Hi.
For a long time I was under an impression that clones are as efficient
in bluestore as snapshots.
But today I finally decided to test it and ... I discovered it was an
utterly wrong impression :) RBD copies the whole 4 MB object even when a
small 4 KB block is modified within it in the child image. In my
all-NVMe cluster this leads to 40 (40!!!) random write iops (bs=4k
iodepth=1) in a fresh RBD clone, which is terrible.
Question of the day: is it possible to reimplement RBD clones using
"sparse objects"? As I understand the support for sparse objects
themselves is already there. So maybe librbd could only write the
modified part to the child image when writing and read "holes" from
parents when reading?
--
Vitaliy Filippov
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