Tag Archives: advance

How to chown and setuid

Something to watch out for.

If you change the owner of a file that has setuid permission, the setuid bit gets removed:

Therefore if you reset the owner of such a file (for instance, after an accidental recursive chown), then you need to reset the permissions afterwards.
If you reset both the owner AND the permissions, it has to be done in the correct order – ownership first, then permissions.

So what will this do?


It turns out (on RHEL5 at any rate) that it changes the permissions first. So the binaries that ought to have the setuid bit were left without it.

The moral of this story is that you should do the work in two passes:


Space utilised one liners


Apparent size is the number of bytes your applications think are in the file. It’s the amount of data that would be transferred over the network (not counting protocol headers) if you decided to send the file over FTP or HTTP. It’s also the result of cat theFile | wc -c, and the amount of address space that the file would take up if you loaded the whole thing using mmap.

Disk usage is the amount of space that can’t be used for something else because your file is occupying that space.

In most cases, the apparent size is smaller than the disk usage because the disk usage counts the full size of the last (partial) block of the file, and apparent size only counts the data that’s in that last block. However, apparent size is larger when you have a sparse file (sparse files are created when you seek somewhere past the end of the file, and then write something there — the OS doesn’t bother to create lots of blocks filled with zeros — it only creates a block for the part of the file you decided to write to).

Source (clarification): http://stackoverflow.com/questions/5694741/why-is-the-output-of-du-often-so-different-from-du-b 

MySQL – How much memory for InnoDB buffer?

Use this to find out how much storage each engine uses:

From the data above, it may seem like a good idea to keep innodb_buffer_pool low, because we have so much MyISAM data.
BUT if I get one query a minute against that MyISAM data, and 500 queries per second against the considerably smaller InnoDB data-set, that doesn’t make sense anymore.
Then we’d want all of the InnoDB data in memory, hence a buffer pool of 512M+

It’s down to data usage and where the hotspots are!

Percona Toolkit to ‘get the hotspots’

Get the hotspots:
you can set NUMPACKETS or keep the tcpdump running for 20 minutes. JUST keep an eye on the file size!

FYI: https://bugs.launchpad.net/percona-toolkit/+bug/1402776  official bug for that database being wrong thing