[Qemu-devel] [PATCH v3 0/2] add read-pattern for block qourum

[Qemu-devel] [PATCH v3 0/2] add read-pattern for block qourum

[Liu Yuan]

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=single,vote-threshold=1

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

Liu Yuan (2):
  block/quorum: add simple read pattern support
  qapi: add read-pattern support for quorum

 block/quorum.c       | 181 +++++++++++++++++++++++++++++++++++++--------------
 qapi/block-core.json |  19 +++++-
 2 files changed, 149 insertions(+), 51 deletions(-)

-- 
1.9.1

[Qemu-devel] [PATCH v3 1/2] 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=single,vote-threshold=1

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

Cc: Benoit Canet <benoit@irqsave.net>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
---
 block/quorum.c | 181 +++++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 131 insertions(+), 50 deletions(-)

diff --git a/block/quorum.c b/block/quorum.c
index d5ee9c0..1443fc5 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 +75,16 @@ typedef struct BDRVQuorumState {
     bool rewrite_corrupted;/* true if the driver must rewrite-on-read corrupted
                             * block if Quorum is reached.
                             */
+
+#define READ_PATTERN_QUORUM 0 /* default */
+#define READ_PATTERN_FIFO   1
+    int read_pattern;      /* fifo: read a single child and try first one
+                            *         first. If error, try next child in an
+                            *         FIFO order specifed by command line.
+                            *         Return error if no child read succeeds.
+                            * quorum: read all the children and do a quorum
+                            *         vote on reads.
+                            */
 } BDRVQuorumState;
 
 typedef struct QuorumAIOCB QuorumAIOCB;
@@ -117,6 +128,7 @@ struct QuorumAIOCB {
 
     bool is_read;
     int vote_ret;
+    int child_iter;             /* which child to read in fifo pattern */
 };
 
 static bool quorum_vote(QuorumAIOCB *acb);
@@ -154,8 +166,10 @@ static void quorum_aio_finalize(QuorumAIOCB *acb)
 
     if (acb->is_read) {
         for (i = 0; i < s->num_children; i++) {
-            qemu_vfree(acb->qcrs[i].buf);
-            qemu_iovec_destroy(&acb->qcrs[i].qiov);
+            if (i <= acb->child_iter) {
+                qemu_vfree(acb->qcrs[i].buf);
+                qemu_iovec_destroy(&acb->qcrs[i].qiov);
+            }
         }
     }
 
@@ -256,6 +270,21 @@ 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 +292,21 @@ 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 == 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 +387,6 @@ 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 +646,62 @@ 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 == 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,
@@ -782,6 +841,11 @@ 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 */ }
     },
 };
@@ -796,6 +860,7 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
     QDict *sub = NULL;
     QList *list = NULL;
     const QListEntry *lentry;
+    const char *read_pattern;
     int i;
     int ret = 0;
 
@@ -827,28 +892,44 @@ 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);
-    if (ret < 0) {
-        goto exit;
+    read_pattern = qemu_opt_get(opts, QUORUM_OPT_READ_PATTERN);
+    if (read_pattern) {
+        if (strcmp(read_pattern, "fifo") == 0) {
+            s->read_pattern = READ_PATTERN_FIFO;
+        } else if (strcmp(read_pattern, "quorum") == 0) {
+            s->read_pattern = READ_PATTERN_QUORUM;
+        } else {
+            error_setg(&local_err,
+                       "Please set read-pattern as fifo or quorum\n");
+            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");
-    }
+    if (s->read_pattern == 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 v3 1/2] block/quorum: add simple read pattern support

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 2582 bytes --]

On 07/15/2014 12:34 AM, Liu Yuan wrote:
> 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.
> 

>                              */
> +
> +#define READ_PATTERN_QUORUM 0 /* default */
> +#define READ_PATTERN_FIFO   1
> +    int read_pattern;      /* fifo: read a single child and try first one
> +                            *         first. If error, try next child in an
> +                            *         FIFO order specifed by command line.
> +                            *         Return error if no child read succeeds.
> +                            * quorum: read all the children and do a quorum
> +                            *         vote on reads.
> +                            */

Please swap the order of your two patches, or squash them back into one.
 If you define the qapi enum first, then you don't need to open-code
these #defines; instead, the qapi code will generate a typedef that puts
the C enum QuorumReadPattern into play, so that this declaration becomes
'QuorumReadPattern read_pattern;'...


