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SubjectRe: [PATCH/RFC] add "failfast" support for raid1/raid10.
Hi Neil,

2016-11-24 5:47 GMT+01:00 NeilBrown <neilb@suse.com>:
> On Sat, Nov 19 2016, Jack Wang wrote:
>
>> 2016-11-18 6:16 GMT+01:00 NeilBrown <neilb@suse.com>:
>>> Hi,
>>>
>>> I've been sitting on these patches for a while because although they
>>> solve a real problem, it is a fairly limited use-case, and I don't
>>> really like some of the details.
>>>
>>> So I'm posting them as RFC in the hope that a different perspective
>>> might help me like them better, or find a better approach.
>>>
>>> The core idea is that when you have multiple copies of data
>>> (i.e. mirrored drives) it doesn't make sense to wait for a read from
>>> a drive that seems to be having problems. It will probably be faster
>>> to just cancel that read, and read from the other device.
>>> Similarly, in some circumstances, it might be better to fail a drive
>>> that is being slow to respond to writes, rather than cause all writes
>>> to be very slow.
>>>
>>> The particular context where this comes up is when mirroring across
>>> storage arrays, where the storage arrays can temporarily take an
>>> unusually long time to respond to requests (firmware updates have
>>> been mentioned). As the array will have redundancy internally, there
>>> is little risk to the data. The mirrored pair is really only for
>>> disaster recovery, and it is deemed better to lose the last few
>>> minutes of updates in the case of a serious disaster, rather than
>>> occasionally having latency issues because one array needs to do some
>>> maintenance for a few minutes. The particular storage arrays in
>>> question are DASD devices which are part of the s390 ecosystem.
>>
>> Hi Neil,
>>
>> Thanks for pushing this feature also to mainline.
>> We at Profitbricks use raid1 across IB network, one pserver with
>> raid1, both legs on 2 remote storages.
>> We've noticed if one remote storage crash , and raid1 still keep
>> sending IO to the faulty leg, even after 5 minutes,
>> md still redirect I/Os, and md refuse to remove active disks, eg:
>
> That make sense. It cannot remove the active disk until all pending IO
> completes, either with an error or with success.
>
> If the target has a long timeout, that can delay progress a lot.
>
>>
>> I tried to port you patch from SLES[1], with the patchset, it reduce
>> the time to ~30 seconds.
>>
>> I'm happy to see this feature upstream :)
>> I will test again this new patchset.
>
> Thanks for your confirmation that this is more generally useful than I
> thought, and I'm always happy to hear for more testing :-)
>
> Thanks,
> NeilBrown

Just want to update test result, so far it's working fine, no regression :)
Will report if anything breaks.

Thanks
Jack

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