[Qemu-devel] [PATCH v6 0/2] add read-pattern for block qourum

[Qemu-devel] [PATCH v6 0/2] add read-pattern for block qourum

[Liu Yuan]

v6:
 - fix a unused warning introduced by last version

v5:
 - simplify a for loop in quorum_aio_finalize()

v4:
 - swap the patch order
 - update comment for fifo pattern in qaip
 - use qapi enumeration in quorum driver instead of manual parsing

v3:
 - separate patch into two, one for quorum and one for qapi for easier review
 - add enumeration for quorum read pattern
 - remove unrelated blank line fix from this patch set

v2:
 - rename single as 'fifo'
 - rename read_all_children as read_quorum_children
 - fix quorum_aio_finalize() for fifo pattern

This patch adds single read pattern to quorum driver and quorum vote is default
pattern.

For now we do a quorum vote on all the reads, it is designed for unreliable
underlying storage such as non-redundant NFS to make sure data integrity at the
cost of the read performance.

For some use cases as following:

        VM
  --------------
  |            |
  v            v
  A            B

Both A and B has hardware raid storage to justify the data integrity on its own.
So it would help performance if we do a single read instead of on all the nodes.
Further, if we run VM on either of the storage node, we can make a local read
request for better performance.

This patch generalize the above 2 nodes case in the N nodes. That is,

vm -> write to all the N nodes, read just one of them. If single read fails, we
try to read next node in FIFO order specified by the startup command.

The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
functionality in the single device/node failure for now. But compared with DRBD
we still have some advantages over it:

- Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
storage. And if A crashes, we need to restart all the VMs on node B. But for
practice case, we can't because B might not have enough resources to setup 20 VMs
at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
images over the data center, we can very likely restart 20 VMs without any
resource problem.

After all, I think we can build a more powerful replicated image functionality
on quorum and block jobs(block mirror) to meet various High Availibility needs.

E.g, Enable single read pattern on 2 children,
  
-drive driver=quorum,children.0.file.filename=0.qcow2,\
children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1

[1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device

Cc: Benoit Canet <benoit@irqsave.net>
Cc: Eric Blake <eblake@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Liu Yuan (2):
  qapi: add read-pattern enum for quorum
  block/quorum: add simple read pattern support

 block/quorum.c       | 177 +++++++++++++++++++++++++++++++++++++--------------
 qapi/block-core.json |  20 +++++-
 2 files changed, 148 insertions(+), 49 deletions(-)

-- 
1.9.1

[Qemu-devel] [PATCH v6 1/2] qapi: add read-pattern enum for quorum

[Liu Yuan]

Cc: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
---
 qapi/block-core.json | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index e378653..42033d9 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1384,6 +1384,19 @@
             'raw': 'BlockdevRef' } }
 
 ##
+# @QuorumReadPattern
+#
+# An enumeration of quorum read patterns.
+#
+# @quorum: read all the children and do a quorum vote on reads
+#
+# @fifo: read only from the first child that has not failed
+#
+# Since: 2.2
+##
+{ 'enum': 'QuorumReadPattern', 'data': [ 'quorum', 'fifo' ] }
+
+##
 # @BlockdevOptionsQuorum
 #
 # Driver specific block device options for Quorum
@@ -1398,12 +1411,17 @@
 # @rewrite-corrupted: #optional rewrite corrupted data when quorum is reached
 #                     (Since 2.1)
 #
+# @read-pattern: #optional choose read pattern and set to quorum by default
+#                (Since 2.2)
+#
 # Since: 2.0
 ##
 { 'type': 'BlockdevOptionsQuorum',
   'data': { '*blkverify': 'bool',
             'children': [ 'BlockdevRef' ],
-            'vote-threshold': 'int', '*rewrite-corrupted': 'bool' } }
+            'vote-threshold': 'int',
+            '*rewrite-corrupted': 'bool',
+            '*read-pattern': 'QuorumReadPattern' } }
 
 ##
 # @BlockdevOptions
-- 
1.9.1

[Qemu-devel] [PATCH v6 2/2] block/quorum: add simple read pattern support

[Liu Yuan]

This patch adds single read pattern to quorum driver and quorum vote is default
pattern.