> @@ -263,6 +292,21 @@ 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 == READ_PATTERN_FIFO) {

...and code like this can compare against the constant
QUORUM_READ_PATTERN_FIFO (which will be generated for you from the .json
file)...


> +    read_pattern = qemu_opt_get(opts, QUORUM_OPT_READ_PATTERN);
> +    if (read_pattern) {
> +        if (strcmp(read_pattern, "fifo") == 0) {
> +            s->read_pattern = READ_PATTERN_FIFO;
> +        } else if (strcmp(read_pattern, "quorum") == 0) {
> +            s->read_pattern = READ_PATTERN_QUORUM;
> +        } else {
> +            error_setg(&local_err,
> +                       "Please set read-pattern as fifo or quorum\n");
> +            ret = -EINVAL;
> +            goto exit;
> +        }

...and instead of open-coding this, you should use parse_enum_option().
Also, by using the generated list here, if you later add a third read
pattern, then this parser code doesn't have to change, but already
automatically covers all valid options encoded in the qapi list.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH v3 1/2] block/quorum: add simple read pattern support

[Liu Yuan]

On Tue, Jul 15, 2014 at 06:14:32AM -0600, Eric Blake wrote:
> On 07/15/2014 12:34 AM, Liu Yuan wrote:
> > 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.
> > 
> 
> >                              */
> > +
> > +#define READ_PATTERN_QUORUM 0 /* default */
> > +#define READ_PATTERN_FIFO   1
> > +    int read_pattern;      /* fifo: read a single child and try first one
> > +                            *         first. If error, try next child in an
> > +                            *         FIFO order specifed by command line.
> > +                            *         Return error if no child read succeeds.
> > +                            * quorum: read all the children and do a quorum
> > +                            *         vote on reads.
> > +                            */
> 
> Please swap the order of your two patches, or squash them back into one.
>  If you define the qapi enum first, then you don't need to open-code
> these #defines; instead, the qapi code will generate a typedef that puts
> the C enum QuorumReadPattern into play, so that this declaration becomes
> 'QuorumReadPattern read_pattern;'...
> 
> 
> > @@ -263,6 +292,21 @@ 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 == READ_PATTERN_FIFO) {
> 
> ...and code like this can compare against the constant
> QUORUM_READ_PATTERN_FIFO (which will be generated for you from the .json
> file)...
> 
> 
> > +    read_pattern = qemu_opt_get(opts, QUORUM_OPT_READ_PATTERN);
> > +    if (read_pattern) {
> > +        if (strcmp(read_pattern, "fifo") == 0) {
> > +            s->read_pattern = READ_PATTERN_FIFO;
> > +        } else if (strcmp(read_pattern, "quorum") == 0) {
> > +            s->read_pattern = READ_PATTERN_QUORUM;
> > +        } else {
> > +            error_setg(&local_err,
> > +                       "Please set read-pattern as fifo or quorum\n");
> > +            ret = -EINVAL;
> > +            goto exit;
> > +        }
> 
> ...and instead of open-coding this, you should use parse_enum_option().
> Also, by using the generated list here, if you later add a third read
> pattern, then this parser code doesn't have to change, but already
> automatically covers all valid options encoded in the qapi list.
> 

That's awesome, thanks for your pointers, Eric.

Yuan

[Qemu-devel] [PATCH v3 2/2] qapi: add read-pattern support for quorum

[Liu Yuan]

Cc: Eric Blake <eblake@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
---
 qapi/block-core.json | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index e378653..22491bc 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1384,6 +1384,18 @@
             'raw': 'BlockdevRef' } }
 
 ##
+# @QuorumReadPattern
+# An enumeration of quorum read pattern.
+#
+# @quorum: read all the children and do a quorum vote on reads
+#
+# @fifo: read a single child and try next child in FIFO order if read fails.
+#
+# Since: 2.2
+##
+{ 'enum': 'QuorumReadPattern', 'data': [ 'quorum', 'fifo' ] }
+
+##
 # @BlockdevOptionsQuorum
 #
 # Driver specific block device options for Quorum
@@ -1398,12 +1410,17 @@
 # @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

Re: [Qemu-devel] [PATCH v3 2/2] qapi: add read-pattern support for quorum

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 1142 bytes --]

On 07/15/2014 12:34 AM, Liu Yuan wrote:
> Cc: Eric Blake <eblake@redhat.com>
> Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> ---
>  qapi/block-core.json | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index e378653..22491bc 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1384,6 +1384,18 @@
>              'raw': 'BlockdevRef' } }
>  
>  ##
> +# @QuorumReadPattern
> +# An enumeration of quorum read pattern.

s/pattern/patterns/

> +#
> +# @quorum: read all the children and do a quorum vote on reads
> +#
> +# @fifo: read a single child and try next child in FIFO order if read fails.

