It's a very broad question -- are you trying to determine something more specific? Notionally, your DB engine will safely journal the changes to disk, commit the changes to the backing table structures, and prune the journal. Your mileage my vary depending on the specific DB engine and its configuration settings. The VM's OS will send write requests addressed by block offset and block counts (e.g. 512 blocks) through the block device hardware (either a slower emulated block device or a faster paravirtualized block device like virtio-blk/virtio-scsi). Within the internals of QEMU, these block-addressed write requests will be delivered to librbd in byte-addressed format (the blocks are converted to absolute byte ranges). librbd will take the provided byte offset and length and quickly calculate which backing RADOS objects are associated with the provided range [1]. If the extent intersects multiple backing objects, the sub-operation is sent to each affected object in parallel. These operations will be sent to the OSDs responsible for handling the object (as per the CRUSH map) -- by default via TCP/IP. The MTU is the maximum size of each IP packet -- larger MTUs allow you to send more data within a single packet [2]. [1] http://docs.ceph.com/docs/master/architecture/#data-striping [2] https://en.wikipedia.org/wiki/Maximum_transmission_unit On Mon, Mar 20, 2017 at 5:24 PM, Alejandro Comisario <alejandro@xxxxxxxxxxx> wrote: > anyone ? > > On Fri, Mar 17, 2017 at 5:40 PM, Alejandro Comisario > <alejandro@xxxxxxxxxxx> wrote: >> Hi, it's been a while since im using Ceph, and still im a little >> ashamed that when certain situation happens, i dont have the knowledge >> to explain or plan things. >> >> Basically what i dont know is, and i will do an exercise. >> >> EXCERCISE: >> a virtual machine running on KVM has an extra block device where the >> datafiles of a database runs (this block device is exposed to the vm >> using libvirt) >> >> facts. >> * the db writes to disk in 8K blocks >> * the connection between the phisical compute node and Ceph has an MTU of 1500 >> * the QEMU RBD driver uses a stipe unit of 2048 kB and a stripe count of 4. >> * everything else is default >> >> So conceptually, if someone can explain me, what happens from the >> momment the DB contained on the VM commits to disk a query of >> 20MBytes, what happens on the compute node, what happens on the >> client's file striping, what happens on the network (regarding >> packages, if other than creating 1500 bytes packages), what happens >> with rados objects, block sizes, etc. >> >> I would love to read this from the bests, mainly because as i said i >> dont understand all the workflow of blocks, objects, etc. >> >> thanks to everyone ! >> >> -- >> Alejandrito > > > > -- > Alejandro Comisario > CTO | NUBELIU > E-mail: alejandro@nubeliu.comCell: +54 9 11 3770 1857 > _ > www.nubeliu.com > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com -- Jason _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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