For now we do a quorum vote on all the reads, it is designed for unreliable
underlying storage such as non-redundant NFS to make sure data integrity at the
cost of the read performance.

For some use cases as following:

        VM
  --------------
  |            |
  v            v
  A            B

Both A and B has hardware raid storage to justify the data integrity on its own.
So it would help performance if we do a single read instead of on all the nodes.
Further, if we run VM on either of the storage node, we can make a local read
request for better performance.

This patch generalize the above 2 nodes case in the N nodes. That is,

vm -> write to all the N nodes, read just one of them. If single read fails, we
try to read next node in FIFO order specified by the startup command.

The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
functionality in the single device/node failure for now. But compared with DRBD
we still have some advantages over it:

- Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
storage. And if A crashes, we need to restart all the VMs on node B. But for
practice case, we can't because B might not have enough resources to setup 20 VMs
at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
images over the data center, we can very likely restart 20 VMs without any
resource problem.

After all, I think we can build a more powerful replicated image functionality
on quorum and block jobs(block mirror) to meet various High Availibility needs.

E.g, Enable single read pattern on 2 children,

-drive driver=quorum,children.0.file.filename=0.qcow2,\
children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1

[1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device

Cc: Benoit Canet <benoit@irqsave.net>
Cc: Eric Blake <eblake@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
---
 block/quorum.c | 177 +++++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 129 insertions(+), 48 deletions(-)

diff --git a/block/quorum.c b/block/quorum.c
index d5ee9c0..c5f1b35 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -24,6 +24,7 @@
 #define QUORUM_OPT_VOTE_THRESHOLD "vote-threshold"
 #define QUORUM_OPT_BLKVERIFY      "blkverify"
 #define QUORUM_OPT_REWRITE        "rewrite-corrupted"
+#define QUORUM_OPT_READ_PATTERN   "read-pattern"
 
 /* This union holds a vote hash value */
 typedef union QuorumVoteValue {
@@ -74,6 +75,8 @@ typedef struct BDRVQuorumState {
     bool rewrite_corrupted;/* true if the driver must rewrite-on-read corrupted
                             * block if Quorum is reached.
                             */
+
+    QuorumReadPattern read_pattern;
 } BDRVQuorumState;
 
 typedef struct QuorumAIOCB QuorumAIOCB;
@@ -117,6 +120,7 @@ struct QuorumAIOCB {
 
     bool is_read;
     int vote_ret;
+    int child_iter;             /* which child to read in fifo pattern */
 };
 
 static bool quorum_vote(QuorumAIOCB *acb);
@@ -143,7 +147,6 @@ static AIOCBInfo quorum_aiocb_info = {
 
 static void quorum_aio_finalize(QuorumAIOCB *acb)
 {
-    BDRVQuorumState *s = acb->common.bs->opaque;
     int i, ret = 0;
 
     if (acb->vote_ret) {
@@ -153,7 +156,8 @@ static void quorum_aio_finalize(QuorumAIOCB *acb)
     acb->common.cb(acb->common.opaque, ret);
 
     if (acb->is_read) {
-        for (i = 0; i < s->num_children; i++) {
+        /* on the quorum case acb->child_iter == s->num_children - 1 */
+        for (i = 0; i <= acb->child_iter; i++) {
             qemu_vfree(acb->qcrs[i].buf);
             qemu_iovec_destroy(&acb->qcrs[i].qiov);
         }
@@ -256,6 +260,21 @@ static void quorum_rewrite_aio_cb(void *opaque, int ret)
     quorum_aio_finalize(acb);
 }
 
