Fetching contributors…
Cannot retrieve contributors at this time
77 lines (53 sloc) 2.58 KB


nbdkit-log-filter - nbdkit log filter


nbdkit --filter=log plugin logfile=FILE [logappend=BOOL] [plugin-args...]


nbdkit-log-filter is a filter that logs all transactions. When used as the first filter, it can show the original client requests; as a later filter, it can show how earlier filters have modified the original request. The log results are placed in a user-specified file; for more details on the log format, see FILES. Note that using nbdkit -v -f produces much more verbose logging details to stderr about every aspect of nbdkit operation, although this requires running nbdkit in the foreground; while the log filter is designed to work even when nbdkit is run as a daemon.


The nbdkit-log-filter requires a single parameter logfile which specifies the path of the file to use for logging. If the file already exists, it will be truncated unless the logappend parameter was specified with a value that can be parsed as a boolean true.


Serve the file disk.img, and log each client transaction in the file disk.log:

nbdkit --filter=log file disk.img logfile=disk.log

Repeat the task, but with the cow (copy-on-write) filter to perform local caching of data served from the original plugin:

nbdkit --filter=cow --filter=log file disk.img logfile=disk.log2

After running a client that performs the same operations under each of the two servers, you can compare disk.log and disk.log2 to see the impact of the caching.


This filter writes to the file specified by the logfile=FILE parameter. All lines include a timestamp, a connection counter, then details about the command. The following actions are logged: Connect, Read, Write, Zero, Trim, Flush, and Disconnect. Except for Connect and Disconnect, an event is logged across two lines for call and return value, to allow tracking duration and tracing any parallel execution, using id for correlation (incremented per action on the connection).

An example logging session of a client that performs a single successful read is:

2018-01-27 20:38:22.959984 connection=1 Connect size=0x400 write=1 flush=1 rotational=0 trim=0 zero=1 fua=1
2018-01-27 20:38:23.001720 connection=1 Read id=1 offset=0x0 count=0x100 ...
2018-01-27 20:38:23.001995 connection=1 ...Read id=1 return=0 (Success)
2018-01-27 20:38:23.044259 connection=1 Disconnect transactions=1


nbdkit(1), nbdkit-file-plugin(1), nbdkit-cow-filter(1), nbdkit-filter(3).


Eric Blake


Copyright (C) 2018 Red Hat Inc.