ZFS Hangs

Kris Kennaway kris at FreeBSD.org
Sun Nov 4 04:26:27 PST 2007

Peter Schuller wrote:
>> It certainly could be a caching effect but the particular problem you
>> were seeing should have been fixed.  Are you still seeing it?
> I am not still seeing it; the machine has since been switched to AMD 64, and 
> RAM increased from 1 GB to 3 GB (and FreeBSD itself upgraded several times 
> since then). I did not look into it in terrible detail; I just observed by 
> tracing that the I/O in question was definitely the package database, and 
> that it was definitely accessing the same files mutiple times. I assumed it 
> was a caching effect at the time, given all the maxvnode tweaking and sizing 
> restrictions put on the ARC. I do know I never saw it after the memory 
> increase.

I think the particular issue was fixed by improvements to the package tools.

> Also, this might have been prior to disabling prefetch. If it was, perhaps the 
> prefetch hosed the cache. (Are the prefetch issues still there, anyone?)

Well the readahead "issues" are just the zfs design, AFAIK.  It does 
aggressive prefetching to improve performance on the assumption that you 
are running with a reasonably fast storage system that can keep up.  On 
crappy low end disk hardware (e.g. single ATA disk) this can easily 
saturate the disk.


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