On 10/10/2012 10:10 AM, Travis Rhoden wrote:
Hey folks, I have two questions about determining how much storage has been used *inside* of an RBD. First, I'm confused by the output of df. I've created, mapped, and mounted a 500GB RBD, and see the following: # df -h /srv/test Filesystem Size Used Avail Use% Mounted on /dev/rbd44 500G 7.5G 467G 2% /srv/test # cd /srv/test # du -sh . 20K . Any ideas way a brand-new, no files added mount shows 7.5GB of used space? Does this happen from the file system formatting (ext4 in this case)? Additionally, 500G - 7.5G != 467G (the number shown as Avail). Why the huge discrepancy? I don't expect the numbers to add up exact due to rounding from kB, MB, GB, etc, but they should be darn close, a la df -h /dev/sda1 Filesystem Size Used Avail Use% Mounted on /dev/sda1 15G 1.7G 13G 12% / Second question, is it possible to know how much storage has been used in the RBD without mounting it and running df or du? For the same RBD as above, I see: # rbd info test rbd image 'test'': size 500 GB in 128000 objects order 22 (4096 KB objects) block_name_prefix: rb.0.18f9.2d9c66c6 parent: (pool -1) Is there perhaps a way to know the number of objects that have been 'used'? Then I could take that and multiply by the object size (4MB).
You can get an upper bound by looking at the number of objects in the image: rados --pool rbd ls | grep -c '^rb\.0\.18f9\.2d9c66c6' Each object represents a section of the block device, but they may not be entirely filled (objects are sparse), so this will probably still be a higher estimate than df. Also note that listing all the objects in a pool is an expensive operation, so it shouldn't be done very often. Josh
I'm running 0.48.1argonaut on Ubuntu 12.04. RBD maps are also on Ubuntu 12.04, with the stock 3.2.0-29-generic kernel. Thanks, - Travis
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