Software Disk Cache Limit Issues
Trent George
soundsampler at sbcglobal.net
Mon Jul 7 23:34:17 PDT 2003
Hi,
Since my last post I found a couple more interesting observations
1/ nswbuf_min has no effect on msdosfs (fat32) mounted filesystems like it did on ffs
this led me to beleive that it may be a metadata issue so:
2/ I experimented with newfs to see if blocksize, fragment size or ufs1, ufs2 effected the
ratio of nswbuf's to usable hirunningspace.
the answer is no newfs settings effected the seeming fixed ratio of 65k of usable
hirunningspace per nswbuf allocated on ffs filesystems
In summary:
a/ the default kernel has a max allocation of 256 nswbuf (in vfs_bio.c)
b/ this can be overridden with options NSWBUF_MIN=xxxx in custom kernel
c/ it seems the effect of a large sysctl vfs.hirunningspace gets capped on ffs filesystems
by the size of nswbuf. and this always seems fixed at 64k per nswbuf
d/ fat32 filesystems seemed immuned from this "capping" effect
it seems the more i try to understand the relationship, the more mysterious it becomes.
it also seems that fat32 performance seems erratic and less re-producable with ffs
Trent
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