Date: Mon, 13 Feb 2012 13:18:08 -0500 From: Gary Palmer <gpalmer@freebsd.org> To: Kevin Day <toasty@dragondata.com> Cc: freebsd-fs@freebsd.org Subject: Re: Simultaneous read-only snapshot mount Message-ID: <20120213181808.GC78733@in-addr.com> In-Reply-To: <A14E351F-9C8C-4239-9ECD-E84F3BBFD9D5@dragondata.com> References: <A14E351F-9C8C-4239-9ECD-E84F3BBFD9D5@dragondata.com>
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On Mon, Feb 13, 2012 at 09:55:36AM -0600, Kevin Day wrote: > > It's now possible/somewhat common for the ability to have two systems can see the same disk(s) at the same time (through iSCSI or storage systems that support dual controllers, etc). > > Suppose system A has a filesystem mounted RW, and takes a snapshot. Can system B access that snapshot via a read only mount, while system A continues to read/write to that filesystem? Or more basically, is everything needed to read a snapshot stable/unchanging enough that a second system can read from it while the filesystem is still being updated? If the snapshot is taken on the filesystem, take the snapshot, mount it on server 1 and export it over NFS (or other network filesystem of your choice) to server 2. Anything else will likely result in undefined behaviour. I don't think any FS guarantees that metadata related to files in a snapshot will not change as the original read/write FS changes. If nothing else, most (all?) times you have to have the original FS mounted to get to the snapshot, which leads to obvious data consistency problems given the original FS is read/write on server1. Alternatively, do a snapshot on the shared storage rather than at the filesystem layer. e.g. export a LUN via FC/iSCSI from a NetApp to server 1, take a snapshot on the volume that contains server 1 and through some commands that escape me right now you can use the LUN in the snapshot and export it to server 2. I'm sure other storage systems can do something similar, I just happen to know offhand NetApp can do it. Gary
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