[dm-devel] Question about iscsi session block

[Donald Williams]

Hello,
   Something else to check is your MPIO configuration.  I have seen this
same symptom when the linux MPIO feature "queue_if_no_path" was enabled

 From the /etc/multipath.conf file showing it enabled.

    failback                immediate
    features                "1 queue_if_no_path"

 Also, in the past some versions of linux multipathd would wait for a
very long time before moving all I/O to the remaining path.

 Regards,
Don


On Tue, Feb 15, 2022 at 10:49 AM Zhengyuan Liu <liuzhengyuang521 at gmail.com>
wrote:

> Hi, all
>
> We have an online server which uses multipath + iscsi to attach storage
> from Storage Server. There are two NICs on the server and for each it
> carries about 20 iscsi sessions and for each session it includes about 50
>  iscsi devices (yes, there are totally about 2*20*50=2000 iscsi block
> devices
>  on the server). The problem is: once a NIC gets faulted, it will take too
> long
> (nearly 80s) for multipath to switch to another good NIC link, because it
> needs to block all iscsi devices over that faulted NIC firstly. The
> callstack is
>  shown below:
>
>     void iscsi_block_session(struct iscsi_cls_session *session)
>     {
>         queue_work(iscsi_eh_timer_workq, &session->block_work);
>     }
>
>  __iscsi_block_session() -> scsi_target_block() -> target_block() ->
>   device_block() ->  scsi_internal_device_block() -> scsi_stop_queue() ->
>  blk_mq_quiesce_queue()>synchronize_rcu()
>
> For all sessions and all devices, it was processed sequentially, and we
> have
> traced that for each synchronize_rcu() call it takes about 80ms, so
> the total cost
> is about 80s (80ms * 20 * 50). It's so long that the application can't
> tolerate and
> may interrupt service.
>
> So my question is that can we optimize the procedure to reduce the time
> cost on
> blocking all iscsi devices?  I'm not sure if it is a good idea to increase
> the
> workqueue's max_active of iscsi_eh_timer_workq to improve concurrency.
>
> Thanks in advance.
>
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