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block: add enable_write_cache flag

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Add a enable_write_cache flag in the block driver state, and use it to
decide if we claim to have a volatile write cache that needs controlled
flushing from the guest.  The flag is off if cache=writethrough is
defined because O_DSYNC guarantees that every write goes to stable
storage, and it is on for cache=none and cache=writeback.

Both scsi-disk and ide now use the new flage, changing from their
defaults of always off (ide) or always on (scsi-disk).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This commit is contained in:
Christoph Hellwig 2009-09-04 19:01:15 +02:00 • committed by Anthony Liguori
parent b8193adbda
commit e900a7b748
5 changed files with 27 additions and 3 deletions
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15
block.c
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@ -408,6 +408,16 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
}
bs->drv = drv;
bs->opaque = qemu_mallocz(drv->instance_size);
/*
* Yes, BDRV_O_NOCACHE aka O_DIRECT means we have to present a
* write cache to the guest. We do need the fdatasync to flush
* out transactions for block allocations, and we maybe have a
* volatile write cache in our backing device to deal with.
*/
if (flags & (BDRV_O_CACHE_WB|BDRV_O_NOCACHE))
bs->enable_write_cache = 1;
/* Note: for compatibility, we open disk image files as RDWR, and
RDONLY as fallback */
if (!(flags & BDRV_O_FILE))
@ -918,6 +928,11 @@ int bdrv_is_sg(BlockDriverState *bs)
return bs->sg;
}
int bdrv_enable_write_cache(BlockDriverState *bs)
{
return bs->enable_write_cache;
}
/* XXX: no longer used */
void bdrv_set_change_cb(BlockDriverState *bs,
void (*change_cb)(void *opaque), void *opaque)