On Fri, 2020-02-07 at 13:20 -0800, Andres Freund wrote: > Hi, > > On 2020-02-08 07:52:43 +1100, Dave Chinner wrote: > > On Fri, Feb 07, 2020 at 12:04:20PM -0500, Jeff Layton wrote: > > > You're probably wondering -- Where are v1 and v2 sets? > > > The basic idea is to track writeback errors at the superblock level, > > > so that we can quickly and easily check whether something bad happened > > > without having to fsync each file individually. syncfs is then changed > > > to reliably report writeback errors, and a new ioctl is added to allow > > > userland to get at the current errseq_t value w/o having to sync out > > > anything. > > > > So what, exactly, can userspace do with this error? It has no idea > > at all what file the writeback failure occurred on or even > > what files syncfs() even acted on so there's no obvious error > > recovery that it could perform on reception of such an error. > > Depends on the application. For e.g. postgres it'd to be to reset > in-memory contents and perform WAL replay from the last checkpoint. Due > to various reasons* it's very hard for us (without major performance > and/or reliability impact) to fully guarantee that by the time we fsync > specific files we do so on an old enough fd to guarantee that we'd see > the an error triggered by background writeback. But keeping track of > all potential filesystems data resides on (with one fd open permanently > for each) and then syncfs()ing them at checkpoint time is quite doable. > > *I can go into details, but it's probably not interesting enough > Do applications (specifically postgresql) need the ability to check whether there have been writeback errors on a filesystem w/o blocking on a syncfs() call? I thought that you had mentioned a specific usecase for that, but if you're actually ok with syncfs() then we can drop that part altogether. > > > > - This adds a new generic fs ioctl to allow userland to scrape the > > > current superblock's errseq_t value. It may be best to present this > > > to userland via fsinfo() instead (once that's merged). I'm fine with > > > dropping the last patch for now and reworking it for fsinfo if so. > > > > What, exactly, is this useful for? Why would we consider exposing > > an internal implementation detail to userspace like this? > > There is, as far as I can tell, so far no way but scraping the kernel > log to figure out if there have been data loss errors on a > machine/fs. Even besides app specific reactions like outlined above, > just generally being able to alert whenever there error count increases > seems extremely useful. I'm not sure it makes sense to expose the > errseq_t bits straight though - seems like it'd enshrine them in > userspace ABI too much? > Yeah, if we do end up keeping it, I'm leaning toward making this fetchable via fsinfo() (once that's merged). If we do that, then we'll split this into a struct with two fields -- the most recent errno and an opaque token that you can keep to tell whether new errors have been recorded since. I think that should be a little cleaner from an API standpoint. Probably we can just drop the ioctl, under the assumption that fsinfo() will be available in 5.7. Cheers, -- Jeff Layton <jlayton@xxxxxxxxxx>
