Using an SSD "disk" for /

[Oliver Fromme]

Oliver Fromme wrote:
 > With /usr/src and /usr/obj both on the same SATA-300 HDD,
 > a buildworld takes 10 minutes and 13 seconds.
 > 
 > With /usr/src and /usr/obj on separate memory disks,
 > a buildworld takes 10 minutes and 4 seconds.
 > 
 > This is stable/8 with empty /etc/src.conf, i.e. a complete
 > buildworld.  The machine is an AMD Phenom II X6 (hexacore).

I think I should give some more information about the
system that I used for the test.

The HDD is accessed via ahci(4) and ada(4) with NCQ.
This might make a small difference, though I haven't
benchmarked it.  src and obj are mounted with noatime.
In the memory disk case, src was even mounted read-only,
and obj was mounted async.

I made sure that there was no swapping / page-out
activity during the memory disk test, so all the data
really stayed in RAM (which should be even faster
than an SSD, I assume).  The system was otherwise idle,
gstat(8) basically showed nothing on ada0 during the
memory disk test.

The following sysctl tunings are in effect:

vfs.hirunningspace=8388608
vfs.lorunningspace=1048576
vfs.read_max=32
vfs.ufs.dirhash_maxmem=33554432
kern.sched.preempt_thresh=220

(I've got more things in /etc/sysctl.conf, but the rest
shouldn't affect file system performance or buildworld,
I think.)

The kernel is running with HZ=300 (unfortunately HZ isn't
dynamic in FreeBSD, so I'm using 300 as a trade-off).
AUDIT and MAC are disabled.

Best regards
   Oliver

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