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interfaces: document that the 'fills-holes-with-zero-bytes' key should be used to detect whether a storage server has that behavior. refs #1528

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david-sarah 2011-09-12 17:28:43 -07:00
parent 4c33d855d1
commit c10099f982
1 changed files with 11 additions and 7 deletions
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src/allmydata/interfaces.py
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@ -214,7 +214,10 @@ class RIStorageServer(RemoteInterface):
between the end of the current data and the beginning of the write between the end of the current data and the beginning of the write
vector will be filled with zero bytes. In earlier versions the vector will be filled with zero bytes. In earlier versions the
contents of this space was unspecified (and might end up containing contents of this space was unspecified (and might end up containing
secrets). secrets). Storage servers with the new zero-filling behavior will
advertise a true value for the 'fills-holes-with-zero-bytes' key
(under 'http://allmydata.org/tahoe/protocols/storage/v1') in their
version information.
Each write vector is accompanied by a 'new_length' argument, which Each write vector is accompanied by a 'new_length' argument, which
can be used to truncate the data. If new_length is not None and it is can be used to truncate the data. If new_length is not None and it is
@ -228,12 +231,13 @@ class RIStorageServer(RemoteInterface):
of Tahoe-LAFS v1.8.3 it no longer works and the new_length is ignored of Tahoe-LAFS v1.8.3 it no longer works and the new_length is ignored
in that case. in that case.
If a storage client can rely on a server being of version v1.8.3 or If a storage client knows that the server supports zero-filling, for
later, it can extend the file efficiently by writing a single zero example from the 'fills-holes-with-zero-bytes' key in its version
byte just before the new end-of-file. Otherwise it must explicitly information, it can extend the file efficiently by writing a single
write zeroes to all bytes between the old and new end-of-file. In any zero byte just before the new end-of-file. Otherwise it must
case it should avoid sending new_length larger than the size of the explicitly write zeroes to all bytes between the old and new
data after applying all write vectors. end-of-file. In any case it should avoid sending new_length larger
than the size of the data after applying all write vectors.
The read vector is used to extract data from all known shares, The read vector is used to extract data from all known shares,
*before* any writes have been applied. The same vector is used for *before* any writes have been applied. The same vector is used for