On Tue, 02/24 15:50, Wen Congyang wrote: > On 02/12/2015 04:44 PM, Fam Zheng wrote: > > On Thu, 02/12 15:40, Wen Congyang wrote: > >> On 02/12/2015 03:21 PM, Fam Zheng wrote: > >>> Hi Congyang, > >>> > >>> On Thu, 02/12 11:07, Wen Congyang wrote: > >>>> +== Workflow == > >>>> +The following is the image of block replication workflow: > >>>> + > >>>> + +----------------------+ +------------------------+ > >>>> + |Primary Write Requests| |Secondary Write Requests| > >>>> + +----------------------+ +------------------------+ > >>>> + | | > >>>> + | (4) > >>>> + | V > >>>> + | /-------------\ > >>>> + | Copy and Forward | | > >>>> + |---------(1)----------+ | Disk Buffer | > >>>> + | | | | > >>>> + | (3) \-------------/ > >>>> + | speculative ^ > >>>> + | write through (2) > >>>> + | | | > >>>> + V V | > >>>> + +--------------+ +----------------+ > >>>> + | Primary Disk | | Secondary Disk | > >>>> + +--------------+ +----------------+ > >>>> + > >>>> + 1) Primary write requests will be copied and forwarded to Secondary > >>>> + QEMU. > >>>> + 2) Before Primary write requests are written to Secondary disk, the > >>>> + original sector content will be read from Secondary disk and > >>>> + buffered in the Disk buffer, but it will not overwrite the > >>>> existing > >>>> + sector content in the Disk buffer. > >>> > >>> I'm a little confused by the tenses ("will be" versus "are") and terms. I > >>> am > >>> reading them as "s/will be/are/g" > >>> > >>> Why do you need this buffer? > >> > >> We only sync the disk till next checkpoint. Before next checkpoint, > >> secondary > >> vm write to the buffer. > >> > >>> > >>> If both primary and secondary write to the same sector, what is saved in > >>> the > >>> buffer? > >> > >> The primary content will be written to the secondary disk, and the > >> secondary content > >> is saved in the buffer. > > > > I wonder if alternatively this is possible with an imaginary "writable > > backing > > image" feature, as described below. > > > > When we have a normal backing chain, > > > > {virtio-blk dev 'foo'} > > | > > | > > | > > [base] <- [mid] <- (foo) > > > > Where [base] and [mid] are read only, (foo) is writable. When we add an > > overlay > > to an existing image on top, > > > > {virtio-blk dev 'foo'} {virtio-blk dev 'bar'} > > | | > > | | > > | | > > [base] <- [mid] <- (foo) <---------------------- (bar) > > > > It's important to make sure that writes to 'foo' doesn't break data for > > 'bar'. > > We can utilize an automatic hidden drive-backup target: > > > > {virtio-blk dev 'foo'} > > {virtio-blk dev 'bar'} > > | > > | > > | > > | > > v > > v > > > > [base] <- [mid] <- (foo) <----------------- (hidden target) > > <--------------- (bar) > > > > v ^ > > v ^ > > v ^ > > v ^ > > >>>> drive-backup sync=none >>>> > > > > So when guest writes to 'foo', the old data is moved to (hidden target), > > which > > remains unchanged from (bar)'s PoV. > > > > The drive in the middle is called hidden because QEMU creates it > > automatically, > > the naming is arbitrary. > > I don't understand this. In which function, the hidden target is created > automatically? >
It's to be determined. This part is only in my mind :) Fam > > > > > It is interesting because it is a more generalized case of image fleecing, > > where the (hidden target) is exposed via NBD server for data scanning (read > > only) purpose. > > > > More interestingly, with above facility, it is also possible to create a > > guest > > visible live snapshot (disk 'bar') of an existing device (disk 'foo') very > > cheaply. Or call it shadow copy if you will. > > > > Back to the COLO case, the configuration will be very similar: > > > > > > {primary wr} > > {secondary vm} > > | > > | > > | > > | > > | > > | > > v > > v > > > > [what] <- [ever] <- (nbd target) <------------ (hidden buf disk) > > <------------- (active disk) > > > > v ^ > > v ^ > > v ^ > > v ^ > > >>>> drive-backup sync=none >>>> > > > > The workflow analogue is: > > > >>>> + 1) Primary write requests will be copied and forwarded to Secondary > >>>> + QEMU. > > > > Primary write requests are forwarded to secondary QEMU as well. > > > >>>> + 2) Before Primary write requests are written to Secondary disk, the > >>>> + original sector content will be read from Secondary disk and > >>>> + buffered in the Disk buffer, but it will not overwrite the > >>>> existing > >>>> + sector content in the Disk buffer. > > > > Before Primary write requests are written to (nbd target), aka the Secondary > > disk, the orignal sector content is read from it and copied to (hidden buf > > disk) by drive-backup. It obviously will not overwrite the data in (active > > disk). > > > >>>> + 3) Primary write requests will be written to Secondary disk. > > > > Primary write requests are written to (nbd target). > > > >>>> + 4) Secondary write requests will be buffered in the Disk buffer and > >>>> it > >>>> + will overwrite the existing sector content in the buffer. > > > > Secondary write request will be written in (active disk) as usual. > > > > Finally, when checkpoint arrives, if you want to sync with primary, just > > drop > > data in (hidden buf disk) and (active disk); when failover happends, if you > > want to promote secondary vm, you can commit (active disk) to (nbd target), > > and > > drop data in (hidden buf disk). > > > > Fam > > . > > >