Thursday, March 12, 2009

The purpose in life of vxfsd

Currently, the #1 Google keyword that has readers stumbling on this blog is "vxfsd" and they spend some time checking out the entry where I tell how to reduce vxfsd usage on 11iv2. It figures; vxfsd often shows up first in top(1) and there is not much documentation on what it does, let alone a man page. I think an explanation of what this this daemon is used for is in order.

Under Unix, an inode is a structure that contains file metadata (size, position on disk, owner, etc). You can find a .pdf with an example inode here. HP-UX puts these inode structures in a cache to accelerate I/O performance. That area is called the inode cache. It is a dynamic cache, meaning that it grows or shrinks its size RAM depending on the needs.

Where does vxfsd come in? It's a daemon whose job is to scan the inode cache, freeing inodes which have not been referenced since a while. The more inodes there are to scan and free, the more it spins and consumes CPU time.

When there is intensive read I/O on a server, such as during a backup, all files are read, so a great bunch of inodes get cached in the progress, with the cache size growing quickly. It is normal under these circumstances to see vxfsd spinning once that I/O pressure is over; it's doing its job, scanning the cache and freeing the inodes that haven't been recently needed. It should slow down once it's done.

As I described in my earlier post, the kernel can be tuned to use a static inode cache, and this will make vxfsd stop needing to scan the cache. It can actually be a good idea to do this on low-end systems (especially VMs) that have few or slow CPUs, as vxfsd can consume a fair percentage of CPU time on these types of systems. The trade off is that you could have lower I/O performance since the cache will no longer be able to grow to meet a sudden I/O demand. I normally don't care about vxfsd on systems that have multiple processors, and let it live with its default settings.

Take care

4 comments:

Purpose in life essay said...

well post, i was looking the same for my purpose in life essay help.

Purpose in Life Essay

Salty Bodice Ripper said...
This comment has been removed by the author.
Salty Bodice Ripper said...

glad i came across you blog, but i dont have kctune on that server, im guessing someones using the veritas FS to transfer some files, just saw the user's ID on the box~

Anonymous said...

very much thanks for d info
perfectly explained what is vxfsd.
I really appreciate ppl writing technical posts eventhough their viewers r less.They are a lot fo help for novices like me.