Hitchhiker's Guide to Openbsd

14.19 - Why aren't we using async mounts?
Question: "I simply do "mount -u -o async /" which makes one package I use (which insists on touching a few 
hundred things from time to time) usable. Why is async mounting frowned upon and not on by default (as it is in 
some other unixen)? Isn't it a much simpler, and therefore, a safer way of improving performance in some 
applications?" 
Answer: "Async mounts are indeed faster than sync mounts, but they are also less safe. What happens in case of a 
power failure? Or a hardware problem? The quest for speed should not sacrifice the reliability and the stability of 
the system. Check the man page for 
mount(8)
." 
async All I/O to the file system should be done 
asynchronously.
This is a dangerous flag to set since it does not 
guaran-
tee to keep a consistent file system structure on the
disk. You should not use this flag unless you are pre-
pared to recreate the file system should your system
crash. The most common use of this flag is to speed up
restore(8) where it can give a factor of two speed in-
crease.
http://www.openbsd.org/faq/faq14.html (33 of 34)9/4/2011 10:02:25 AM

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14 - Disk Setup
On the other hand, when you are dealing with temp data that you can recreate from scratch after a crash, you can 
gain speed by using a separate partition for that data only, mounted async. Again, do this only if you don't mind 
the loss of all the data in the partition when something goes wrong. For this reason, 
mfs(8)
 partitions are mounted 
asynchronously, as they will get wiped and recreated on a reboot anyway. 
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15 - The OpenBSD packages and ports system
 
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