Add bdrv_aio_multiwrite

One performance problem of qcow2 during the initial image growth are
sequential writes that are not cluster aligned. In this case, when a first
requests requires to allocate a new cluster but writes only to the first
couple of sectors in that cluster, the rest of the cluster is zeroed - just
to be overwritten by the following second request that fills up the cluster.

Let's try to merge sequential write requests to the same cluster, so we can
avoid to write the zero padding to the disk in the first place.

As a nice side effect, also other formats take advantage of dealing with less
and larger requests.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

block.c

@@ -1354,6 +1354,189 @@ BlockDriverAIOCB *bdrv_aio_writev(BlockDriverState *bs, int64_t sector_num,
     return ret;
 }
 
+
+typedef struct MultiwriteCB {
+    int error;
+    int num_requests;
+    int num_callbacks;
+    struct {
+        BlockDriverCompletionFunc *cb;
+        void *opaque;
+        QEMUIOVector *free_qiov;
+        void *free_buf;
+    } callbacks[];
+} MultiwriteCB;
+
+static void multiwrite_user_cb(MultiwriteCB *mcb)
+{
+    int i;
+
+    for (i = 0; i < mcb->num_callbacks; i++) {
+        mcb->callbacks[i].cb(mcb->callbacks[i].opaque, mcb->error);
+        qemu_free(mcb->callbacks[i].free_qiov);
+        qemu_free(mcb->callbacks[i].free_buf);
+    }
+}
+
+static void multiwrite_cb(void *opaque, int ret)
+{
+    MultiwriteCB *mcb = opaque;
+
+    if (ret < 0) {
+        mcb->error = ret;
+        multiwrite_user_cb(mcb);
+    }
+
+    mcb->num_requests--;
+    if (mcb->num_requests == 0) {
+        if (mcb->error == 0) {
+            multiwrite_user_cb(mcb);
+        }
+        qemu_free(mcb);
+    }
+}
+
+static int multiwrite_req_compare(const void *a, const void *b)
+{
+    return (((BlockRequest*) a)->sector - ((BlockRequest*) b)->sector);
+}
+
+/*
+ * Takes a bunch of requests and tries to merge them. Returns the number of
+ * requests that remain after merging.
+ */
+static int multiwrite_merge(BlockDriverState *bs, BlockRequest *reqs,
+    int num_reqs, MultiwriteCB *mcb)
+{
+    int i, outidx;
+
+    // Sort requests by start sector
+    qsort(reqs, num_reqs, sizeof(*reqs), &multiwrite_req_compare);
+
+    // Check if adjacent requests touch the same clusters. If so, combine them,
+    // filling up gaps with zero sectors.
+    outidx = 0;
+    for (i = 1; i < num_reqs; i++) {
+        int merge = 0;
+        int64_t oldreq_last = reqs[outidx].sector + reqs[outidx].nb_sectors;
+
+        // This handles the cases that are valid for all block drivers, namely
+        // exactly sequential writes and overlapping writes.
+        if (reqs[i].sector <= oldreq_last) {
+            merge = 1;
+        }
+
+        // The block driver may decide that it makes sense to combine requests
+        // even if there is a gap of some sectors between them. In this case,
+        // the gap is filled with zeros (therefore only applicable for yet
+        // unused space in format like qcow2).
+        if (!merge && bs->drv->bdrv_merge_requests) {
+            merge = bs->drv->bdrv_merge_requests(bs, &reqs[outidx], &reqs[i]);
+        }
+
+        if (merge) {
+            size_t size;
+            QEMUIOVector *qiov = qemu_mallocz(sizeof(*qiov));
+            qemu_iovec_init(qiov,
+                reqs[outidx].qiov->niov + reqs[i].qiov->niov + 1);
+
+            // Add the first request to the merged one. If the requests are
+            // overlapping, drop the last sectors of the first request.
+            size = (reqs[i].sector - reqs[outidx].sector) << 9;
+            qemu_iovec_concat(qiov, reqs[outidx].qiov, size);
+
+            // We might need to add some zeros between the two requests
+            if (reqs[i].sector > oldreq_last) {
+                size_t zero_bytes = (reqs[i].sector - oldreq_last) << 9;
+                uint8_t *buf = qemu_blockalign(bs, zero_bytes);
+                memset(buf, 0, zero_bytes);
+                qemu_iovec_add(qiov, buf, zero_bytes);
+                mcb->callbacks[i].free_buf = buf;
+            }
+
+            // Add the second request
+            qemu_iovec_concat(qiov, reqs[i].qiov, reqs[i].qiov->size);
+
+            reqs[outidx].nb_sectors += reqs[i].nb_sectors;
+            reqs[outidx].qiov = qiov;
+
+            mcb->callbacks[i].free_qiov = reqs[outidx].qiov;
+        } else {
+            outidx++;
+            reqs[outidx].sector     = reqs[i].sector;
+            reqs[outidx].nb_sectors = reqs[i].nb_sectors;
+            reqs[outidx].qiov       = reqs[i].qiov;
+        }
+    }
+
+    return outidx + 1;
+}
+
+/*
+ * Submit multiple AIO write requests at once.
+ *
+ * On success, the function returns 0 and all requests in the reqs array have
+ * been submitted. In error case this function returns -1, and any of the
+ * requests may or may not be submitted yet. In particular, this means that the
+ * callback will be called for some of the requests, for others it won't. The
+ * caller must check the error field of the BlockRequest to wait for the right
+ * callbacks (if error != 0, no callback will be called).
+ *
+ * The implementation may modify the contents of the reqs array, e.g. to merge
+ * requests. However, the fields opaque and error are left unmodified as they
+ * are used to signal failure for a single request to the caller.
+ */
+int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
+{
+    BlockDriverAIOCB *acb;
+    MultiwriteCB *mcb;
+    int i;
+
+    if (num_reqs == 0) {
+        return 0;
+    }
+
+    // Create MultiwriteCB structure
+    mcb = qemu_mallocz(sizeof(*mcb) + num_reqs * sizeof(*mcb->callbacks));
+    mcb->num_requests = 0;
+    mcb->num_callbacks = num_reqs;
+
+    for (i = 0; i < num_reqs; i++) {
+        mcb->callbacks[i].cb = reqs[i].cb;
+        mcb->callbacks[i].opaque = reqs[i].opaque;
+    }
+
+    // Check for mergable requests
+    num_reqs = multiwrite_merge(bs, reqs, num_reqs, mcb);
+
+    // Run the aio requests
+    for (i = 0; i < num_reqs; i++) {
+        acb = bdrv_aio_writev(bs, reqs[i].sector, reqs[i].qiov,
+            reqs[i].nb_sectors, multiwrite_cb, mcb);
+
+        if (acb == NULL) {
+            // We can only fail the whole thing if no request has been
+            // submitted yet. Otherwise we'll wait for the submitted AIOs to
+            // complete and report the error in the callback.
+            if (mcb->num_requests == 0) {
+                reqs[i].error = EIO;
+                goto fail;
+            } else {
+                mcb->error = EIO;
+                break;
+            }
+        } else {
+            mcb->num_requests++;
+        }
+    }
+
+    return 0;
+
+fail:
+    free(mcb);
+    return -1;
+}
+
 void bdrv_aio_cancel(BlockDriverAIOCB *acb)
 {
     acb->pool->cancel(acb);

