04.02.2020 18:49, Eric Blake wrote:
On 2/4/20 9:35 AM, Vladimir Sementsov-Ogievskiy wrote:
31.01.2020 20:44, Eric Blake wrote:
Having two slightly-different function names for related purposes is
unwieldy, especially since I envision adding yet another notion of
zero support in an upcoming patch. It doesn't help that
bdrv_has_zero_init() is a misleading name (I originally thought that a
driver could only return 1 when opening an already-existing image
known to be all zeroes; but in reality many drivers always return 1
because it only applies to a just-created image). Refactor all uses
to instead have a single function that returns multiple bits of
information, with better naming and documentation.
Sounds good
No semantic change, although some of the changes (such as to qcow2.c)
require a careful reading to see how it remains the same.
...
diff --git a/include/block/block.h b/include/block/block.h
index 6cd566324d95..a6a227f50678 100644
--- a/include/block/block.h
+++ b/include/block/block.h
Hmm, header file in the middle of the patch, possibly you don't use
[diff]
orderFile = scripts/git.orderfile
in git config.. Or it is broken.
I do have it set up, so I'm not sure why it didn't work as planned. I'll make
sure v2 follows the order I intended.
@@ -85,6 +85,28 @@ typedef enum {
BDRV_REQ_MASK = 0x3ff,
} BdrvRequestFlags;
+typedef enum {
+ /*
+ * bdrv_known_zeroes() should include this bit if the contents of
+ * a freshly-created image with no backing file reads as all
+ * zeroes without any additional effort. If .bdrv_co_truncate is
+ * set, then this must be clear if BDRV_ZERO_TRUNCATE is clear.
I understand that this is preexisting logic, but could I ask: why? What's wrong
if driver can guarantee that created file is all-zero, but is not sure about
file resizing? I agree that it's normal for these flags to have the same value,
but what is the reason for this restriction?..
For _this_ patch, my goal is to preserve pre-existing practice. Where we think
pre-existing practice is wrong, we can then improve it in other patches (see
patch 6, for example).
This is OK, of course, I'm just trying to understand existing logic.
I _think_ the reason for this original limitation is as follows: If an image
can be resized, we could choose to perform 'create(size=0),
truncate(size=final)' instead of 'create(size=final)', and we want to guarantee
the same behavior. If truncation can't guarantee a zero read, then why is
creation doing so?
If we want to guarantee the same behavior, we should restrict any difference
between these flags :)
But as I did not write the original patch, I would welcome Max's input with
regards to the thought behind commit ceaca56f.
So, the only possible combination of flags, when they differs, is create=0 and
truncate=1.. How is it possible?
qcow2 had that mode, at least before patch 5.
yes, it reported even for encrypted images truncate=1...
+ * Since this bit is only reliable at image creation, a driver may
+ * return this bit even for existing images that do not currently
+ * read as zero.
+ */
+ BDRV_ZERO_CREATE = 0x1,
+
+ /*
+ * bdrv_known_zeroes() should include this bit if growing an image
+ * with PREALLOC_MODE_OFF (either with no backing file, or beyond
+ * the size of the backing file) will read the new data as all
+ * zeroes without any additional effort. This bit only matters
+ * for drivers that set .bdrv_co_truncate.
+ */
+ BDRV_ZERO_TRUNCATE = 0x2,
+} BdrvZeroFlags;
+
...
--
Best regards,
Vladimir
