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Re: [Qemu-devel] [PATCH 2/3] block/backup: avoid copying less than full
From: |
Fam Zheng |
Subject: |
Re: [Qemu-devel] [PATCH 2/3] block/backup: avoid copying less than full target clusters |
Date: |
Sun, 14 Feb 2016 14:49:19 +0800 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Fri, 02/12 18:06, John Snow wrote:
> During incremental backups, if the target has a cluster size that is
> larger than the backup cluster size and we are backing up to a target
> that cannot (for whichever reason) pull clusters up from a backing image,
> we may inadvertantly create unusable incremental backup images.
>
> For example:
>
> If the bitmap tracks changes at a 64KB granularity and we transmit 64KB
> of data at a time but the target uses a 128KB cluster size, it is
> possible that only half of a target cluster will be recognized as dirty
> by the backup block job. When the cluster is allocated on the target
> image but only half populated with data, we lose the ability to
> distinguish between zero padding and uninitialized data.
>
> This does not happen if the target image has a backing file that points
> to the last known good backup.
>
> Even if we have a backing file, though, it's likely going to be faster
> to just buffer the redundant data ourselves from the live image than
> fetching it from the backing file, so let's just always round up to the
> target granularity.
>
> Signed-off-by: John Snow <address@hidden>
> ---
> block/backup.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/block/backup.c b/block/backup.c
> index fcf0043..62faf81 100644
> --- a/block/backup.c
> +++ b/block/backup.c
> @@ -568,9 +568,16 @@ void backup_start(BlockDriverState *bs, BlockDriverState
> *target,
> job->on_target_error = on_target_error;
> job->target = target;
> job->sync_mode = sync_mode;
> - job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
> - sync_bitmap : NULL;
> - job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
> + if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
> + BlockDriverInfo bdi;
> +
> + bdrv_get_info(job->target, &bdi);
> + job->sync_bitmap = sync_bitmap;
> + job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT,
> + bdi.cluster_size);
Why not just do it for all sync modes?
Fam
> + } else {
> + job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
> + }
> job->sectors_per_cluster = job->cluster_size / BDRV_SECTOR_SIZE;
> job->common.len = len;
> job->common.co = qemu_coroutine_create(backup_run);
> --
> 2.4.3
>