RepoMirrors/ceph
1
0
Fork 0
mirror of https://github.com/ceph/ceph synced 2025-01-02 00:52:22 +00:00
Code Issues Packages Projects Releases Wiki Activity

crimson/onode-staged-tree: correct the node size equation

Browse Source 
Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>
This commit is contained in:
Yingxin Cheng 2021-05-26 14:26:59 +08:00
parent 9e8bb747ca
commit 88fd967b78
1 changed files with 10 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
src/crimson/os/seastore/onode_manager/staged-fltree/node_layout.h
View File

@ -625,24 +625,27 @@ class NodeLayoutT final : public InternalNodeImpl, public LeafNodeImpl {
* - I > 2
* This means the largest possible insert_size must be smaller than 1/2 of
* the node_block_size, which is better than strategy A.
*
* In order to avoid "double split", there is another side-effect we need
* to take into consideration: if split happens with snap-gen indexes, the
* according ns-oid string needs to be copied to the right node. That is
* to say: right_node_size + string_size < node_block_size.
*
* Say that the largest allowed string size is 1/S of the largest allowed
* insert_size N/I KiB. If we go with stragety B, the equation should be
* changed to:
* - right_node_size ~= (I+2)/2I + 1/(I*S) < 1
* - I > 2 + 2/S (S > 1)
* insert_size N/I KiB. If we go with stragety B, and when split happens
* with snap-gen indexes and split just overflow the target_split_size:
* - left_node_size ~= target_split_size - 1/2 * (1/I - 1/IS)
* ~= 1/2 + 1/2IS
* - right_node_size ~= 1 + 1/I - left_node_size + 1/IS
* ~= 1/2 + 1/I + 1/2IS < 1
* - I > 2 + 1/S (S > 1)
*
* Now back to NODE_BLOCK_SIZE calculation, if we have limits of at most
* X KiB ns-oid string and Y KiB of value to store in this BTree, then:
* - largest_insert_size ~= X+Y KiB
* - 1/S == X/(X+Y)
* - I > (4X+2Y)/(X+Y)
* - node_block_size(N) == I * insert_size > 4X+2Y KiB
* - I > (3X+2Y)/(X+Y)
* - node_block_size(N) == I * insert_size > 3X+2Y KiB
*
* In conclusion,
* (TODO) the current node block size (4 KiB) is too small to