> On 02 Nov 2015, at 11:59, Wido den Hollander <w...@42on.com> wrote: > > > > On 02-11-15 11:56, Jan Schermer wrote: >> Can those hints be disabled somehow? I was battling XFS preallocation >> the other day, and the mount option didn't make any difference - maybe >> because those hints have precedence (which could mean they aren't >> working as they should), maybe not. >> > > This config option? > > OPTION(rbd_enable_alloc_hint, OPT_BOOL, true) // when writing a object, > it will issue a hint to osd backend to indicate the expected size object > need > > Found in src/common/config_opts.h > > Wido >
Thanks, but can this option be set for a whole OSD by default? Jan >> In particular, when you fallocate a file, some number of blocks will be >> reserved without actually allocating the blocks. When you then dirty a >> block with write and flush, metadata needs to be written (in journal, >> synchronously) <- this is slow with all drives, and extremely slow with >> sh*tty drives (doing benchmark on such a file will yield just 100 write >> IOPs, but when you allocate the file previously with dd if=/dev/zero it >> will have 6000 IOPs!) - and there doesn't seem to be a way to disable it >> in XFS. Not sure if hints should help or if they are actually causing >> the problem (I am not clear on whether they preallocate metadata blocks >> or just block count). Ext4 does the same thing. >> >> Might be worth looking into? >> >> Jan >> >> >>> On 31 Oct 2015, at 19:36, Gregory Farnum <gfar...@redhat.com >>> <mailto:gfar...@redhat.com>> wrote: >>> >>> On Friday, October 30, 2015, mad Engineer <themadengin...@gmail.com >>> <mailto:themadengin...@gmail.com>> wrote: >>> >>> i am learning ceph,block storage and read that each object size is >>> 4 Mb.I am not clear about the concepts of object storage still >>> what will happen if the actual size of data written to block is >>> less than 4 Mb lets say 1 Mb.Will it still create object with 4 mb >>> size and keep the rest of the space free and unusable? >>> >>> >>> No, it will only take up as much space as you write (plus some >>> metadata). Although I think RBD passes down io hints suggesting the >>> object's final size will be 4MB so that the underlying storage (eg >>> xfs) can prevent fragmentation. >>> -Greg >>> _______________________________________________ >>> ceph-users mailing list >>> ceph-users@lists.ceph.com <mailto:ceph-users@lists.ceph.com> >>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >> >> >> >> _______________________________________________ >> ceph-users mailing list >> ceph-users@lists.ceph.com >> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >> > _______________________________________________ > ceph-users mailing list > ceph-users@lists.ceph.com > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