block.h

@@ -87,6 +87,21 @@ BlockDriverAIOCB *bdrv_aio_writev(BlockDriverState *bs, int64_t sector_num,
                                   BlockDriverCompletionFunc *cb, void *opaque);
 void bdrv_aio_cancel(BlockDriverAIOCB *acb);
 
+typedef struct BlockRequest {
+    /* Fields to be filled by multiwrite caller */
+    int64_t sector;
+    int nb_sectors;
+    QEMUIOVector *qiov;
+    BlockDriverCompletionFunc *cb;
+    void *opaque;
+
+    /* Filled by multiwrite implementation */
+    int error;
+} BlockRequest;
+
+int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs,
+    int num_reqs);
+
 /* sg packet commands */
 int bdrv_ioctl(BlockDriverState *bs, unsigned long int req, void *buf);
 BlockDriverAIOCB *bdrv_aio_ioctl(BlockDriverState *bs,

block_int.h

@@ -70,6 +70,12 @@ struct BlockDriver {
         int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,
         BlockDriverCompletionFunc *cb, void *opaque);
 
+    int (*bdrv_aio_multiwrite)(BlockDriverState *bs, BlockRequest *reqs,
+        int num_reqs);
+    int (*bdrv_merge_requests)(BlockDriverState *bs, BlockRequest* a,
+        BlockRequest *b);
+
+
     const char *protocol_name;
     int (*bdrv_truncate)(BlockDriverState *bs, int64_t offset);
     int64_t (*bdrv_getlength)(BlockDriverState *bs);

cutils.c

@@ -151,6 +151,31 @@ void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len)
     ++qiov->niov;
 }
 
+/*
+ * Copies iovecs from src to the end dst until src is completely copied or the
+ * total size of the copied iovec reaches size. The size of the last copied
+ * iovec is changed in order to fit the specified total size if it isn't a
+ * perfect fit already.
+ */
+void qemu_iovec_concat(QEMUIOVector *dst, QEMUIOVector *src, size_t size)
+{
+    int i;
+    size_t done;
+
+    assert(dst->nalloc != -1);
+
+    done = 0;
+    for (i = 0; (i < src->niov) && (done != size); i++) {
+        if (done + src->iov[i].iov_len > size) {
+            qemu_iovec_add(dst, src->iov[i].iov_base, size - done);
+            break;
+        } else {
+            qemu_iovec_add(dst, src->iov[i].iov_base, src->iov[i].iov_len);
+        }
+        done += src->iov[i].iov_len;
+    }
+}
+
 void qemu_iovec_destroy(QEMUIOVector *qiov)
 {
     assert(qiov->nalloc != -1);

qemu-common.h

@@ -223,6 +223,7 @@ typedef struct QEMUIOVector {
 void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint);
 void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov);
 void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len);
+void qemu_iovec_concat(QEMUIOVector *dst, QEMUIOVector *src, size_t size);
 void qemu_iovec_destroy(QEMUIOVector *qiov);
 void qemu_iovec_reset(QEMUIOVector *qiov);
 void qemu_iovec_to_buffer(QEMUIOVector *qiov, void *buf);