Re: [Qemu-block] [Patch v12 resend 05/10] docs: block replication's description
On Mon, Jan 04, 2016 at 02:03:16PM +0800, Wen Congyang wrote: > On 12/23/2015 05:26 PM, Stefan Hajnoczi wrote: > > On Wed, Dec 02, 2015 at 01:31:46PM +0800, Wen Congyang wrote: > >> +== Failure Handling == > >> +There are 6 internal errors when block replication is running: > >> +1. I/O error on primary disk > >> +2. Forwarding primary write requests failed > >> +3. Backup failed > >> +4. I/O error on secondary disk > >> +5. I/O error on active disk > >> +6. Making active disk or hidden disk empty failed > >> +In case 1 and 5, we just report the error to the disk layer. In case 2, 3, > >> +4 and 6, we just report block replication's error to FT/HA manager (which > >> +decides when to do a new checkpoint, when to do failover). > >> +There is no internal error when doing failover. > > > > Not sure this is true. > > > > Below it says the following for failover: "We will flush the Disk buffer > > into Secondary Disk and stop block replication". Flushing the disk > > buffer can result in I/O errors. This means that failover operations > > are not guaranteed to succeed. > > We don't use mirror job now. We may use it in the next version. > Is there any way to know the I/O error when the mirror job is running? > Get the job's status? Block jobs have an error status which is exposed via QMP. The block job emits a QMP event notifying the client. If the client issues query-block-jobs it will also see the iostatus field. I'm not aware of an internal API to monitor QMP events. It would be possible to add it but first I wonder why you want to use mirror? > > In practice I think this is similar to a successful failover followed by > > immediately getting I/O errors on the new Primary Disk. It means that > > right after failover there is another failure and the system may not be > > able to continue. > > Block replication is not designed for such case. For example, we don't do > failover on primary disk's failure. In such case, we just report the error > to the disk layer(It is the case 1 in the above Failure Handling). > > Sorry for the late reply. Your mail is sent at 2015-12-23, but I receive > it at 2016-01-04 What is supposed to happen when flushing the Disk Buffer into the Secondary Disk fails? Stefan signature.asc Description: PGP signature
Re: [Qemu-block] [Patch v12 resend 05/10] docs: block replication's description
* Stefan Hajnoczi (stefa...@redhat.com) wrote: > On Wed, Dec 02, 2015 at 01:31:46PM +0800, Wen Congyang wrote: > > +== Failure Handling == > > +There are 6 internal errors when block replication is running: > > +1. I/O error on primary disk > > +2. Forwarding primary write requests failed > > +3. Backup failed > > +4. I/O error on secondary disk > > +5. I/O error on active disk > > +6. Making active disk or hidden disk empty failed > > +In case 1 and 5, we just report the error to the disk layer. In case 2, 3, > > +4 and 6, we just report block replication's error to FT/HA manager (which > > +decides when to do a new checkpoint, when to do failover). > > +There is no internal error when doing failover. > > Not sure this is true. > > Below it says the following for failover: "We will flush the Disk buffer > into Secondary Disk and stop block replication". Flushing the disk > buffer can result in I/O errors. This means that failover operations > are not guaranteed to succeed. > > In practice I think this is similar to a successful failover followed by > immediately getting I/O errors on the new Primary Disk. It means that > right after failover there is another failure and the system may not be > able to continue. Yes, I think that's true. > So this really only matters in the case where there is a new Secondary > ready after failover. In that case the user might expect failover to > continue to the new Secondary (Host 3): > >[X][X] > Host 1 <-> Host 2 <-> Host 3 Since COLO is just doing a 1+1 redundency, I think it's not expecting to cope with a double host failure; it's going to take some time (seconds?) to sync Host 3 back in when you add it after a failover and the aim would be not to have distrubed the application for that long, so it should already be running on Host 2 during that resync. Dave -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
Re: [Qemu-block] [Patch v12 resend 05/10] docs: block replication's description
On 12/23/2015 05:26 PM, Stefan Hajnoczi wrote: > On Wed, Dec 02, 2015 at 01:31:46PM +0800, Wen Congyang wrote: >> +== Failure Handling == >> +There are 6 internal errors when block replication is running: >> +1. I/O error on primary disk >> +2. Forwarding primary write requests failed >> +3. Backup failed >> +4. I/O error on secondary disk >> +5. I/O error on active disk >> +6. Making active disk or hidden disk empty failed >> +In case 1 and 5, we just report the error to the disk layer. In case 2, 3, >> +4 and 6, we just report block replication's error to FT/HA manager (which >> +decides when to do a new checkpoint, when to do failover). >> +There is no internal error when doing failover. > > Not sure this is true. > > Below it says the following for failover: "We will flush the Disk buffer > into Secondary Disk and stop block replication". Flushing the disk > buffer can result in I/O errors. This means that failover operations > are not guaranteed to succeed. We don't use mirror job now. We may use it in the next version. Is there any way to know the I/O error when the mirror job is running? Get the job's status? > > In practice I think this is similar to a successful failover followed by > immediately getting I/O errors on the new Primary Disk. It means that > right after failover there is another failure and the system may not be > able to continue. Block replication is not designed for such case. For example, we don't do failover on primary disk's failure. In such case, we just report the error to the disk layer(It is the case 1 in the above Failure Handling). Sorry for the late reply. Your mail is sent at 2015-12-23, but I receive it at 2016-01-04 > > So this really only matters in the case where there is a new Secondary > ready after failover. In that case the user might expect failover to > continue to the new Secondary (Host 3): > >[X][X] > Host 1 <-> Host 2 <-> Host 3 >
Re: [Qemu-block] [Patch v12 resend 05/10] docs: block replication's description
On Wed, Dec 02, 2015 at 01:31:46PM +0800, Wen Congyang wrote: > +== Failure Handling == > +There are 6 internal errors when block replication is running: > +1. I/O error on primary disk > +2. Forwarding primary write requests failed > +3. Backup failed > +4. I/O error on secondary disk > +5. I/O error on active disk > +6. Making active disk or hidden disk empty failed > +In case 1 and 5, we just report the error to the disk layer. In case 2, 3, > +4 and 6, we just report block replication's error to FT/HA manager (which > +decides when to do a new checkpoint, when to do failover). > +There is no internal error when doing failover. Not sure this is true. Below it says the following for failover: "We will flush the Disk buffer into Secondary Disk and stop block replication". Flushing the disk buffer can result in I/O errors. This means that failover operations are not guaranteed to succeed. In practice I think this is similar to a successful failover followed by immediately getting I/O errors on the new Primary Disk. It means that right after failover there is another failure and the system may not be able to continue. So this really only matters in the case where there is a new Secondary ready after failover. In that case the user might expect failover to continue to the new Secondary (Host 3): [X][X] Host 1 <-> Host 2 <-> Host 3 signature.asc Description: PGP signature
