Date: Sat, 19 Oct 2002 10:13:51 +0200 From: Jens Rehsack <rehsack@liwing.de> To: Dan Nelson <dnelson@allantgroup.com> Cc: Mike Thompson <mike@atomz.com>, freebsd-questions@FreeBSD.ORG Subject: Re: Multiple FreeBSD SCSI Hosts Message-ID: <3DB1143F.1F4E49F1@liwing.de> References: <4.3.2.7.2.20021018114625.00c974a0@pop.atomz.com> <20021018193431.GC38243@dan.emsphone.com>
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Dan Nelson wrote: > > In the last episode (Oct 18), Mike Thompson said: > > I'm interested in setting up two FreeBSD systems in a very simple > > clustering type configuration around centralized external SCSI > > storage devices for hardware redundancy. The specific configuration > > I'm thinking about is shown in the simple diagram below: > > > > +-----------+ +----------+ +-----------+ > > | FreeBSD | | | | FreeBSD | > > | Host 1 | | External | | Host 2 | > > | (primary) | | SCSI | |(secondary)| > > | | |RAID/DRIVE| | | > > | SCSI r/w | | | | SCSI r/o | > > +-----+-----+ +---+--+---+ +-----+-----+ > > | | | | > > +---------------+ +---------------+ > > Whoa. Deja vu here. > > http://groups.google.com/groups?threadm=Pine.LNX.4.44.0210101808560.3631-100000@scribble.fsn.hu > > Your primary problem is that the disk cache on Host2 does not know > about changes made to the filesystem by Host1. > > > 1. Do the FreeBSD SCSI drivers support such a configuration by > > implementing the SCSI RESERVE/RELEASE commands to lock access to the > > SCSI bus? If so, which drivers for which specific SCSI adapters? > > Reserve/release looks like it is implemented for tape devices on > open(), but not for disks (since it's legal to mount slice 1 on machine > A but slice 2 on machine B, for example). Reserve/release is probably > not useful in this situation anyway, as I don't know how easy it is to > break a reservation (when host1 goes down, host2 has to break the > reservation before it can acces the disk). > > > 2. Many external raid controllers describe themselves as host dual > > capability. Is the type of configuration they are describing? > > Possibly. Most of the time it's to allow multiple hosts to access > their own private storage, but you can usually force a volume to be > visible on multiple ports. > > > 3. Will the secondary server still experience a corrupted file system > > because of write caching by the primary server? If so, would it be > > possible to configure the primary server to write through the cache? > > Big time. The problem is not so much write caching on the primary, > it's read caching on the secondary. Without a shared-storage > filesystem, you're pretty much limited to NFS mounts (which are not all > that bad). > > As for failover detection, one good way is to dedicate a small > partition (da0s1a, for example) as a heartbeat partition. Host 1 > writes a timestamp to /dev/da0s1a block 1 every second, Host 2 does the > same thing on block 2. Each host then reads the other host's block, > and if there are no updates for (say) 10 seconds, you can assume that > the other host is dead, do a fsck, and mount the volume. Using the raw > disk device bypasses any caches on the systems. Hi folks, I thought about a similar configuration, too. I found a manufactor for host controller (SCSI, Fibre Channel), who can deliver: ICP Vortex: http://www.icp-vortex.com/ - they have cluster ready host controller, but they always mirror, so you need 2 disk stacks. Maybe there is required additional software for take over, but by the way, it would even in your situation ... Bye, Jens > -- > Dan Nelson > dnelson@allantgroup.com > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-questions" in the body of the message -- L i W W W i Jens Rehsack L W W W L i W W W W i nnn gggg LiWing IT-Services L i W W W W i n n g g LLLL i W W i n n g g Friesenstraße 2 gggg 06112 Halle g g g Tel.: +49 - 3 45 - 5 17 05 91 ggg e-Mail: <rehsack@liwing.de> Fax: +49 - 3 45 - 5 17 05 92 http://www.liwing.de/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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