On Dec 15, 2014, at 2:42 PM, Gregory Farnum <greg@xxxxxxxxxxx> wrote: > On Mon, Dec 15, 2014 at 10:54 AM, Atchley, Scott <atchleyes@xxxxxxxx> wrote: >> Hi all, >> >> For a given file in cephfs, I would like to determine: >> >> 1) the number of PGs >> 2) the PG IDs >> 3) the offsets handles by each PG >> 4) the stripe unit (i.e. bytes per block of data) >> >> preferably using a C API. >> >> I found the getfattr command line tool which provides (1) and (4). >> >> It appears that cephfs command line tool provides some of the info, but the manpage is not clear as to what is provided by show_layout versus show_location. Does show_layout provide (1) and (4)? Does show_location include (2) and (3)? >> >> I found a Java CephFileExtent() that provides (1) and a list of OSDs for the stripe unit that contains the provided file extent. > > What exactly are you trying to accomplish here? I can imagine some > limited scenarios where you want to map to PGs, but in general you're > a lot more interested in either the objects associated with the file, > or the OSDs that host them. I think you can get objects and OSDs out > of the latest libcephfs api. We implemented a file transfer tool for Lustre that transfers data by object rather than by file. The scheduling of the object transfers is done in a way to defer transfers from congested servers in hopes that they will be less congested in the near future (given our bulk-synchronous workload, congestion of a set of servers is generally temporary). I would rather know OSDs versus PGs, but I did not assume that the info was available. >> On the architecture page (http://ceph.com/docs/next/architecture/) under DATA STRIPING, does cephfs client use the same set of PGs used for Object Set 1 for Object Set 2 or does it use a different PGs? > > Assuming you mean does it use the same PGs for each file's objects, > the answer is no, definitely not. > -Greg No, I assume the Object Set 1 on that page indicates four objects on four different PGs. Within Object Set 1, Object 0 holds stripe units 0, 4, 8, and 12. Object 1 holds stripe units 1, 5, 9, and 13 and so on. Each Object 0-3 are stored on different PGs. Is this correct so far? My question above is about Object Set 2, which holds Objects 4-7? Is Object 4 on the same PG as Object 0? Is Object 5 on the same PG as Object 1? Etc? Or does Object Set 2 (holding Objects 4-7) use different PGs? Scott-- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
