Hi! While using Cassandra, I've seen this log messages when running some test cases (which insert lots of columns in 4 rows).
I've tryied Cassandra 1.0.10, 1.1.0, 1.1.1-SNAPSHOT and 1.2.0-SNAPSHOT (built from git). I'm using the default configuration, Oracle jdk 1.6.0_32, Ubuntu 12.04 and pycassa.

Since I'm very new to Cassandra (I'm just starting to learn it) I don't know if I'm doing something wrong, or maybe there are some bugs in the several versions of Cassandra I've tested.

cassandra-1.0.10

 - IOException: unable to mkdirs

cassandra-1.1.0

 - IOException: Unable to create directory

cassandra-1.1.1-SNAPSHOT

 - IOException: Unable to create directory

cassandra-1.2.0-SNAPSHOT

 - IOException: Unable to create directory

 - CLibrary.java (line 191) Unable to create hard link (...) 
      command output: ln: failed to create hard link `(...)/lolog_tests-Logs_by_app-ia-3-Summary.db': File exists



Thanks in advance!
Horacio


------------------------------------------------------------
system-cassandra-1.0.10.log
------------------------------------------------------------

ERROR [MutationStage:1] 2012-06-02 20:37:41,115 AbstractCassandraDaemon.java (line 139) Fatal exception in thread Thread[MutationStage:1,5,main]
java.io.IOError: java.io.IOException: unable to mkdirs /var/lib/cassandra/data/lolog_tests/snapshots/1338680261112-Logs_by_app
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1433)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1462)
at org.apache.cassandra.db.ColumnFamilyStore.truncate(ColumnFamilyStore.java:1657)
at org.apache.cassandra.db.TruncateVerbHandler.doVerb(TruncateVerbHandler.java:50)

ERROR [MutationStage:11] 2012-06-02 20:37:55,730 AbstractCassandraDaemon.java (line 139) Fatal exception in thread Thread[MutationStage:11,5,main]
java.io.IOError: java.io.IOException: unable to mkdirs /var/lib/cassandra/data/lolog_tests/snapshots/1338680275729-Logs_by_app
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1433)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1462)
at org.apache.cassandra.db.ColumnFamilyStore.truncate(ColumnFamilyStore.java:1657)
at org.apache.cassandra.db.TruncateVerbHandler.doVerb(TruncateVerbHandler.java:50)

ERROR [MutationStage:19] 2012-06-02 20:37:57,395 AbstractCassandraDaemon.java (line 139) Fatal exception in thread Thread[MutationStage:19,5,main]
java.io.IOError: java.io.IOException: unable to mkdirs /var/lib/cassandra/data/lolog_tests/snapshots/1338680277394-Logs_by_app
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1433)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1462)
at org.apache.cassandra.db.ColumnFamilyStore.truncate(ColumnFamilyStore.java:1657)
at org.apache.cassandra.db.TruncateVerbHandler.doVerb(TruncateVerbHandler.java:50)

ERROR [MutationStage:20] 2012-06-02 20:41:26,666 AbstractCassandraDaemon.java (line 139) Fatal exception in thread Thread[MutationStage:20,5,main]
java.io.IOError: java.io.IOException: unable to mkdirs /var/lib/cassandra/data/lolog_tests/snapshots/1338680486666-Logs_by_app
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1433)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1462)
at org.apache.cassandra.db.ColumnFamilyStore.truncate(ColumnFamilyStore.java:1657)
at org.apache.cassandra.db.TruncateVerbHandler.doVerb(TruncateVerbHandler.java:50)

------------------------------------------------------------
system-cassandra-1.1.0.log
------------------------------------------------------------

ERROR [MutationStage:1] 2012-06-02 20:45:15,609 AbstractCassandraDaemon.java (line 134) Exception in thread Thread[MutationStage:1,5,main]
java.io.IOError: java.io.IOException: Unable to create directory /var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338680715606-Logs_by_app
at org.apache.cassandra.db.Directories.getOrCreate(Directories.java:384)
at org.apache.cassandra.db.Directories.getSnapshotDirectory(Directories.java:186)
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1440)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1483)


ERROR [MutationStage:9] 2012-06-02 20:45:30,728 AbstractCassandraDaemon.java (line 134) Exception in thread Thread[MutationStage:9,5,main]
java.io.IOError: java.io.IOException: Unable to create directory /var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338680730727-Logs_by_app
at org.apache.cassandra.db.Directories.getOrCreate(Directories.java:384)
at org.apache.cassandra.db.Directories.getSnapshotDirectory(Directories.java:186)
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1440)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1483)


