If the RBD object map feature is enabled, the read request would never even be sent to the OSD if the client knows the backing object doesn't exist. However, if the object map feature is disabled, the read request will be sent to the OSD.
The OSD isn't my area of expertise, but I can try to explain what occurs to the best of my knowledge. There is a small in-memory cache for object contexts with the OSD PG -- which includes a whiteout flag to indicate the object is deleted. I believe that the whiteout flag is only really used on a cache tier to avoid having to attempt to promote a known non-existent object. Therefore, in the common case the OSD would query the non-existent object from the object store (FileStore or BlueStore).
In FileStore, it will attempt to open the associated backing file for object. If the necessary dentries are cached in the kernel, I'd expect that the -ENOENT error would be bubbled back to RBD w/o a disk hit. Otherwise, the kernel would need to read the associated dentries from disk to determine that the object is missing.
In BlueStore, there is another in-memory cache for onodes that can quickly detect a missing object. If the object isn't in the cache, the associated onode will be looked up within the backing RocksDB. If the RocksDB metadata scan for the object's onode fails since the object is missing, the -ENOENT error would be bubbled back to the client.
On Tue, May 2, 2017 at 1:24 PM, Prashant Murthy <pmurthy@xxxxxxxxxxxxxx> wrote:
I wanted to add that I was particularly interested about the behavior with filestore, but was also curious how this works on bluestore.PrashantOn Mon, May 1, 2017 at 10:04 PM, Prashant Murthy <pmurthy@xxxxxxxxxxxxxx> wrote:Hi all,I was wondering what happens when reads are issued to an RBD device with no previously written data. Can somebody explain how such requests flow from rbd (client) into OSDs and whether any of these reads would hit the disks at all or whether OSD metadata would recognize that there is no data at the offsets requested and returns a bunch of zeros back to the client?Thanks,Prashant--
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