+static BlockDriverAIOCB *read_fifo_child(QuorumAIOCB *acb);
+
+static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
+{
+    int i;
+    assert(dest->niov == source->niov);
+    assert(dest->size == source->size);
+    for (i = 0; i < source->niov; i++) {
+        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
+        memcpy(dest->iov[i].iov_base,
+               source->iov[i].iov_base,
+               source->iov[i].iov_len);
+    }
+}
+
 static void quorum_aio_cb(void *opaque, int ret)
 {
     QuorumChildRequest *sacb = opaque;
@@ -263,6 +282,21 @@ static void quorum_aio_cb(void *opaque, int ret)
     BDRVQuorumState *s = acb->common.bs->opaque;
     bool rewrite = false;
 
+    if (acb->is_read && s->read_pattern == QUORUM_READ_PATTERN_FIFO) {
+        /* We try to read next child in FIFO order if we fail to read */
+        if (ret < 0 && ++acb->child_iter < s->num_children) {
+            read_fifo_child(acb);
+            return;
+        }
+
+        if (ret == 0) {
+            quorum_copy_qiov(acb->qiov, &acb->qcrs[acb->child_iter].qiov);
+        }
+        acb->vote_ret = ret;
+        quorum_aio_finalize(acb);
+        return;
+    }
+
     sacb->ret = ret;
     acb->count++;
     if (ret == 0) {
@@ -343,19 +377,6 @@ static bool quorum_rewrite_bad_versions(BDRVQuorumState *s, QuorumAIOCB *acb,
     return count;
 }
 
-static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
-{
-    int i;
-    assert(dest->niov == source->niov);
-    assert(dest->size == source->size);
-    for (i = 0; i < source->niov; i++) {
-        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
-        memcpy(dest->iov[i].iov_base,
-               source->iov[i].iov_base,
-               source->iov[i].iov_len);
-    }
-}
-
 static void quorum_count_vote(QuorumVotes *votes,
                               QuorumVoteValue *value,
                               int index)
@@ -615,34 +636,62 @@ free_exit:
     return rewrite;
 }
 
-static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
-                                         int64_t sector_num,
-                                         QEMUIOVector *qiov,
-                                         int nb_sectors,
-                                         BlockDriverCompletionFunc *cb,
-                                         void *opaque)
+static BlockDriverAIOCB *read_quorum_children(QuorumAIOCB *acb)
 {
-    BDRVQuorumState *s = bs->opaque;
-    QuorumAIOCB *acb = quorum_aio_get(s, bs, qiov, sector_num,
-                                      nb_sectors, cb, opaque);
+    BDRVQuorumState *s = acb->common.bs->opaque;
     int i;
 
-    acb->is_read = true;
-
     for (i = 0; i < s->num_children; i++) {
-        acb->qcrs[i].buf = qemu_blockalign(s->bs[i], qiov->size);
-        qemu_iovec_init(&acb->qcrs[i].qiov, qiov->niov);
-        qemu_iovec_clone(&acb->qcrs[i].qiov, qiov, acb->qcrs[i].buf);
+        acb->qcrs[i].buf = qemu_blockalign(s->bs[i], acb->qiov->size);
+        qemu_iovec_init(&acb->qcrs[i].qiov, acb->qiov->niov);
+        qemu_iovec_clone(&acb->qcrs[i].qiov, acb->qiov, acb->qcrs[i].buf);
     }
 
     for (i = 0; i < s->num_children; i++) {
-        bdrv_aio_readv(s->bs[i], sector_num, &acb->qcrs[i].qiov, nb_sectors,
-                       quorum_aio_cb, &acb->qcrs[i]);
+        bdrv_aio_readv(s->bs[i], acb->sector_num, &acb->qcrs[i].qiov,
+                       acb->nb_sectors, quorum_aio_cb, &acb->qcrs[i]);
     }
 
