linux-kernel - Re: [PATCH/RFC] add "failfast" support for raid1/raid10.

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Date:   Thu, 24 Nov 2016 15:47:19 +1100
From:   NeilBrown <neilb@...e.com>
To:     Jack Wang <jack.wang.usish@...il.com>
Cc:     Shaohua Li <shli@...nel.org>,
        linux-raid <linux-raid@...r.kernel.org>,
        linux-block@...r.kernel.org, Christoph Hellwig <hch@....de>,
        linux-kernel@...r.kernel.org, hare@...e.de
Subject: Re: [PATCH/RFC] add "failfast" support for raid1/raid10.

On Sat, Nov 19 2016, Jack Wang wrote:

> 2016-11-18 6:16 GMT+01:00 NeilBrown <neilb@...e.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

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