25.05.2017 19:32, Eric Blake wrote:
On 05/25/2017 10:26 AM, Vladimir Sementsov-Ogievskiy wrote:
Current comment is not clear enough: which sparseness is meant, coming
from sparse image format or from sparse file system?
For example, if we have qcow2 above raw file on non-sparse file system,
this function will say nothing about unallocated (by qcow2 layer)
clusters.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
block.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/block.c b/block.c
index 50ba264143..ba22fc0dfb 100644
--- a/block.c
+++ b/block.c
@@ -3388,8 +3388,8 @@ int bdrv_truncate(BdrvChild *child, int64_t offset, Error
**errp)
}
/**
- * Length of a allocated file in bytes. Sparse files are counted by actual
Yay for getting rid of bad grammar (s/a /an /)
- * allocated space. Return < 0 if error or unknown.
+ * Size of allocated in underlying file system area. Sparseness is taken into
Doesn't read well. Maybe: s/Size of allocated/Allocation size/ ?
+ * account for sparse file systems. Return < 0 if error or unknown.
I still don't get what we are trying to present.
If we have the following 6 qcow2 file clusters backed by the underlying
lseek(SEEK_DATA/HOLE) file system contents:
BDRV_BLOCK_UNALLOCATED N/A
BDRV_BLOCK_ZERO_PLAIN N/A
BDRV_BLOCK_ZERO_ALLOC hole
BDRV_BLOCK_ZERO_ALLOC data
BDRV_BLOCK_DATA hole
BDRV_BLOCK_DATA data
Then is our answer the size of all qcow2 allocations regardless of
underlying status (4, due to clusters 3-6), or the size of only the
clusters that read from the backing file (2, due to clusters 5-6), or
the size of only the clusters that currently occupy space in the file
system (2, due to clusters 4 and 6), or the size of clusters that are
not provably read-as-zero (1, due to cluster 6)?
Does the answer change if you can have underlying holes happen at a
smaller granularity than clusters?
What happens for compressed clusters?
I think we still need to do a better job at writing a precise comment.
bdrv_get_allocated_file_size is not related to qcow2, as qcow2 doesn't
support it. So, it is finally just raw_get_allocated_file_size, which
returns st.st_blocks * 512 after fstat(s->fd).
It will correspond to qcow2 sparseness if qcow2 discarded corresponding
clusters in bs->file and if underlying fs is sparse.
--
Best regards,
Vladimir