ERROR [MutationStage:17] 2012-06-02 20:45:31,967 AbstractCassandraDaemon.java (line 134) Exception in thread Thread[MutationStage:17,5,main]
java.io.IOError: java.io.IOException: Unable to create directory /var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338680731966-Logs_by_app
at org.apache.cassandra.db.Directories.getOrCreate(Directories.java:384)
at org.apache.cassandra.db.Directories.getSnapshotDirectory(Directories.java:186)
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1440)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1483)

------------------------------------------------------------
system-cassandra-1.1.1-SNAPSHOT.log
------------------------------------------------------------

ERROR [MutationStage:4] 2012-06-02 22:19:55,821 AbstractCassandraDaemon.java (line 134) Exception in thread Thread[MutationStage:4,5,main]
java.io.IOError: java.io.IOException: Unable to create directory /var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338686395804-Logs_by_app
at org.apache.cassandra.db.Directories.getOrCreate(Directories.java:386)
at org.apache.cassandra.db.Directories.getSnapshotDirectory(Directories.java:173)
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1488)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1540)

ERROR [MutationStage:9] 2012-06-02 22:20:07,892 AbstractCassandraDaemon.java (line 134) Exception in thread Thread[MutationStage:9,5,main]
java.io.IOError: java.io.IOException: Unable to create directory /var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338686407891-Logs_by_app
at org.apache.cassandra.db.Directories.getOrCreate(Directories.java:386)
at org.apache.cassandra.db.Directories.getSnapshotDirectory(Directories.java:173)
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1488)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1540)

------------------------------------------------------------
system-cassandra-1.2.0-SNAPSHOT.log
------------------------------------------------------------

ERROR [MutationStage:3] 2012-06-02 20:47:23,371 AbstractCassandraDaemon.java (line 133) Exception in thread Thread[MutationStage:3,5,main]
java.io.IOError: java.io.IOException: Unable to create directory /var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338680843363-Logs_by_app
at org.apache.cassandra.db.Directories.getOrCreate(Directories.java:386)
at org.apache.cassandra.db.Directories.getSnapshotDirectory(Directories.java:173)
at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1472)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1524)

ERROR [MutationStage:20] 2012-06-02 20:47:40,361 CLibrary.java (line 191) Unable to create hard link
java.io.IOException: Exception while executing the command: ln /var/lib/cassandra/data/lolog_tests/Logs_by_app/lolog_tests-Logs_by_app-ia-3-Summary.db /var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338680860345-Logs_by_app/lolog_tests-Logs_by_app-ia-3-Summary.db, command error Code: 1, command output: ln: failed to create hard link `/var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338680860345-Logs_by_app/lolog_tests-Logs_by_app-ia-3-Summary.db': File exists

at org.apache.cassandra.utils.FBUtilities.exec(FBUtilities.java:561)
at org.apache.cassandra.utils.CLibrary.createHardLinkWithExec(CLibrary.java:187)
at org.apache.cassandra.utils.CLibrary.createHardLink(CLibrary.java:150)

ERROR [MutationStage:20] 2012-06-02 20:47:40,364 AbstractCassandraDaemon.java (line 133) Exception in thread Thread[MutationStage:20,5,main]
java.io.IOError: java.io.IOException: Exception while executing the command: ln /var/lib/cassandra/data/lolog_tests/Logs_by_app/lolog_tests-Logs_by_app-ia-3-Summary.db /var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338680860345-Logs_by_app/lolog_tests-Logs_by_app-ia-3-Summary.db, command error Code: 1, command output: ln: failed to create hard link `/var/lib/cassandra/data/lolog_tests/Logs_by_app/snapshots/1338680860345-Logs_by_app/lolog_tests-Logs_by_app-ia-3-Summary.db': File exists

at org.apache.cassandra.db.ColumnFamilyStore.snapshotWithoutFlush(ColumnFamilyStore.java:1486)
at org.apache.cassandra.db.ColumnFamilyStore.snapshot(ColumnFamilyStore.java:1524)
at org.apache.cassandra.db.ColumnFamilyStore.truncate(ColumnFamilyStore.java:1761)