DAOS-18164 bio: bump default cluster_sz

[NiuYawei]

Bump default BS cluster size from 32MB to 128MB in md-on-ssd mode, make the cluster size configurable through DAOS_BS_CLUSTER_MB.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Errors are Unable to load ticket data
https://daosio.atlassian.net/browse/DAOS-18164

[daosbuild3]

Test stage Unit Test bdev on EL 8.8 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-17334/2/testReport/

[daosbuild3]

Test stage Unit Test bdev with memcheck on EL 8.8 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-17334/2/testReport/

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-17334/2/testReport/

[daosbuild3]

Test stage Functional Hardware Medium Verbs Provider MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-17334/2/execution/node/1330/log

[rpadma2]

Performed the testing on daos_mdonssd_cluster. Stopping and starting the cluster after rank exclusion doesn't take much time with this PR.

rpadma2@aurora-daos-0799:/scratchbox/daos/clusters/daos_mdonssd_cluster> dmg system query
Rank                      State    
----                      -----    
[0-3,5-16,18-24,26-37,39] Stopped  
[4,17,25,38]              Excluded 
rpadma2@aurora-daos-0799:/scratchbox/daos/clusters/daos_mdonssd_cluster> date
Thu 08 Jan 2026 07:56:53 PM UTC
rpadma2@aurora-daos-0799:/scratchbox/daos/clusters/daos_mdonssd_cluster> dmg system query
Rank                      State    
----                      -----    
[0-3,5-16,18-24,26-37,39] Stopped  
[4,17,25,38]              Excluded 

rpadma2@aurora-daos-0799:/scratchbox/daos/clusters/daos_mdonssd_cluster> dmg system query
Rank   State  
----   -----  
[0-39] Joined 

rpadma2@aurora-daos-0799:/scratchbox/daos/clusters/daos_mdonssd_cluster> date
Thu 08 Jan 2026 07:57:12 PM UTC
I have set  DAOS_BS_CLUSTER_MB=256 on the daos_server.yml:
- env_pass_through:
  - LD_LIBRARY_PATH
  env_vars:
  - DAOS_MD_CAP=1024
  - DAOS_DMA_INIT_PCT=20
  - DAOS_BS_CLUSTER_MB=256