On 10.08.19 21:31, Vladimir Sementsov-Ogievskiy wrote: > backup_cow_with_offload can transfer more than one cluster. Let > backup_cow_with_bounce_buffer behave similarly. It reduces the number > of IO requests, since there is no need to copy cluster by cluster. > > Logic around bounce_buffer allocation changed: we can't just allocate > one-cluster-sized buffer to share for all iterations. We can't also > allocate buffer of full-request length it may be too large, so > BACKUP_MAX_BOUNCE_BUFFER is introduced. And finally, allocation logic > is to allocate a buffer sufficient to handle all remaining iterations > at the point where we need the buffer for the first time. > > Bonus: get rid of pointer-to-pointer. > > Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]> > --- > block/backup.c | 65 +++++++++++++++++++++++++++++++------------------- > 1 file changed, 41 insertions(+), 24 deletions(-) > > diff --git a/block/backup.c b/block/backup.c > index d482d93458..65f7212c85 100644 > --- a/block/backup.c > +++ b/block/backup.c > @@ -27,6 +27,7 @@ > #include "qemu/error-report.h" > > #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16) > +#define BACKUP_MAX_BOUNCE_BUFFER (64 * 1024 * 1024) > > typedef struct CowRequest { > int64_t start_byte; > @@ -98,44 +99,55 @@ static void cow_request_end(CowRequest *req) > qemu_co_queue_restart_all(&req->wait_queue); > } > > -/* Copy range to target with a bounce buffer and return the bytes copied. If > - * error occurred, return a negative error number */ > +/* > + * Copy range to target with a bounce buffer and return the bytes copied. If > + * error occurred, return a negative error number > + * > + * @bounce_buffer is assumed to enough to store
s/to/to be/
> + * MIN(BACKUP_MAX_BOUNCE_BUFFER, @end - @start) bytes
> + */
> static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
> int64_t start,
> int64_t end,
> bool is_write_notifier,
> bool *error_is_read,
> - void **bounce_buffer)
> + void *bounce_buffer)
> {
> int ret;
> BlockBackend *blk = job->common.blk;
> - int nbytes;
> + int nbytes, remaining_bytes;
> int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
>
> assert(QEMU_IS_ALIGNED(start, job->cluster_size));
> - bdrv_reset_dirty_bitmap(job->copy_bitmap, start, job->cluster_size);
> - nbytes = MIN(job->cluster_size, job->len - start);
> - if (!*bounce_buffer) {
> - *bounce_buffer = blk_blockalign(blk, job->cluster_size);
> - }
> + bdrv_reset_dirty_bitmap(job->copy_bitmap, start, end - start);
> + nbytes = MIN(end - start, job->len - start);
>
> - ret = blk_co_pread(blk, start, nbytes, *bounce_buffer, read_flags);
> - if (ret < 0) {
> - trace_backup_do_cow_read_fail(job, start, ret);
> - if (error_is_read) {
> - *error_is_read = true;
> +
> + remaining_bytes = nbytes;
> + while (remaining_bytes) {
> + int chunk = MIN(BACKUP_MAX_BOUNCE_BUFFER, remaining_bytes);
> +
> + ret = blk_co_pread(blk, start, chunk, bounce_buffer, read_flags);
> + if (ret < 0) {
> + trace_backup_do_cow_read_fail(job, start, ret);
> + if (error_is_read) {
> + *error_is_read = true;
> + }
> + goto fail;
> }
> - goto fail;
> - }
>
> - ret = blk_co_pwrite(job->target, start, nbytes, *bounce_buffer,
> - job->write_flags);
> - if (ret < 0) {
> - trace_backup_do_cow_write_fail(job, start, ret);
> - if (error_is_read) {
> - *error_is_read = false;
> + ret = blk_co_pwrite(job->target, start, chunk, bounce_buffer,
> + job->write_flags);
> + if (ret < 0) {
> + trace_backup_do_cow_write_fail(job, start, ret);
> + if (error_is_read) {
> + *error_is_read = false;
> + }
> + goto fail;
> }
> - goto fail;
> +
> + start += chunk;
> + remaining_bytes -= chunk;
> }
>
> return nbytes;
> @@ -301,9 +313,14 @@ static int coroutine_fn backup_do_cow(BackupBlockJob
> *job,
> }
> }
> if (!job->use_copy_range) {
> + if (!bounce_buffer) {
> + size_t len = MIN(BACKUP_MAX_BOUNCE_BUFFER,
> + MAX(dirty_end - start, end - dirty_end));
> + bounce_buffer = blk_try_blockalign(job->common.blk, len);
> + }
If you use _try_, you should probably also check whether it succeeded.
Anyway, I wonder whether it’d be better to just allocate this buffer
once per job (the first time we get here, probably) to be of size
BACKUP_MAX_BOUNCE_BUFFER and put it into BackupBlockJob. (And maybe add
a buf-size parameter similar to what the mirror jobs have.)
Max
> ret = backup_cow_with_bounce_buffer(job, start, dirty_end,
> is_write_notifier,
> - error_is_read,
> &bounce_buffer);
> + error_is_read,
> bounce_buffer);
> }
> if (ret < 0) {
> break;
>
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