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binary complains that it is obsolete for getting layout information, but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;
}
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