On 2/4/20 11:34 AM, Max Reitz wrote:
+++ b/include/block/block.h
@@ -105,6 +105,16 @@ typedef enum {
* for drivers that set .bdrv_co_truncate.
*/
BDRV_ZERO_TRUNCATE = 0x2,
+
+ /*
+ * bdrv_known_zeroes() should include this bit if an image is
+ * known to read as all zeroes when first opened; this bit should
+ * not be relied on after any writes to the image.
Is there a good reason for this? Because to me this screams like we are
going to check this flag without ensuring that the image has actually
not been written to yet. So if it’s generally easy for drivers to stop
reporting this flag after a write, then maybe we should do so.
In patch 15 (implementing things in qcow2), I actually wrote the driver to
return live results, rather than just open-time results, in part because
writing the bit to persistent storage in qcow2 means that the bit must be
accurate, without relying on the block layer's help.
But my pending NBD patch (not posted yet, but will be soon), the proposal I'm
making for the NBD protocol itself is just open-time, not live, and so it would
be more work than necessary to make the NBD driver report live results.
But it seems like it should be easy enough to also patch the block layer itself
to guarantee that callers of bdrv_known_zeroes() cannot see this bit set if the
block layer has been used in any non-zero transaction, by repeating the same
logic as used in qcow2 to kill the bit (any write/write_compressed/bdrv_copy
clear the bit, any trim clears the bit if the driver does not guarantee trim
reads as zero, any truncate clears the bit if the driver does not guarantee
truncate reads as zero, etc). Basically, the block layer would cache the
results of .bdrv_known_zeroes during .bdrv_co_open, bdrv_co_pwrite() and
friends would update that cache, and and bdrv_known_zeroes() would report the
cached value rather than a fresh call to .bdrv_known_zeroes.
Are we worried enough about clients of this interface to make the block layer
more robust? (From the maintenance standpoint, the more the block layer
guarantees, the easier it is to write code that uses the block layer; but there
is the counter-argument that making the block layer track whether an image has
been modified means a [slight] penalty to every write request to update the
boolean.)