Re: [Qemu-devel] [PATCH v5 2/2] block/quorum: add simple read pattern support

[Benoît Canet]

The Friday 15 Aug 2014 à 13:05:17 (+0800), 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.
> 
> 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 <address@hidden>
> Cc: Eric Blake <address@hidden>
> Cc: Kevin Wolf <address@hidden>
> Cc: Stefan Hajnoczi <address@hidden>
> Signed-off-by: Liu Yuan <address@hidden>
> ---
>  block/quorum.c | 176 
> ++++++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 129 insertions(+), 47 deletions(-)
> 
> diff --git a/block/quorum.c b/block/quorum.c
> index d5ee9c0..1235d7c 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 */

I don't understand what "fifo pattern" could mean for a bunch of disk
as they are not forming a queue.

Maybe round-robin is more suitable but your code does not implement
round-robin since it will alway start from the first disk.

Your code is scanning the disks set it's a scan pattern.

That said is it a problem that the first disk will be accessed more often than 
the other ?
You will have to care to insert disks in different order on each QEMU to spread 
the load.

Shouldn't the code try to spread the load by circling on the disk like a real 
round robin pattern ?

Best regards

Benoît

>  };
>  
>  static bool quorum_vote(QuorumAIOCB *acb);
> @@ -153,7 +157,8 @@ 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 +261,21 @@ 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 +283,21 @@ 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 +378,6 @@ 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 +637,62 @@ 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 +832,33 @@ 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 +900,37 @@ 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
>