     return &acb->common;
 }
 
+static BlockDriverAIOCB *read_fifo_child(QuorumAIOCB *acb)
+{
+    BDRVQuorumState *s = acb->common.bs->opaque;
+
+    acb->qcrs[acb->child_iter].buf = qemu_blockalign(s->bs[acb->child_iter],
+                                                     acb->qiov->size);
+    qemu_iovec_init(&acb->qcrs[acb->child_iter].qiov, acb->qiov->niov);
+    qemu_iovec_clone(&acb->qcrs[acb->child_iter].qiov, acb->qiov,
+                     acb->qcrs[acb->child_iter].buf);
+    bdrv_aio_readv(s->bs[acb->child_iter], acb->sector_num,
+                   &acb->qcrs[acb->child_iter].qiov, acb->nb_sectors,
+                   quorum_aio_cb, &acb->qcrs[acb->child_iter]);
+
+    return &acb->common;
+}
+
+static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
+                                          int64_t sector_num,
+                                          QEMUIOVector *qiov,
+                                          int nb_sectors,
+                                          BlockDriverCompletionFunc *cb,
+                                          void *opaque)
+{
+    BDRVQuorumState *s = bs->opaque;
+    QuorumAIOCB *acb = quorum_aio_get(s, bs, qiov, sector_num,
+                                      nb_sectors, cb, opaque);
+    acb->is_read = true;
+
+    if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
+        acb->child_iter = s->num_children - 1;
+        return read_quorum_children(acb);
+    }
+
+    acb->child_iter = 0;
+    return read_fifo_child(acb);
+}
+
 static BlockDriverAIOCB *quorum_aio_writev(BlockDriverState *bs,
                                           int64_t sector_num,
                                           QEMUIOVector *qiov,
@@ -782,10 +831,33 @@ static QemuOptsList quorum_runtime_opts = {
             .type = QEMU_OPT_BOOL,
             .help = "Rewrite corrupted block on read quorum",
         },
+        {
+            .name = QUORUM_OPT_READ_PATTERN,
+            .type = QEMU_OPT_STRING,
+            .help = "Allowed pattern: quorum, fifo. Quorum is default",
+        },
         { /* end of list */ }
     },
 };
 
+static int parse_read_pattern(const char *opt)
+{
+    int i;
+
+    if (!opt) {
+        /* Set quorum as default */
+        return QUORUM_READ_PATTERN_QUORUM;
+    }
+
+    for (i = 0; i < QUORUM_READ_PATTERN_MAX; i++) {
+        if (!strcmp(opt, QuorumReadPattern_lookup[i])) {
+            return i;
+        }
+    }
+
+    return -EINVAL;
+}
+
 static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
                        Error **errp)
 {
@@ -827,28 +899,37 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     s->threshold = qemu_opt_get_number(opts, QUORUM_OPT_VOTE_THRESHOLD, 0);
-
-    /* and validate it against s->num_children */
-    ret = quorum_valid_threshold(s->threshold, s->num_children, &local_err);
+    ret = parse_read_pattern(qemu_opt_get(opts, QUORUM_OPT_READ_PATTERN));
     if (ret < 0) {
+        error_setg(&local_err, "Please set read-pattern as fifo or quorum\n");
         goto exit;
     }
+    s->read_pattern = ret;
 
-    /* is the driver in blkverify mode */
-    if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
-        s->num_children == 2 && s->threshold == 2) {
-        s->is_blkverify = true;
-    } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
-        fprintf(stderr, "blkverify mode is set by setting blkverify=on "
-                "and using two files with vote_threshold=2\n");
-    }
+    if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
+        /* and validate it against s->num_children */
+        ret = quorum_valid_threshold(s->threshold, s->num_children, &local_err);
+        if (ret < 0) {
+            goto exit;
+        }
 
-    s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE, false);
-    if (s->rewrite_corrupted && s->is_blkverify) {
-        error_setg(&local_err,
-                   "rewrite-corrupted=on cannot be used with blkverify=on");
-        ret = -EINVAL;
-        goto exit;
+        /* is the driver in blkverify mode */
+        if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
+            s->num_children == 2 && s->threshold == 2) {
+            s->is_blkverify = true;
+        } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
+            fprintf(stderr, "blkverify mode is set by setting blkverify=on "
+                    "and using two files with vote_threshold=2\n");
+        }
+
+        s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE,
+                                                 false);
+        if (s->rewrite_corrupted && s->is_blkverify) {
+            error_setg(&local_err,
+                       "rewrite-corrupted=on cannot be used with blkverify=on");
+            ret = -EINVAL;
+            goto exit;
+        }
     }
 
