turbo boost emerge --sync and cvs up on gentoo-x86

November 17, 2012

I while ago I used seekwatcher to find out that portage operations are seek-bound and better handled by small loop device. I had some problems creating such btrfs loop device, due to nonworking nocow loop devices, but now I have the solution!

The recipe:

for btrfs host filesystem: make sure you have at least >=sys-fs/e2fsprogs-1.42.6 which has chattr/lsattr with nocow support.

for btrfs host filesystem: mark a directory with loop device as nocow:

$ mkdir -p /subvolumes/nocow-images/
$ chattr +C /subvolumes/nocow-images/
$ sudo lsattr -ld /subvolumes/nocow-images/
/subvolumes/nocow-images/    No_COW

Build small btrfs loop device:

dd if=/dev/zero of="$image" bs=1M count=1K
mkfs.btrfs           \
 -d single -m single \
 -L "$label"         \
 -l 32k -n 32k       \
 "$image"

for btrfs host filesystem: make sure created file is nocow:

$ lsattr -l /subvolumes/nocow-images/gentoo-32k.img
/subvolumes/nocow-images/gentoo-32k.img No_COW
$ /usr/sbin/filefrag /subvolumes/nocow-images/gentoo-32k.img
/subvolumes/nocow-images/gentoo-32k.img: 1 extent found

And mount it:

/subvolumes/nocow-images/gentoo-32k.img         /gentoo-32k     btrfs           loop,nodatasum,noatime,nodiratime    0       0

Amend /etc/portage/make.conf like that:
```
PORTDIR=/gentoo-32k/portage
```
and so on.
Have fun!

Some notes:

Larger leaf sizes allow better inlining of small files and speeding up read pattern
Smaller loop device allow faster reading-out the whole loop file in RAM reducing all the I/O.
Disabling datasums and atimes for loop contents we leave only raw data there.

Some numbers:

$ time emerge --sync # 72 seconds
...
real    1m12.111s
user    0m1.706s
sys     0m1.819s

gentoo-x86 $ time cvs up # ~4 minutes
...
real    4m35.208s
user    0m3.656s
sys     0m14.226s