On 2012-04-27, at 7:13 AM, Dave Chinner wrote: > Have a look at fs/xfs/xfs_dinode.h. There's a bunch of flags defined > at the bottom of the file. > > Stuff like the "nodefrag", "nodump", and "prealloc" bits seem fairly > generic - they are for indicating that files are to be avoided for > defrag or backup purposes, the prealloc bit indicates that fallocate > has been used to reserve space on the inode (finding files that space > can be punched out of safely), and so on. There is already the FS_NODUMP_FL in the standard FS_IOC_GETFLAGS ioctl and I expect this to be in statxat() also. In ext4 there was also an EXT4_EOFBLOCKS_FL added for inodes with fallocate'd data beyond EOF, but Eric thought it was a pain to maintain and it has been deprecated in ext4 and e2fsprogs recently. > Currently these things are queried and manipulated by ioctls > (XFS_IOC_FSX[GS]ETATTR) along with extent size hints, project > quotas, etc. but I think there's some wider use for many of the > flags, which is why I was asking is there's any thought to this sort > of flag being exposed by the VFS. > > Historically the flags exposed by the VFS are those used by extN - I > see little reason why we should favour one filesystem's flags over > any others in an extended stat interface if they are generically > useful.... Sure, they started as ext4 flags because the "lsattr" and "chattr" tools were using this ioctl/flags, but have become more generic in recent years. FS_NOTAIL_FL was added for Reiserfs, and FS_NOCOW_FL was added for another filesystem (maybe Btrfs?). I'm not against adding more flags here that are generically useful, and recommended that statxat() have a 64-bit st_ioc_flags, since there are already 22 FS_*_FL flags defined today. >> Either you can add some of them to the ioc flags (which may be >> impractical, I grant you) or we'd have to add an arbitrary fs-type >> specific field and specify the host fs (the provision of which might >> not be a bad idea in and of itself) to tell userspace how to interpret them. > > Well, that's the complexity, isn't it. I have no good answer to > that... > >>> Along the same lines, filesytsems can have different allocation >>> constraints to IO the filesystem block size - ext4 with it's >>> bigalloc hack, XFS with it's per-inode extent size hints and the >>> realtime device, etc. Then there's optimal IO characteristics >>> (e.g. geometery hints like stripe unit/stripe width for the >>> allocation policy of that given file) that applications could >>> use if they were present rather than having to expose them >>> through ioctls that nobody even knows about... >> >> Yeah... Not representable by one number. You'd have to unset a >> flag to say you were providing this information. >> >> However, providing a whole bunch of hints about I/O characteristics >> is probably beyond this syscall - especially if it isn't constant >> over the length of a file. That's specialist knowledge that most >> applications don't need to know. >> Having a generic way to retrieve it, though, may be a good idea. > > We're continually talking about applications giving us usage hints > on what IO they are going to do so the storage can optimise the IO. > IO is still a GIGO problem, though, and the idea of geometry hints > is to enable us to tell the application to do well formed IO. i.e. > less garbage. > > XFS has ioctls to expose filesystem geometry, optimal IO sizes, the > alignment limits for direct IO, etc, and they are very useful to > applications that care about high performance IO. A lot of this can > be distilled down to a simple set of geometries, and generally > speaking they don't change mid way through a file.... > >> OTOH, there's plenty of uncommitted space, so if we can condense >> the hints down to something small, we could perhaps add it later - >> but from your paragraph above, it doesn't sound like it'll be small. > > Allocation block size, minimum sane IO size (to avoid page cache RMW > cycles or DIO zeroing), minimum prefered IO size (e.g. stripe unit), > optimal IO size for bandwidth (e.g. stripe width). I don't think > there's much more than that which will be really usable by > applications. I think this is a minimal set that makes sense, and is manageable for both the interface and for users. Even if it isn't 100% correct for every file of every filesystem, it still makes sense for many systems. I'd suggest st_frsize (like BSD statvfs() f_frsize) would be the minimum fragment or page size, st_iosize (BSD f_iosize) could be the optimal IO size, and "st_stripesize" for the minimum preferred RAID/chunk size. One could argue that "st_blksize" is used for the "optimal IO size" on Linux today, but this is an overloaded term. It _appears_ to represent the filesystem blocksize, which it usually is not, and on BSD st_bsize means the minimum blocksize and has a confusingly similar name. Since any application using this API needs to do some extra coding already, we may as well give the structure members good names that are not ambiguous. >>> Perhaps also exposing the project ID for quota purposes, like we >>> do UID and GID. That way we wouldn't need a filesystem specific >>> ioctl to read it.... >> >> Is this an XFS only thing? If so, can it be generalised? > > Right now it is, but there's been patches in the past to introduce > project quotas to ext4. That didn't go far because it was done in a > way that was semantically different to XFS (for no reason that I > could understand) and nobody wanted two different sets of semantics > for the "same" feature. The most common use of project quotas is to > implement sub-tree quotas, which is probably of more interest to > btrfs folks as it is an exact match for per-subvolume quotas. > > So, yes, I do see it as something generically useful - it's a > feature that a lot of people use XFS specifically for.... I'd agree. There was the tree quota project for ext4, and I've also heard this is available in other filesystems. Cheers, Andreas -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
