what is the command in Linux

[Sudhakar_16]

It’s a command in linux, to check for system performance, which will cause the system to reboot writing every thing (what happens during the shutdown and in the startup) into a log which can be placed in a tar file.

[Ivan Ferreira]

If you install the sysstat package, you will get:

sar
iostat
mpstat

Sar will log performance metris to /var/log/sa

Then you have

vmstat
top

[Mike Stroyan]

Maybe you are thinking of bootchart.
http://www.bootchart.org/

or diskstat
http://ds9a.nl/diskstat/

[dirk dierickx]

or perhaps the boot.log file found in /var/log on RH systems.

[Delrish]

for boot process you can use dmesg.
for performance monitoring you can use sar or top