Inconsistent on whether you are ending a line with '.'.  I'm not quite
sure if this reads well; maybe:

@fifo: read only from the first child that has not failed

I also wonder if 'single' might be a better name than 'fifo', especially
if we later add a 'round-robin' for read load balancing.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH v3 2/2] qapi: add read-pattern support for quorum

[Liu Yuan]

On Tue, Jul 15, 2014 at 06:22:14AM -0600, Eric Blake wrote:
> On 07/15/2014 12:34 AM, Liu Yuan wrote:
> > Cc: Eric Blake <eblake@redhat.com>
> > Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> > ---
> >  qapi/block-core.json | 19 ++++++++++++++++++-
> >  1 file changed, 18 insertions(+), 1 deletion(-)
> > 
> > diff --git a/qapi/block-core.json b/qapi/block-core.json
> > index e378653..22491bc 100644
> > --- a/qapi/block-core.json
> > +++ b/qapi/block-core.json
> > @@ -1384,6 +1384,18 @@
> >              'raw': 'BlockdevRef' } }
> >  
> >  ##
> > +# @QuorumReadPattern
> > +# An enumeration of quorum read pattern.
> 
> s/pattern/patterns/
> 
> > +#
> > +# @quorum: read all the children and do a quorum vote on reads
> > +#
> > +# @fifo: read a single child and try next child in FIFO order if read fails.
> 
> Inconsistent on whether you are ending a line with '.'.  I'm not quite
> sure if this reads well; maybe:
> 
> @fifo: read only from the first child that has not failed
> 
> I also wonder if 'single' might be a better name than 'fifo', especially
> if we later add a 'round-robin' for read load balancing.

Both fifo and round-robin in our context indicate a single read.

but either one looks okay to me. We need to reach agreement on the name before I
send the next version, single or fifo or any other suggested, Benoit, Kevin,
Stefan?

Thanks
Yuan

Re: [Qemu-devel] [PATCH v3 2/2] qapi: add read-pattern support for quorum

[Liu Yuan]

On Wed, Jul 16, 2014 at 10:22:20AM +0800, Liu Yuan wrote:
> On Tue, Jul 15, 2014 at 06:22:14AM -0600, Eric Blake wrote:
> > On 07/15/2014 12:34 AM, Liu Yuan wrote:
> > > Cc: Eric Blake <eblake@redhat.com>
> > > Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> > > ---
> > >  qapi/block-core.json | 19 ++++++++++++++++++-
> > >  1 file changed, 18 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/qapi/block-core.json b/qapi/block-core.json
> > > index e378653..22491bc 100644
> > > --- a/qapi/block-core.json
> > > +++ b/qapi/block-core.json
> > > @@ -1384,6 +1384,18 @@
> > >              'raw': 'BlockdevRef' } }
> > >  
> > >  ##
> > > +# @QuorumReadPattern
> > > +# An enumeration of quorum read pattern.
> > 
> > s/pattern/patterns/
> > 
> > > +#
> > > +# @quorum: read all the children and do a quorum vote on reads
> > > +#
> > > +# @fifo: read a single child and try next child in FIFO order if read fails.
> > 
> > Inconsistent on whether you are ending a line with '.'.  I'm not quite
> > sure if this reads well; maybe:
> > 
> > @fifo: read only from the first child that has not failed
> > 
> > I also wonder if 'single' might be a better name than 'fifo', especially
> > if we later add a 'round-robin' for read load balancing.
> 
> Both fifo and round-robin in our context indicate a single read.
> 
> but either one looks okay to me. We need to reach agreement on the name before I
> send the next version, single or fifo or any other suggested, Benoit, Kevin,
> Stefan?

Cc others mentioned.