On 06/27/2014 07:40 PM, Russell Coker wrote: > On Fri, 27 Jun 2014 18:34:34 Goffredo Baroncelli wrote: >> I don't think that it is possible to mount the _same device_ at the _same >> time_ on two different machines. And this doesn't depend by the filesystem. > > If you use a clustered filesystem then you can safely mount it on multiple > machines. > > If you use a non-clustered filesystem it can still mount and even appear to > work for a while. It's surprising how many writes you can make to a dual- > mounted filesystem that's not designed for such things before you get a > totally broken filesystem. > > On Fri, 27 Jun 2014 13:15:16 Austin S Hemmelgarn wrote: >> The reason it appears to work when using iSCSI and not with directly >> connected parallel SCSI or SAS is that iSCSI doesn't provide low level >> hardware access. > > I've tried this with dual-attached FC and had no problems mounting. In what > way is directly connected SCSI different from FC? > FC is actually it's own networking stack (and you can even run (in theory) other protocols like IP and ATM on top of it), whereas parallel SCSI is just a multi-drop bus, and SAS is just a tree-structured bus with point-to-point communications emulated on top of it. In other words, parallel SCSI has topological constraints like RS-485, SAS has topology constraints like USB, and FC has topology constraints like Ethernet.
Secondarily, most filesystems on Linux will let you mount them multiple times on separate hosts (ext4 has features to prevent this, but they are expensive and therefore turned off by default, I think XFS might have similar features, but I'm not sure). BTRFS should in theory be more resilient than most because of the COW nature (as long as it's only a few commit cycles, you should still be able to recover most of the data just fine).
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