On Wed, Apr 18, 2018 at 06:04:30PM +0800, Craig Ringer wrote: > On 18 April 2018 at 05:19, Bruce Momjian <br...@momjian.us> wrote: > > On Tue, Apr 10, 2018 at 05:54:40PM +0100, Greg Stark wrote: > >> On 10 April 2018 at 02:59, Craig Ringer <cr...@2ndquadrant.com> wrote: > >> > >> > Nitpick: In most cases the kernel reserves disk space immediately, > >> > before returning from write(). NFS seems to be the main exception > >> > here. > >> > >> I'm kind of puzzled by this. Surely NFS servers store the data in the > >> filesystem using write(2) or the in-kernel equivalent? So if the > >> server is backed by a filesystem where write(2) preallocates space > >> surely the NFS server must behave as if it'spreallocating as well? I > >> would expect NFS to provide basically the same set of possible > >> failures as the underlying filesystem (as long as you don't enable > >> nosync of course). > > > > I don't think the write is _sent_ to the NFS at the time of the write, > > so while the NFS side would reserve the space, it might get the write > > request until after we return write success to the process. > > It should be sent if you're using sync mode. > > >From my reading of the docs, if you're using async mode you're already > open to so many potential corruptions you might as well not bother. > > I need to look into this more re NFS and expand the tests I have to > cover that properly.
So, if sync mode passes the write to NFS, and NFS pre-reserves write space, and throws an error on reservation failure, that means that NFS will not corrupt a cluster on out-of-space errors. So, what about thin provisioning? I can understand sharing _free_ space among file systems, but once a write arrives I assume it reserves the space. Is the problem that many thin provisioning systems don't have a sync mode, so you can't force the write to appear on the device before an fsync? -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + As you are, so once was I. As I am, so you will be. + + Ancient Roman grave inscription +
