Date: Tue, 15 Jun 2010 20:12:58 +0200 From: Bernd Walter <ticso@cicely7.cicely.de> To: Pawel Jakub Dawidek <pjd@freebsd.org> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS l2arc and HAST ? newbie question Message-ID: <20100615181258.GO87112@cicely7.cicely.de> In-Reply-To: <20100615155348.GA1922@garage.freebsd.pl> References: <4C177E69.3020204@gibfest.dk> <20100615155348.GA1922@garage.freebsd.pl>
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On Tue, Jun 15, 2010 at 05:53:48PM +0200, Pawel Jakub Dawidek wrote: > On Tue, Jun 15, 2010 at 03:21:45PM +0200, Thomas Steen Rasmussen wrote: > > Hello list, > > > > I am playing with HAST in order to build some redundant storage > > for a mailserver, using ZFS as the filesystem. > > I have the following zpool layout before stating the HAST experiments: > > > > NAME STATE READ WRITE CKSUM > > tank ONLINE 0 0 0 > > raidz2 ONLINE 0 0 0 > > label/hd4 ONLINE 0 0 0 > > label/hd5 ONLINE 0 0 0 > > label/hd6 ONLINE 0 0 0 > > label/hd7 ONLINE 0 0 0 > > logs ONLINE 0 0 0 > > mirror ONLINE 0 0 0 > > label/ssd0s1 ONLINE 0 0 0 > > label/ssd1s1 ONLINE 0 0 0 > > cache > > label/ssd0s2 ONLINE 0 0 0 > > label/ssd1s2 ONLINE 0 0 0 > > > > As I understand it, to accomplish this with HAST I will need to make a > > HAST resource for each physical disk, like so: > > > > NAME STATE READ WRITE CKSUM > > tank ONLINE 0 0 0 > > raidz2 ONLINE 0 0 0 > > hast/hahd4 ONLINE 0 0 0 > > hast/hahd5 ONLINE 0 0 0 > > hast/hahd6 ONLINE 0 0 0 > > hast/hahd7 ONLINE 0 0 0 > > > > But what about slog and cache devices, currently on SSD disks for > > performance reasons ? It doesn't really make sense to synchronize > > a cache disk over the network, does it ? > > No, it doesn't. Cache is forgotten on import anyway, so don't bother. > You have to be careful, though, because you probably need to remove old > cache device from the pool after import on secondary and add local disk. Unless conditions have changed a missing cache device seem to be without any sign of problem. I have been running with USB-sticks for L2ARC and also tested with missing USB devices at boot time (without reimporting). Even unplugging a device during access wasn't a problem - at least not with ZFS. ZFS didn't even complain when I had a blocked USB and every access to a specific cache device just timed out - in fact I didn't even notice any slowness. Pawel, do you know if there is any chance that ZFS can boot with warm L2ARC? I is mentioned in several articles, but my system starts with empty cache. My system is already quite old, so probably it is already in. > > Could I build the zpool with the SSD disks directly (without > > HAST) and would ZFS survive an export/import on the other host, > > when the cache disks are suddently different ? I am thinking cache > > only here, not slog. > > It simply won't find cache disks, you will need to do what I described > above. > > > Do SSD l2arc / slog even make any sense when I am "deliberately" > > slowing down the filsystem with network redundancy anyway ? > > Forget about HAST for L2ARC. In case of SLOG it can still be faster over > the network than pool with local SATA disks without SLOG. As usual the > best way to verify this is to test it for your workload:) > > > Oh, and is there any problems using labels for HAST devices ? My > > controller likes to give new device names to disks now and then, > > and it has been a blessing to use labels instead of device names, > > so I'd like to continue doing that when using HAST. > > Use labeled providers in hast.conf, but there is no need to label HAST > providers (/dev/hast/<name>). I'm very interested to hear about your results with HAST/ZFS combo because I have a possible use case. -- B.Walter <bernd@bwct.de> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.
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