On Sat, May 25, 2013 at 08:12:31PM +1000, Russell Coker wrote:
options zfs zfs_arc_max=536870912
I just checked the system in question, it still has the above in the modules configuration from my last tests. "free" reports that 9G of RAM is used as cache so things seem to be getting cached anyway.
yeah, but it'll be caching everything *except* for ZFS. zfsonlinux doesn't use linux's own caching, it uses ARC and L2ARC. it's not additional to linux's caching, it's instead of. this is on the TODO list to change in the future, but I don't think anyone's even working on it right now. lower priority than lots of other tasks
does btrfs use significantly less RAM than zfs? i suppose it would, as it uses the linux cache whereas ZFS has its separate ARC.
Yes. On any sort of modern system you won't notice a memory use impact of it.
One Xen DomU has 192M of RAM assigned to it and BTRFS memory use isn't a problem.
ok, cool.
The system in question doesn't have serious load (it's used as a box I can ssh to to test other systems and for occasional OpenVPN use) and it could be that it gives less performance because of BTRFS. But the fact that it works at all sets it apart from ZFS.
yeah, well, execpt in special circumstances (like a file-server where i'm going to be throwing lots of RAM at the VM anyway), i wouldn't use ZFS as a VM's filesystem. i'd use ext4 or xfs on a zvol.
Also note that dpkg calls sync() a lot and thus gives poor performance when installing packages on BTRFS. As an aside, I'm proud to have filed the bug report against dpkg which led to this.
dpkg's sync() fetish gives pretty poor performance on most filesystems. for an exciting time: # apt-get install eatmydata # ln -s /usr/bin/eatmydata /usr/local/sbin/dpkg (i used to do this on my home system with an xfs / before i got an SSD. dangerous, but *much* faster. YMMV but i never suffered any data loss or corruption of dpkg's status file etc from it - low risk of a problem but potentially catastrophic if power-failure/crash/whatever does occur)
If snapshots HAVE to be run at the same time then you'll probably have other problems.
true. as i said, it was speculation. of the 'what could possibly go wrong?' kind :) and i won't know whether it is a problem or not until i try it. craig -- craig sanders <cas@taz.net.au>