Add support for use_md_swap. When this variable is set in a

portbuild.conf, builds will each be done in a separate swap-backed md.
This dramatically improves build performance since

* Every transaction is not written to disk, so disk bandwidth is not
  a bottleneck

* Multiple builds do not contend with each other for the same set of
  filesystem locks and other per-device resources

The size of the md devices is controlled by the md_size variable.  '2g'
seems to be a good size.

Currently we mdconfig -u each device after each port build, since
otherwise dirty blocks accumulate and the md eventually uses a full
amount of backing store (2g in the above example).  This is a problem
if there is unsufficient swap backing to accomodate them all.

XXX This should be made configurable to avoid the performance penalty on
systems that do have enough swap backing

1 files changed, 18 insertions, 4 deletions

diff --git a/Tools/portbuild/scripts/claim-chroot b/Tools/portbuild/scripts/claim-chroot
index 3fd4a70..c03283f 100755
--- a/Tools/portbuild/scripts/claim-chroot
+++ b/Tools/portbuild/scripts/claim-chroot
@@ -40,10 +40,24 @@ done
 chrootnum=$$
 # If we didn't find a pre-existing directory, create and claim a new one.
 while [ ${found} != 1 ]; do
-  chrootnum=$(($chrootnum+1))
-  chroot=${chrootdir}/${chrootnum}
-  mkdir -p ${chroot} 2>/dev/null || continue
-  mkdir ${chroot}/used 2>/dev/null || continue
+  if [ "${use_md_swap}" = "1" ]; then
+    unit=$(mdconfig -a -t swap -s ${md_size})
+    newfs /dev/${unit} > /dev/null
+    chrootnum=$(echo ${unit} | sed 's,md,,')
+    chroot=${chrootdir}/${chrootnum}
+    mkdir -p ${chroot}/used 2>/dev/null || continue
+    # Need to make sure that used/ is also present after mounting the fresh md so as to not leave open any races
+    mount /dev/${unit} ${chroot}/used
+    mkdir ${chroot}/used/used
+    touch ${chroot}/used/used/${pkgname}
+    umount ${chroot}/used
+    mount /dev/${unit} ${chroot}/
+  else
+    chrootnum=$(($chrootnum+1))
+    chroot=${chrootdir}/${chrootnum}
+    mkdir -p ${chroot} 2>/dev/null || continue
+    mkdir ${chroot}/used 2>/dev/null || continue
+  fi
   touch ${chroot}/used/${pkgname}
   touch ${chroot}/.notready
   found=1