On Wed, May 15, 2019 at 9:24 AM Eric Sandeen <sandeen@xxxxxxxxxxx> wrote: > >> For example, xfs_db is not the right tool for probing online, active > >> filesystems. It is not coherent with the active kernel filesystem, > >> and is quite capable of walking off into la-la land as a result of > >> mis-parsing the inconsistent filesystem that is on disk underneath > >> active mounted filesystems. This does not make for a robust, usable > >> tool, let alone one that can make use of things like rmap for > >> querying usage and ownership information really quickly. > > > > I see your point, that the FS is constantly changing and that we might > > see an inconsistent view. But if we are generating bucketed > > histograms we are anyways approximating the stats. > > I think that Dave's "inconsistency" concern is literal - if the on-disk > metadata is not consistent, you may wander into what looks like corruption > if you try to traverse every inode while mounted. > > It's pretty much never valid for userspace to try to traverse or read > the filesystem while mounted. Sure, I understand this point. Then can we: 1) Abort scan if the we detect "corrupt" metadata, the user would then either restart the scan or decide not to. 2) Have a mechanism which detects if the FS changed will scan was in progress and tell the user the results might be stale? > > >> To solve this problem, we now have the xfs_spaceman tool and the > >> GETFSMAP ioctl for running usage queries on mounted filesystems. > >> That avoids all the coherency and crash problems, and for rmap > >> enabled filesystems it does not require scanning the entire > >> filesystem to work out this information (i.e. it can all be derived > >> from the contents of the rmap tree). > >> > >> So I'd much prefer that new online filesystem queries go into > >> xfs-spaceman and use GETFSMAP so they can be accelerated on rmap > >> configured filesystems rather than hoping xfs_db will parse the > >> entire mounted filesystem correctly while it is being actively > >> changed... > > > > Good to know, I wasn't aware of this tool. However I seems like I > > don't have that ioctl in my systems yet :( > > It was added in 2017, in kernel-4.12 I believe. > What kernel did you test? Yeap, that's it we tested in 4.11. -- Jorge E Guerra D
