In message <20090714201940.GF27582@shell>, Valerie Aurora writes: > On Tue, Jul 14, 2009 at 02:19:26PM -0400, Erez Zadok wrote: > > In message <20090714174828.GE27582@shell>, Valerie Aurora writes: [...] > > IMHO it's not worth at this stage to try and solve that problem in an > > end-to-end manner (client to server). For a unioning layer to have to worry > > about every possible change in any of the layers below it, is no different > > than for every possible network-filesystem client to be able to guarantee > > that nothing ever changes on the server unexpectedly: they don't, so why > > should you have to solve this problem now? Not that I don't think it's an > > important problem---I just don't see why *you* should have to solve this and > > not the network-filesystem community: whatever solution that can come up, > > can be applicable to any unioning layer. In the mean time, do the best you > > can (e.g., ESTALE, readonly superblocks, etc.). > > Okay, so my best idea for a solution is to introduce a new NFS mount > option that means the server promises that the exported file system is > read-only (using superblock read-only count scheme locally). E.g.: How would the server be able to guarantee that? Are you planning to change the protocol or implementation somehow? Are you assuming that the server will be running linux w/ special r/o sb support? If so, it won't work on other platforms (NFS is supposed to be interoperable in principle :-) Without a protocol change, such an option (if I understood you), is at best a server promise to "behave nice." In dealing with Unionfs, I've already had to face some really annoying cases related to this. For example, when the client mounts read-write, but the server does *any* combination of these two: 1. export readonly or readwrite 2. the native f/s exported can be locally mounted r/o or r/w. Turns out that servers in that case will return any of EROFS, EACESS, EPERM, and even ESTALE. So this is annoying to have to detect: a true permission error should be returned to the user in unionfs, but a readonly access should result in a copyup. I don't believe this behavior was standardized in v2/v3. So, in retrospect, it would be *great* if I had a client-side mount option that could guarantee that the server is exporting/mounting readonly. But I feel that for such a client-side option to work, some sort of information has to flow b/t the server and the client to validate this readonly assertion. > /etc/exports: > /client_root_fs thin-client-*.local.domain(server_ro,no_root_squash) > > Trond, is this super-gross or totally reasonable? Seems like we add > new NFS mount options at the drop of a hat. > > -VAL Erez. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
