[sheepdog-users] Discard support

[Hitoshi Mitake]

At Thu, 21 May 2015 18:43:13 +0200 (CEST),
Walid Moghrabi wrote:
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> Hi, 
> 
> Just a question : is discard support safe and proven ? 
> 
> I activated discard support on my VDIs and I can see with dog vdi list that discard is effective when I run a "fstrim /" in my VMs but, in Sheepdog logs, here is what I get on some nodes : 
> 
> May 21 18:40:42 ERROR [gway 370139] wait_forward_request(438) fail 251f930000033f, No object found 
> May 21 18:40:42 ERROR [gway 370139] wait_forward_request(438) fail 251f930000033f, No object found 
> May 21 18:40:42 ERROR [gway 370139] wait_forward_request(438) fail 251f930000033f, No object found 
> May 21 18:40:42 ERROR [gway 370139] wait_forward_request(438) fail 251f930000033f, No object found 
> May 21 18:40:42 ERROR [gway 370139] wait_forward_request(438) fail 251f930000033f, No object found 
> May 21 18:40:42 ERROR [gway 370139] wait_forward_request(438) fail 251f930000033f, No object found 
> May 21 18:40:42 ERROR [io 370197] sd_remove_object(466) failed to remove object 251f930000033f, No object found 
> May 21 18:40:42 ERROR [io 370197] local_discard_obj(901) failed to remove 251f930000033f 
> 
> What should I think of it ? 

I checked the implementation of discard, and it is completely
broken. In short, it can cause data loss. Please never use it.

It can cause data loss in a below case: issue discard from VM to a
VDI, which is a cloned one. If the discarded object is a one of parent
VDI (a snapshot which is a parent of the cloned VDI), it will remove
the data from the snapshot.

Thanks,
Hitoshi

> 
> Regards, 
> 
> Walid 
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