One of the items I regularly recommend to my clients is to have a "permanent" Perfmon collector log, where a basic set of counters is collected every five minutes, with the log file cycled every day. This generates 1.5MB-2.0MB of files daily (a trivial amount of storage even if maintained for several years) and daily cycling makes the logs easier to handle when troubleshooting issues. This is also useful for something most people don't have - a baseline over time - but to use it this way requires some serious finagling as you will see.
The way I set up the collector by default, the collector writes to a file in a designated directory, and I use a couple of batch files that use the logman command to stop and start the collector, and then a pair of Windows Scheduled Tasks, one to stop and start the collector each night (effectively cycling to a new output file) and one to start the collector on server start-up (so that a server reboot doesn't kill the collector altogether). I know there are ways to configure a collector to cycle to new files each day, but they are version-specific based on your version of Windows, while the batch file/scheduled task method works across O/S versions.
The net effect of this method is that you end up with a directory with a large stack of blg files like this:
You can see a file for each day plus a new file any time the Server restarts. This is great when you are looking for info of what happened on the night of 05/24/2014, but not so much when you need to know what happened over the last thirty days.
Relog is a command line utility for manipulating perfmon output files (either .blg or .csv). It is frequently used to filter existing files by counter or to convert .blg files into .csv files, both of which are described in this blog post and this one by John Pertell (blog/@jayape). A lesser known use of relog is its ability to combine individual files into a single larger file, like this:
relog day1.blg day2.blg day3.blg -f BIN -o Day1_2_3.blg
...which would combine the three files day1, day2, and day3 into a single large file Day_1_2_3.
This is a necessary evil for many types of analysis since it can be difficult to wade through multiple perfmon log files at once. Most tools claim to support it, but most of them also have vague disclaimers that amount to YMMV. The most reliable way to handle this situation is to use relog to combine the files prior to beginning the analysis.
In the given situation, I had 43 files covering a little more than a month to analyze. I hadn't collected this many files at once before, but didn't really think about it as any big deal. I used notepad to mock together the relog command (not for the faint of heart):
C:\PerfLogs\Blog>relog Ntirety-SQLServerPerf_000066.blg Ntirety-SQLServerPerf_000067.blg Ntirety-SQLServerPerf_000068.blg Ntirety-SQLServerPerf_000069.blg Ntirety-SQLServerPerf_000070.blg Ntirety-SQLServerPerf_000071.blg Ntirety-SQLServerPerf_000072.blg Ntirety-SQLServerPerf_000073.blg Ntirety-SQLServerPerf_000074.blg Ntirety-SQLServerPerf_000075.blg Ntirety-SQLServerPerf_000076.blg Ntirety-SQLServerPerf_000077.blg Ntirety-SQLServerPerf_000078.blg Ntirety-SQLServerPerf_000079.blg Ntirety-SQLServerPerf_000080.blg Ntirety-SQLServerPerf_000081.blg Ntirety-SQLServerPerf_000082.blg Ntirety-SQLServerPerf_000083.blg Ntirety-SQLServerPerf_000084.blg Ntirety-SQLServerPerf_000085.blg Ntirety-SQLServerPerf_000086.blg Ntirety-SQLServerPerf_000087.blg Ntirety-SQLServerPerf_000088.blg Ntirety-SQLServerPerf_000089.blg Ntirety-SQLServerPerf_000090.blg Ntirety-SQLServerPerf_000091.blg Ntirety-SQLServerPerf_000092.blg Ntirety-SQLServerPerf_000093.blg Ntirety-SQLServerPerf_000094.blg Ntirety-SQLServerPerf_000095.blg Ntirety-SQLServerPerf_000096.blg Ntirety-SQLServerPerf_000097.blg Ntirety-SQLServerPerf_000098.blg Ntirety-SQLServerPerf_000099.blg Ntirety-SQLServerPerf_000100.blg Ntirety-SQLServerPerf_000101.blg Ntirety-SQLServerPerf_000102.blg Ntirety-SQLServerPerf_000103.blg Ntirety-SQLServerPerf_000104.blg Ntirety-SQLServerPerf_000105.blg Ntirety-SQLServerPerf_000106.blg Ntirety-SQLServerPerf_000107.blg Ntirety-SQLServerPerf_000108.blg -f BIN -o Combined.blg
...and went to run it from the command prompt. Much to my surprise I received the following:
C:\PerfLogs\Blog>relog Ntirety-SQLServerPerf_000095.blg Ntirety-SQLServerPerf_000096.blg Ntirety-SQLServerPerf_000097.blg Ntirety-SQLServerPerf_000098.blg Ntirety-SQLServerPerf_000099.blg Ntirety-SQLServerPerf_000100.blg Ntirety-SQLServerPerf_000101.blg Ntirety-SQLServerPerf_000102.blg Ntirety-SQLServerPerf_000103.blg Ntirety-SQLServerPerf_000104.blg Ntirety-SQLServerPerf_000105.blg Ntirety-SQLServerPerf_000106.blg Ntirety-SQLServerPerf_000107.blg Ntirety-SQLServerPerf_000108.blg -f BIN -o Combined2.blg
C:\Perflogs\Blog>relog Combined1.blg Combined2.blg -f BIN -o Combined.blg