I have been experimenting with dirhash settings, and have scoured the internet
for other peoples' experience with it.
(I found the performance improvement in compiling has forestalled the need to
add an SSD drive. ;-)
I believe that increasing the following values by 10 would benefit most FreeBSD
users without disadvantage.
vfs.ufs.dirhash_maxmem: 2097152 to 20971520
vfs.ufs.dirhash_reclaimage: 5 to 50 or 60
FreeBSD administrators who have adjusted vfs.ufs.dirhash_maxmem for file server
use have found
8 megs to be about minimum to satisfy vfs.ufs.dirhash_mem usage,
while I have found compiling larger packages such as Firefox brings
vfs.ufs.dirhash_mem up to about 13 megs.
Setting vfs.ufs.dirhash_maxmem much larger than 20 megs results in
vfs.ufs.dirhash_lowmemcount events
scavenging excessive amounts of the directory cache, at 10% per event.
I believe vfs.ufs.dirhash_reclaimage of about a minute is sufficient to reclaim
memory without losing active directory cache,
particularly when compiling; however my experience in operations leads me to
believe this is also an optimal default for file servers.
As the worst case scenario is that the directory cache will be over cleansed and
the memory returned to the kernel,
I would like to suggest that these defaults be raised by a factor of 10 for
FreeBSD 9.2 or 10.0.
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