nbdkit-log-filter - nbdkit log filter
nbdkit --filter=log PLUGIN
[logfile=FILE | logscript=SCRIPT] [logappend=BOOL]
[PLUGIN-ARGS...]nbdkit-log-filter is a filter that logs all transactions to a file or external script.
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.
When using logfile=FILE, logs are written to a log file with the format described in "LOG FILE FORMAT" below.
When using logscript=SCRIPT, logs invoke the external script. See "LOG SCRIPT" below.
An alternative to this filter is simply to run nbdkit with the -f and -v flags which enable verbose debugging to stderr. This logs many aspects of nbdkit operation, but requires running nbdkit in the foreground. The log filter uses a more parsimonious and more easily parsable format and works when nbdkit runs in the background.
logfile or logscript or both can be given. If neither then the filter is inactive.
The file where the log is written. See "LOG FILE FORMAT" below.
(nbdkit ≥ 1.24)
Log lines invoke an external script. See "LOG SCRIPT" below.
(nbdkit ≥ 1.8)
This only affects logfile. If false (the default), if the file already exists it will be truncated. If true, the filter appends to the existing log file.
Serve the file disk.img, and log each client transaction in the file disk.log:
nbdkit --filter=log file disk.img logfile=disk.logRepeat 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.log2After 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.
An example logging session of a client that requests an export list before performing a single successful read is:
2020-08-06 02:07:23.080415 ListExports id=1 readonly=0 tls=0 ...
2020-08-06 02:07:23.080502 ...ListExports id=1 exports=("") return=0
2020-08-06 02:07:23.080712 connection=1 Connect export="" tls=0 size=0x400 minsize=0x1 prefsize=0x200 maxsize=0xffffffff write=1 flush=1 rotational=0 trim=1 zero=2 fua=2 extents=1 cache=2 fast_zero=1
2020-08-06 02:07:23.080907 connection=1 Read id=1 offset=0x0 count=0x200 ...
2020-08-06 02:07:23.080927 connection=1 ...Read id=1 return=0
2020-08-06 02:07:23.081255 connection=1 Disconnect transactions=1All lines start with a timestamp in YYYY-MM-DD HH:MM:ZZ.MS format.
For connected calls, connection=N is present to distinguish between clients.
The action follows. Currently the following actions are logged: ListExports, Ready, Fork, Preconnect, Connect, Read, Write, Zero, Trim, Extents, Cache, Flush and Disconnect.
Some actions are logged across two lines showing the call and return value. Because nbdkit handles requests in parallel different requests may be intermingled. Use the id=N field for correlation, it is unique per connection.
Strings and lists are shell-quoted.
If logscript=SCRIPT is given on the command line then log entries are passed to the external script.
The script is passed several shell variables:
$actThe action name, like "Read", "Write" etc.
$connectionThe connection ID identifying the client, only for connected calls like "Read".
$errorFor messages of type "LEAVE" which fail ($return = -1), this contains the errno as a string, for example "EIO".
$idThe transaction ID, used to correlate actions which are split into two messages "ENTER" and "LEAVE".
$returnFor messages of type "LEAVE" this is the return code, usually 0 for success and -1 if there was an error.
$typeThe message type: "ENTER", "LEAVE" or "PRINT".
Other parameters like offset=N are turned into shell variables $offset etc.
Note the return value of the script is ignored. Log scripts cannot modify or interrupt request processing.
The script:
nbdkit -f --filter=log null 10M \
logscript='echo $connection $type $id $act $offset >&2'might print lines like:
PRINT Ready
1 ENTER 1 Read 0x0
1 ENTER 2 Write 0x200
1 LEAVE 2 Write
1 LEAVE 1 Readcorresponding to log file lines:
Ready thread_model=3
connection=1 Read id=1 offset=0x0 count=0x200 ...
connection=1 Write id=2 offset=0x200 count=0x200 ...
connection=1 ...Write id=2
connection=1 ...Read id=1This script will trigger a message when any client reads:
nbdkit -f --filter=log memory 10M \
logscript='
if [ "$act" = "Read" -a "$type" = "ENTER" ]; then
echo Client is reading $count bytes from $offset >&2
fi
'logfile=FILE parameterThis filter writes to the file specified by the logfile=FILE parameter.
The filter.
Use nbdkit --dump-config to find the location of $filterdir.
nbdkit-log-filter first appeared in nbdkit 1.4.
nbdkit(1), nbdkit-file-plugin(1), nbdkit-cow-filter(1), nbdkit-filter(3), nbdkit-stats-filter(1).
Eric Blake
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