[sheepdog-users] Discard support

[Hitoshi Mitake]

At Sun, 24 May 2015 20:47:54 +0900,
Hitoshi Mitake wrote:
> 
> At Thu, 21 May 2015 18:43:13 +0200 (CEST),
> Walid Moghrabi wrote:
> > 
> > [1  <multipart/alternative (7bit)>]
> > [1.1  <text/plain; utf-8 (7bit)>]
> > 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.

Seems that the data loss is not caused. It is my mistake. But the
implementation has a serious bug because it doesn't reload inode in
qemu driver. I don't suggest to use discard anyway.

Thanks,
Hitoshi

> 
> Thanks,
> Hitoshi
> 
> > 
> > Regards, 
> > 
> > Walid 
> > [1.2  <text/html; utf-8 (quoted-printable)>]
> > 
> > [2  <text/plain; us-ascii (7bit)>]
> > -- 
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