Known Issues

Below is a list of known issues in recent releases of Tahoe, and how to manage them.

issues in Tahoe v1.1.0, released 2008-06-10

issue 1: server out of space when writing mutable file

If a v1.0 or v1.1.0 storage server runs out of disk space then its attempts to write data to the local filesystem will fail. For immutable files, this will not lead to any problem (the attempt to upload that share to that server will fail, the partially uploaded share will be deleted from the storage server's "incoming shares" directory, and the client will move on to using another storage server instead).

If the write was an attempt to modify an existing mutable file, however, a problem will result: when the attempt to write the new share fails due to insufficient disk space, then it will be aborted and the old share will be left in place. If enough such old shares are left, then a subsequent read may get those old shares and see the file in its earlier state, which is a "rollback" failure. With the default parameters (3-of-10), six old shares will be enough to potentially lead to a rollback failure.

how to manage it

Make sure your Tahoe storage servers don't run out of disk space. This means refusing storage requests before the disk fills up. There are a couple of ways to do that with v1.1.

First, there is a configuration option named "sizelimit" which will cause the storage server to do a "du" style recursive examination of its directories at startup, and then if the sum of the size of files found therein is greater than the "sizelimit" number, it will reject requests by clients to write new immutable shares.

However, that can take a long time (something on the order of a minute of examination of the filesystem for each 10 GB of data stored in the Tahoe server), and the Tahoe server will be unavailable to clients during that time.

Another option is to set the "readonly_storage" configuration option on the storage server before startup. This will cause the storage server to reject all requests to upload new immutable shares.

Note that neither of these configurations affect mutable shares: even if sizelimit is configured and the storage server currently has greater space used than allowed, or even if readonly_storage is configured, servers will continue to accept new mutable shares and will continue to accept requests to overwrite existing mutable shares.

Mutable files are typically used only for directories, and are usually much smaller than immutable files, so if you use one of these configurations to stop the influx of immutable files while there is still sufficient disk space to receive an influx of (much smaller) mutable files, you may be able to avoid the potential for "rollback" failure.

A future version of Tahoe will include a fix for this issue. Here is [​http://allmydata.org/pipermail/tahoe-dev/2008-May/000630.html the mailing list discussion] about how that future version will work.

issues in Tahoe v1.1.0 and v1.0.0

issue 2: pyOpenSSL and/or Twisted defect resulting false alarms in the unit tests

The combination of Twisted v8.1.0 and pyOpenSSL v0.7 causes the Tahoe v1.1 unit tests to fail, even though the behavior of Tahoe itself which is being tested is correct.

how to manage it

If you are using Twisted v8.1.0 and pyOpenSSL v0.7, then please ignore XYZ in XYZ. Downgrading to an older version of Twisted or pyOpenSSL will cause those false alarms to stop happening.

issues in Tahoe v1.0.0, released 2008-03-25

(Tahoe v1.0 was superceded by v1.1 which was released 2008-06-10.)

issue 3: server out of space when writing mutable file

In addition to the problems caused by insufficient disk space described above, v1.0 clients which are writing mutable files when the servers fail to write to their filesystem are likely to think the write succeeded, when it in fact failed. This can cause data loss.

how to manage it

Upgrade client to v1.1, or make sure that servers are always able to write to their local filesystem (including that there is space available) as described in "issue 1" above.

issue 4: server out of space when writing immutable file

Tahoe v1.0 clients are using v1.0 servers which are unable to write to their filesystem during an immutable upload will correctly detect the first failure, but if they retry the upload without restarting the client, or if another client attempts to upload the same file, the second upload may appear to succeed when it hasn't, which can lead to data loss.

how to manage it

Upgrading either or both of the client and the server to v1.1 will fix this issue. Also it can be avoided by ensuring that the servers are always able to write to their local filesystem (including that there is space available) as described in "issue 1" above.

issue 5: large directories or mutable files in a specific range of sizes

If a client attempts to upload a large mutable file with a size greater than about 3,139,000 and less than or equal to 3,500,000 bytes then it will fail but appear to succeed, which can lead to data loss.

(Mutable files larger than 3,500,000 are refused outright). The symptom of the failure is very high memory usage (3 GB of memory) and 100% CPU for about 5 minutes, before it appears to succeed, although it hasn't.

Directories are stored in mutable files, and a directory of approximately 9000 entries may fall into this range of mutable file sizes (depending on the size of the filenames or other metadata associated with the entries).

how to manage it

This was fixed in v1.1, under ticket #379. If the client is upgraded to v1.1, then it will fail cleanly instead of falsely appearing to succeed when it tries to write a file whose size is in this range. If the server is also upgraded to v1.1, then writes of mutable files whose size is in this range will succeed. (If the server is upgraded to v1.1 but the client is still v1.0 then the client will still suffer this failure.)

issue 6: pycryptopp defect resulting in data corruption

Versions of pycryptopp earlier than pycryptopp-0.5.0 had a defect which, when compiled with some compilers, would cause AES-256 encryption and decryption to be computed incorrectly. This could cause data corruption. Tahoe v1.0 required, and came with a bundled copy of, pycryptopp v0.3.

how to manage it

You can detect whether pycryptopp-0.3 has this failure when it is compiled by your compiler. Run the unit tests that come with pycryptopp-0.3: unpack the "pycryptopp-0.3.tar" file that comes in the Tahoe v1.0 misc/dependencies directory, cd into the resulting pycryptopp-0.3.0 directory, and execute python ./setup.py test. If the tests pass, then your compiler does not trigger this failure.

Tahoe v1.1 requires, and comes with a bundled copy of, pycryptopp v0.5.1, which does not have this defect.