Fancy Read-Ahead

[Keri MacNeil]

At the HP Technology Forum a couple of weeks ago I attended a performance session. On of the attendees asked why "Fancy Read Ahead" was turned on by default in HPUX 11.0, but turned off by default in HPUX 11.11. From the discussion, it appears to be a good way to improve performance on storage devices.

I found the setting in the kernel, vx_fancyra_enable, but can't find any documentation on what it does and how it affects HPUX 11.11. Anyone have experiences with this parameter? Pros/Cons?

[Pete Randall]

Michael,

The only comment I have is that, as I suspected from the name, a quick check in SAM tells me it is VxFS only!

Other than that, read-ahead buffering tends to benefit most seguential filesets - truly random reads generally fare poorly.


Pete

[A. Clay Stephenson]

If you enable this, make sure that you install PHKL_33258. The patch releases notes explain the bug in fancy read aheads that was fixed. Basically, fancy read aheads allows the system to recognize patterns other than purely sequential reads (normal read ahead) and take advantage of them. For example, a backwards read of a file might be recognized and previous records would automatically be prefetched w/o a specific read() being issued. Whether or not this feature helps or hurts is highly data dependent so the only possible advice is 1) gather some metrics 2) change the settings 3) gather more metrics 4) compare metrics.

[Kent Ostby]

There was a bad patch PHKL_23240 that affected this as well.

[James R. Ferguson · Accepted solution]

Hi Michael:

According to Technical Knowledge Base document #JFS35KBQA00000349, 'vx_fancyra_enable' is obsolete in JFS 3.5.

The tunable was enabled by default in 11.0 and disabled by default in 11.11.

There is a short, whitepaper on JFS tuning, here:

http://docs.hp.com/en/5576/JFS_Tuning.pdf

Regards!

...JRF...