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Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_boun
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once |
Date: |
Mon, 12 Aug 2019 17:10:28 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 |
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 <address@hidden>
> ---
> 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;
>
signature.asc
Description: OpenPGP digital signature
- [Qemu-devel] [PATCH v3 0/7] backup improvements, Vladimir Sementsov-Ogievskiy, 2019/08/10
- [Qemu-devel] [PATCH v3 5/7] block/backup: fix backup_cow_with_offload for last cluster, Vladimir Sementsov-Ogievskiy, 2019/08/10
- [Qemu-devel] [PATCH v3 2/7] block/backup: refactor write_flags, Vladimir Sementsov-Ogievskiy, 2019/08/10
- [Qemu-devel] [PATCH v3 1/7] block/backup: deal with zero detection, Vladimir Sementsov-Ogievskiy, 2019/08/10
- [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Vladimir Sementsov-Ogievskiy, 2019/08/10
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once,
Max Reitz <=
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Vladimir Sementsov-Ogievskiy, 2019/08/12
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Max Reitz, 2019/08/12
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Vladimir Sementsov-Ogievskiy, 2019/08/12
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Vladimir Sementsov-Ogievskiy, 2019/08/13
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Max Reitz, 2019/08/13
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Vladimir Sementsov-Ogievskiy, 2019/08/13
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Max Reitz, 2019/08/13
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Vladimir Sementsov-Ogievskiy, 2019/08/13
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Max Reitz, 2019/08/13
- Re: [Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once, Vladimir Sementsov-Ogievskiy, 2019/08/13