     /* allocate the children BlockDriverState array */
-- 
1.9.1

Re: [Qemu-devel] [PATCH v6 0/2] add read-pattern for block qourum

[Liu Yuan]

On Mon, Aug 18, 2014 at 05:41:03PM +0800, Liu Yuan wrote:
> v6:
>  - fix a unused warning introduced by last version
> 

Hi Stefan and Kevin,

   Benoît Canet has added Reviewed-by tag to both patches, could one of you pick
this patch set?

Thanks
Yuan

Re: [Qemu-devel] [PATCH v6 0/2] add read-pattern for block qourum

[Liu Yuan]

On Mon, Aug 25, 2014 at 10:46:21AM +0800, Liu Yuan wrote:
> On Mon, Aug 18, 2014 at 05:41:03PM +0800, Liu Yuan wrote:
> > v6:
> >  - fix a unused warning introduced by last version
> > 
> 
> Hi Stefan and Kevin,
> 
>    Benoît Canet has added Reviewed-by tag to both patches, could one of you pick
> this patch set?
> 

ping....

Re: [Qemu-devel] [PATCH v6 0/2] add read-pattern for block qourum

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 3497 bytes --]

On Mon, Aug 18, 2014 at 05:41:03PM +0800, Liu Yuan wrote:
> v6:
>  - fix a unused warning introduced by last version
> 
> v5:
>  - simplify a for loop in quorum_aio_finalize()
> 
> v4:
>  - swap the patch order
>  - update comment for fifo pattern in qaip
>  - use qapi enumeration in quorum driver instead of manual parsing
> 
> v3:
>  - separate patch into two, one for quorum and one for qapi for easier review
>  - add enumeration for quorum read pattern
>  - remove unrelated blank line fix from this patch set
> 
> v2:
>  - rename single as 'fifo'
>  - rename read_all_children as read_quorum_children
>  - fix quorum_aio_finalize() for fifo pattern
> 
> This patch adds single read pattern to quorum driver and quorum vote is default
> pattern.
> 
> For now we do a quorum vote on all the reads, it is designed for unreliable
> underlying storage such as non-redundant NFS to make sure data integrity at the
> cost of the read performance.
> 
> For some use cases as following:
> 
>         VM
>   --------------
>   |            |
>   v            v
>   A            B
> 
> Both A and B has hardware raid storage to justify the data integrity on its own.
> So it would help performance if we do a single read instead of on all the nodes.
> Further, if we run VM on either of the storage node, we can make a local read
> request for better performance.
> 
> This patch generalize the above 2 nodes case in the N nodes. That is,
> 
> vm -> write to all the N nodes, read just one of them. If single read fails, we
> try to read next node in FIFO order specified by the startup command.
> 
> The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
> functionality in the single device/node failure for now. But compared with DRBD
> we still have some advantages over it:
> 
> - Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
> storage. And if A crashes, we need to restart all the VMs on node B. But for
> practice case, we can't because B might not have enough resources to setup 20 VMs
> at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
> images over the data center, we can very likely restart 20 VMs without any
> resource problem.
> 
> After all, I think we can build a more powerful replicated image functionality
> on quorum and block jobs(block mirror) to meet various High Availibility needs.
> 
> E.g, Enable single read pattern on 2 children,
>   
> -drive driver=quorum,children.0.file.filename=0.qcow2,\
> children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1
> 
> [1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device
> 
> Cc: Benoit Canet <benoit@irqsave.net>
> Cc: Eric Blake <eblake@redhat.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Liu Yuan (2):
>   qapi: add read-pattern enum for quorum
>   block/quorum: add simple read pattern support
> 
>  block/quorum.c       | 177 +++++++++++++++++++++++++++++++++++++--------------
>  qapi/block-core.json |  20 +++++-
>  2 files changed, 148 insertions(+), 49 deletions(-)

I dropped the \n from the error_setg() error message while merging.
Please do not use \n with error_setg().

