Performance: JFS and full file system

[Jeff_Traigle]

Been a while since I took the performance and tuning course. Have a situation to investigate now that involves a JFS file system (delaylog only mount option) that is a bit more than 90% full containing application and data files. Haven't gathered any performance metrics on disk activity, fragmentation, or anything else yet, but had a general question about any free space and performance correlation on JFS. The course material mentions low free space being a problem for HFS because of fragments being moved to create free 8kB block. No mention of this issue for JFS. Since JFS allocates space by extents instead of blocks, does it avoid performance problems when a file system has limited free space remaining or am I reading too much into what's not said?

[Dave Olker · Accepted solution]

Hi Jeff,

Mark Ray gave a talk about this very topic at HP World this year. The proceedings are now posted to the HP World web site. Mark's session was entitled "JFS Tuning and Performance" and it covered topics like fragmentation, limited disk space, etc.

Here is the link to Mark's slides:

ftp://198.151.251.239/pub/conference/hpworld2004/proceedings/3344.pdf

I found his session to be very informative and well worth attending. He usually presents every year, so try to catch his sessions in 2005.

Regards,

Dave

[Bill Hassell]

HFS and VxFS read performance is unaffected by free space in the filesystem. HFS write performance is dramatically slowed at about 85-90% full, but only when creating new blocks. Writing inside a preallocated file (like most databases) is unaffected by a full filesystem. VxFS has less overhead when searching for empty space to allocate and the impact is not nearly as dramatic as HFS but just like HFS, searching for freespace will carry an overhead.

So the first check is to see whether files are being read (no impact), written inside preallocated space (no impact) or creating new files or extending existing files (big impact). Of course, this won't last for long since there is only 10% freespace and it will likely become 0% fairly rapidly and the performance issue is now a failure.

There is also the corencase of rapidly deleting and adding lots of files, like in a Usenet server. In this case, performance will definitelybe affected by running 85-90% of capacity.

[Dave Olker]

Hi Jeff,

One other note, when Mark gave his presentation at HP World, he not only spoke from the slides I mentioned in my previous post, he also handed out a white paper that discussed all of the topics on his slides in greater detail.

Here is the link to the latest version of his JFS Tuning and Performance white paper:

http://www.docs.hp.com/hpux/onlinedocs/5576/JFS_Tuning.pdf

Between the white paper and the slide deck, you will have a good deal of information on JFS filesystem tuning and performance.

Regards,

Dave

[Jeff_Traigle]

Thanks, Dave and Bill. Wish I'd been able to attend HP World, but, as a contractor, I'm considered transient and not worthy of spending training money on. :) Oh well. I'll read through Mark's presentation and white paper.

They are definitely writing to the file system when the slowness occurs. Obviously, the limited space is a problem in itself that will need to be addressed. Once the metrics are collected, I should be able to figure out if it's just a result of the file system issue or something else in addition to it.

[Alzhy]

Hmm. I seemed missed this session... but reading throughthe PDF's.. it does seem OJFS (VxFS) 3.5 may have some advantages over the earlier versions.

Does anyone know if HP-UX 11i v 2.0 is bundled with VxFS 3.5 or 4.0 ?

[Dave Olker]

Hi Nelson,

Page 3 of the technical paper I listed above contains a table that shows each HP-UX OS and their accompanying version of VxFS and default disk layout.

For 11i v2 (11.23), the version of VxFS is 3.5 and the default disk layout is 5.

Regards,

Dave