Re: cephfs "obsolescence" and object location

[John Spray <john.spray@redhat.com>]

Since you're only looking up the ID of the first object, it's really 
simple.  It's just the hex printed inode number followed by 
".00000000".  That's not guaranteed to always be the case in the future, 
but it's likely to be true longer than the deprecated ioctls exist.  If 
I was you, I would hard-code the object naming convention rather than 
writing in a dependency on the ioctl.

As Greg says, you can also query all the layout stuff (via supported 
interfaces) and do the full calculation of object names for arbitrary 
offsets into the file if you need to.

John


On 22/06/2015 22:18, Bill Sharer wrote:
> I'm currently running giant on gentoo and was wondering about the 
> stability of the api for mapping MDS files to rados objects.  The 
> cephfs binary complains that it is obsolete for getting layout 
> information, but it also provides object location info.  AFAICT this 
> is the only way to map files in a cephfs filesystem to object 
> locations if I want to take advantage of the "UFO" nature of ceph's 
> stores in order to access via both cephfs and rados methods.
>
> I have a content store that scans files, calculates their sha1hash and 
> then stores them on a cephfs filesystem tree with their filenames set 
> to their sha1hash name.  I can then build views of this content using 
> an external local filesystem and symlinks pointing into the cephfs 
> store.  At the same time, I want to be able to use this store via 
> rados either through the gateway or my own software that is rados 
> aware.  The store is being treated as a write-once, read-many style 
> system.
>
> Towards this end, I started writing a QT4 based library that includes 
> this little Location routine (which currently works) to grab the rados 
> object location from a hash object in this store. I'm just wondering 
> whether this is all going to break horribly in the future when ongoing 
> MDS development decides to break the code I borrowed from cephfs :-)
>
>
>
> QString Shastore::Location(const QString hash) {
>     QString result = "";
>     QString cache_path = this->dbcache + "/" + hash.left(2) + "/" + 
> hash.mid(2,2) + "/" + hash;
>     QFile cache_file(cache_path);
>     if (cache_file.exists()) {
>         if (cache_file.open(QIODevice::ReadOnly)) {
>             /*
>              * Ripped from cephfs code, grab the handle and use the 
> ceph version of ioctl to
>              * rummage through the file's xattrs for rados location.  
> cephfs whines about being
>              * obsolete to get layout this way, but this appears to be 
> only way to get location.
>              * This may all break horribly in a future release since 
> MDS is undergoing heavy development
>              *
>              *  cephfs lets user pass file_offset in argv but it 
> defaults to 0.  Presumably this is the "first"
>              *  extent of the pile of extents (4mb each?) and shards 
> for the file.  If user wants to jump
>              *  elsewhere with a non-zero offset, the resulting rados 
> object location may be different
>              */
>             int fd = cache_file.handle();
>             struct ceph_ioctl_dataloc location;
>             location.file_offset = 0;
>             int err = ioctl(fd, CEPH_IOC_GET_DATALOC, (unsigned 
> long)&location);
>             if (err) {
>                 qDebug() << "Location: Error getting rados location 
> for " << cache_path;
>             } else {
>                 result = QString(location.object_name);
>             }
>             cache_file.close();
>         } else {
>             qDebug() << "Location: unable to open " << cache_path << " 
> readonly";
>         }
>     } else {
>         qDebug() << "Location: cache file " << cache_path << " does 
> not exist";
>     }
>     return result;
> }

Since you're only looking at