Please extend the quorum qemu-iotests to cover the new fifo read
pattern.  You can send the tests as a separate patch series.

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

[-- Attachment #2: Type: application/pgp-signature, Size: 473 bytes --]

Re: [Qemu-devel] [PATCH v6 0/2] add read-pattern for block qourum

[Liu Yuan]

On Thu, Aug 28, 2014 at 11:33:08AM +0100, Stefan Hajnoczi wrote:
> On Mon, Aug 18, 2014 at 05:41:03PM +0800, Liu Yuan wrote:
> > v6:
> >  - fix a unused warning introduced by last version
> > 
> > v5:
> >  - simplify a for loop in quorum_aio_finalize()
> > 
> > v4:
> >  - swap the patch order
> >  - update comment for fifo pattern in qaip
> >  - use qapi enumeration in quorum driver instead of manual parsing
> > 
> > v3:
> >  - separate patch into two, one for quorum and one for qapi for easier review
> >  - add enumeration for quorum read pattern
> >  - remove unrelated blank line fix from this patch set
> > 
> > v2:
> >  - rename single as 'fifo'
> >  - rename read_all_children as read_quorum_children
> >  - fix quorum_aio_finalize() for fifo pattern
> > 
> > This patch adds single read pattern to quorum driver and quorum vote is default
> > pattern.
> > 
> > For now we do a quorum vote on all the reads, it is designed for unreliable
> > underlying storage such as non-redundant NFS to make sure data integrity at the
> > cost of the read performance.
> > 
> > For some use cases as following:
> > 
> >         VM
> >   --------------
> >   |            |
> >   v            v
> >   A            B
> > 
> > Both A and B has hardware raid storage to justify the data integrity on its own.
> > So it would help performance if we do a single read instead of on all the nodes.
> > Further, if we run VM on either of the storage node, we can make a local read
> > request for better performance.
> > 
> > This patch generalize the above 2 nodes case in the N nodes. That is,
> > 
> > vm -> write to all the N nodes, read just one of them. If single read fails, we
> > try to read next node in FIFO order specified by the startup command.
> > 
> > The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
> > functionality in the single device/node failure for now. But compared with DRBD
> > we still have some advantages over it:
> > 
> > - Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
> > storage. And if A crashes, we need to restart all the VMs on node B. But for
> > practice case, we can't because B might not have enough resources to setup 20 VMs
> > at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
> > images over the data center, we can very likely restart 20 VMs without any
> > resource problem.
> > 
> > After all, I think we can build a more powerful replicated image functionality
> > on quorum and block jobs(block mirror) to meet various High Availibility needs.
> > 
> > E.g, Enable single read pattern on 2 children,
> >   
> > -drive driver=quorum,children.0.file.filename=0.qcow2,\
> > children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1
> > 
> > [1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device
> > 
> > Cc: Benoit Canet <benoit@irqsave.net>
> > Cc: Eric Blake <eblake@redhat.com>
> > Cc: Kevin Wolf <kwolf@redhat.com>
> > Cc: Stefan Hajnoczi <stefanha@redhat.com>
> > Liu Yuan (2):
> >   qapi: add read-pattern enum for quorum
> >   block/quorum: add simple read pattern support
> > 
> >  block/quorum.c       | 177 +++++++++++++++++++++++++++++++++++++--------------
> >  qapi/block-core.json |  20 +++++-
> >  2 files changed, 148 insertions(+), 49 deletions(-)
> 
> I dropped the \n from the error_setg() error message while merging.
> Please do not use \n with error_setg().

Thanks for your fix.
 
> Please extend the quorum qemu-iotests to cover the new fifo read
> pattern.  You can send the tests as a separate patch series.
> 

Okay, will do later.

Yuan