin Wilck wrote:
> On Mo, 2021-07-05 at 15:11 +0200, Hannes Reinecke wrote:
>> On 7/5/21 3:02 PM, Paolo Bonzini wrote:
>>> On 02/07/21 16:21, Martin Wilck wrote:
>>>>> SG_IO gives you raw access to the SCSI logic unit, and you get
>>>>> to
>>>>> keep the pieces if anything goes wrong.
>>>>
>>>> That's a very fragile user space API, on the fringe of being
>>>> useless
>>>> IMO.
>>>
>>> Indeed. If SG_IO is for raw access to an ITL nexus, it shouldn't
>>> be supported at all by mpath devices. If on the other hand SG_IO is
>>> for raw access to a LUN, there is no reason for it to not support
>>> failover.
>>
>> Or we look at IO_URING_OP_URING_CMD, to send raw cdbs via io_uring.
>> And delegate SG_IO to raw access to an ITL nexus.
>> Doesn't help with existing issues, but should get a clean way
>> forward.
>
> I still have to understand how this would help with the retrying
> semantics. Wouldn't we get the exact same problem if a path error
> occurs?
Also, how would the URING_CMD API differ from SG_IO modulo one being a
ioctl and one being io_uring-based? In the end what you have to do is
1) send a CDB and optionally some data 2) get back a status and
optionally some data and sense. Whether the intended use of the API is
for an ITL nexus or a LUN doesn't really matter. So, what is the
rationale for "SG_IO is for a nexus" in the first place, if you think
that "raw CDBs for a LUN" is a useful operation? You can still use
DM_TABLE_STATUS (iirc) to address a specific ITL nexus if desired.
Besides the virtualization case, think of udev rules that use SG_IO to
retrieve the device identification page for an mpath device and create
corresponding symlinks in /dev. They would fail if the first path is
not responding, which is not desirable.
Paolo
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next prev parent reply other threads:[~2021-07-06 10:13 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-28 15:15 [dm-devel] [PATCH v5 0/3] scsi/dm: dm_blk_ioctl(): implement failover for SG_IO on dm-multipath mwilck
2021-06-28 15:15 ` [dm-devel] [PATCH v5 1/3] scsi: scsi_ioctl: export __scsi_result_to_blk_status() mwilck
2021-06-28 15:15 ` [dm-devel] [PATCH v5 2/3] scsi: scsi_ioctl: add sg_io_to_blk_status() mwilck
2021-06-28 15:15 ` [dm-devel] [PATCH v5 3/3] dm mpath: add CONFIG_DM_MULTIPATH_SG_IO - failover for SG_IO mwilck
2021-07-01 7:56 ` Christoph Hellwig
2021-07-01 10:35 ` Martin Wilck
2021-07-01 11:34 ` Christoph Hellwig
2021-07-02 14:21 ` Martin Wilck
2021-07-05 13:02 ` Paolo Bonzini
2021-07-05 13:11 ` Hannes Reinecke
2021-07-05 13:48 ` Martin Wilck
2021-07-06 10:13 ` Paolo Bonzini [this message]
2021-07-01 11:06 ` Paolo Bonzini
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