On Tue, Jan 04, 2011 at 09:46:39AM -0800, Patrick J. LoPresti wrote: > Hey, what's the worst that could happen? That's just asking for trouble. ;) > I recently learned that some of my colleagues have configured two > Linux systems to simultaneously mount a single XFS partition residing > on shared storage. Specifically, "system R" has the partition mounted > read-only while "system W" has it mounted read/write. > > I told them that this sounds like a very bad idea because XFS is not a > clustered file system. But they are skeptical because "it seems to be > working fine". I need to know what the actual risks are and whether > they can be mitigated. Ok, so it will appear to work fine most of the time... > This partition holds large amounts of essentially archival data; that > is, it is read frequently but written rarely. When they do want to > write to it, they do so via system W and then reboot system R. You could probably just run "echo 3 > /proc/sys/vm/drop_caches" or just umount/mount the device again to get the same effect as rebooting. > I am no expert on XFS, but there are essentially two risks that I can see: > > Risk 1: When making changes via system W, the view of the file system > from system R can become corrupted or inconsistent. My colleagues are > aware of this and believe they can live with it, as long as the > underlying file system is not being damaged ("we can just reboot"). Yup, so long as system R does not cache anything, or the caches are dropped after system W writes, you should be fine. However, there is a window between system W starting to write and system R being rebooted that system R could read inconsistent metadata and/or data. There's not much you can do about that apart from take system R offline while system W is writing. > Risk 2: Any time the file system is mounted, even read-only, it will > replay the journal if it is non-empty. (At least, I believe this is > true. Could one of you please confirm or deny?) So if machine R > should reboot while the journal is non-empty, it will replay it, > causing fairly unpredictable on-disk corruption. Yup. > Here are my questions. > > 1) When can a read-only XFS mount write to the disk, exactly? Log recovery only. Use mount -o ro,norecovery to avoid that. > 2) If I do a "sync" on machine W (and perform no further writes), will > that truncate the journal? FYI, the journal cannot be truncated - it is a fixed size circular log. To get the log clean, I'd freeze the filesystem on system W while system R mounts. e.g: system W system R unmount <fs> write data freeze fs mount -o ro,norecovery <fs> unfreeze fs > 3) What am I missing? 1. NFS/CIFS. No need for shared access to the block device. NFs works pretty well for read only access, especially if you put a dedicated 10GbE link between the two machines... 2. Snapshots. If you must share the block device, snapshot the active filesystem and mount that readonly on system R - the snapshot will be unchanging. When system W knows a snapshot is unmounted and finished with, it can delete it. That is: system W system R write data .... write data snapshot <fs.ss2> umount <fs.ss1> mount -o ro,norecovery <fs.ss2> delete snapshot <fs.ss1> .... write data ..... write data snapshot <fs.ss3> umount <fs.ss2> mount -o ro,norecovery <fs.ss3> delete snapshot <fs.ss2> .... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs