In message <199706111729.KAA09130@lestat.nas.nasa.gov> Jason Thorpe writes:
: Async metadata writes do give "better performance", but IMO are the
: wrong way to get better performance.  If you crash before your metadata
: has been written, there is no consistent information from which you can
: repair the file system.  Basically, it's bloody dangerous, especially if
: you consider your data important.

There are cases where it is a big win.  For example if you are doing a
make world on FreeBSD, then if you have a separate partition for
/usr/obj, it can be a huge win (my make world tests showed that I got
a 45 minute improvement in build time from 2:15 to 1:30).  In this
case who cares if you crash and lose /usr/obj.  You can always get it
back by doing another make world.  Also, your window for failure is
limited to those periods when you are actively updating yourfile
system.  So when you move the the install phase (assuming it is also
mounted noatime), then no updates happen to /usr/obj and if you crash
you can still finish the install.

It is also great for a news server.  It is a huge win on a news
partition, which many people consider to be completely expendable
data.

Having run async on various systems for some time now, I've only once
had a crash where we had to newfs the file system.  Other times the
damage was no worse than "usual" with a system crash.  While the
potential for great damage is higher with async, the probaility that
I've seen which that damage happens seems to be low enough for most
people to be an acceptible risk, especially when coupled with other
methods of data protections: daily backups, monthy level 0's, CVS,
etc.

Having said all that, it does take a lot of experience to know what an
acceptible risk is.  My personal email spool area isn't mounted async,
but the disk that has all my mailing lists sorted to it is async.   I
don't care if I lose mail from a mailing list on a crash, I can get it
from the archives.  I do care if I lose personal email, however.

Like everything else, it is what level of risk are you confortable
with.

Warner