[dm-devel] [PATCH RFC] dm-raid1: keep writing after leg failure

[Lidong Zhong]

Hi Brassow,
Thanks for your review.
>>> On 4/22/2015 at 05:49 AM, in message
<D7A1AF2A-21CC-4B39-9221-511A312B2AA5 at redhat.com>, Brassow Jonathan
<jbrassow at redhat.com> wrote: 

> > On Apr 2, 2015, at 10:51 PM, Lidong Zhong <lzhong at suse.com> wrote: 
> >  
> > Currently if there is a leg failure, the bio will be put into the hold 
> > list until userspace replace/remove the leg. Here we are trying to make 
> > dm-raid1 ignore the failure and keep the following bios going on. 
> > This is because there maybe a temporary path failure in clvmd 
> > which leads to cluster raid1 remove/replace the fake device failure. And 
> > it takes a long time to do the full sync if we readd the device back. 
> > --- 
>  
> Just be aware that there are failure scenarios that this does not handle as  
> well.  Like a reboot causing the legs to return but not the log.  It’s a  
In this scenario the raid1 can not even get activated. I think it's beyond what 
dm-raid1 could proceed.

> remote possibility, but something the old model considered.  The new RAID  
> targets which leverage the MD personalities are much better (e.g. ‘lvcreate  
> —type raid1’), but they don’t support active-active cluster setups. 
>  
> I’ve seen a couple things on the linux-raid mailing list suggesting that  
> someone (Goldwyn Rodrigues) may be thinking about making RAID1/10  
> cluster-aware…  LVM should be able to take advantage of this if it became a  
> reality. 
>  
> Are you interested in providing your use-case for cluster mirroring (vs  
> opting for active/passive mirroring)? 
The most common use case would be cLCM providing shared storage with 
ocfs2/gfs2 running on it. Currently it's the only way to provide clustered raid1.
Of course it's a better choice for what Goldwyn is doing now.But how long it will
take to become stable enough. Also, the patch series would be an enhancement 
for dm-raid1. 

Regards,
Lidong
>  
>  brassow 
>  
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