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From: | Wen Congyang |
Subject: | Re: [Qemu-devel] [Qemu-block] [PATCH COLO-Block v6 07/16] Add new block driver interface to connect/disconnect the remote target |
Date: | Mon, 22 Jun 2015 21:56:24 +0800 |
User-agent: | Mozilla/5.0 (Windows NT 6.2; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 |
At 2015/6/22 20:39, Stefan Hajnoczi Wrote:
On Sat, Jun 20, 2015 at 11:31:52AM +0800, Wen Congyang wrote:At 2015/6/19 18:49, Stefan Hajnoczi Wrote:On Fri, Jun 19, 2015 at 08:54:56AM +0800, Wen Congyang wrote:On 06/19/2015 12:06 AM, Stefan Hajnoczi wrote:On Thu, Jun 18, 2015 at 10:36:39PM +0800, Wen Congyang wrote:At 2015/6/18 20:55, Stefan Hajnoczi Wrote:On Thu, Jun 18, 2015 at 04:49:12PM +0800, Wen Congyang wrote:+void bdrv_connect(BlockDriverState *bs, Error **errp) +{ + BlockDriver *drv = bs->drv; + + if (drv && drv->bdrv_connect) { + drv->bdrv_connect(bs, errp); + } else if (bs->file) { + bdrv_connect(bs->file, errp); + } else { + error_setg(errp, "this feature or command is not currently supported"); + } +} + +void bdrv_disconnect(BlockDriverState *bs) +{ + BlockDriver *drv = bs->drv; + + if (drv && drv->bdrv_disconnect) { + drv->bdrv_disconnect(bs); + } else if (bs->file) { + bdrv_disconnect(bs->file); + } +}Please add doc comments describing the semantics of these commands.Where should it be documented? In the header file?block.h doesn't document prototypes in the header file, please document the function definition in block.c. (QEMU is not consistent here, some places do it the other way around.)Why are these operations needed when there is already a bs->drv == NULL case which means the BDS is not ready for read/write?The purpos is that: don't connect to nbd server when opening a nbd client. connect/disconnect to nbd server when we need to do it. IIUC, if bs->drv is NULL, it means that the driver is ejected? Here, connect/disconnect means that connect/disconnect to remote target(The target may be in another host).Connect/disconnect puts something on the QEMU command-line that isn't ready at startup time. How about using monitor commands to add objects when needed instead? That is cleaner because it doesn't introduce a new state (which is only implemented for nbd).The problem is that, nbd client is one child of quorum, and quorum must have more than one child. The nbd server is not ready until colo is running.A monitor command to hot add/remove quorum children solves this problem and could also be used in other scenarios (e.g. user decides to take a quorum child offline).For replication case, we always do checkpoint again and again after migration. If the disk is not synced before migration, we will use disk mirgation or mirror job to sync it. So we cannot start block replication when migration is running. We need that nbd client is not ready when migration is running, and it is ready between migration ends and checkpoint begins. Using a monotir command add the nbd client will cause larger downtime. So if the nbd client has been added(only not connect to the nbd server), we can connect to nbd server automatically.I'm not sure I understand you. Can you rephrase this?
We only use block replication after migration because we assume that the disk has been synced before block replication is started, so we only forward new write requests via nbd.
The NBD connect should not be performed during downtime, regardless of whether a monitor command is used or not.
Why? Thanks Wen Congyang
Stefan
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