HCX - OSAM Densync Process Delay Due to Sparse Files

Products

• VMware HCX

Issue/Introduction

• HCX OS Assisted Migration (OSAM) encounters significant delays.
• The delay occurs primarily during the densync operation in the OSAM migration process.
• Replication of sparse files (like /var/log/lastlog) using OSAM migration process runs into delays.
• While the actual file size is only a few KB, it shows an inflated size in GB.

Environment

HCX

Cause

The densync process is responsible for replicating the files during the HCX OSAM migration process. The sparse file replication is handled by densync using an option --smart-inplace which combines the --sparse and --inplace option from rsync.

When the densync process encounters a sparse file during the replication, it enumerates the contents of an empty block to zero's and on the target side the sparse file is recreated. Though the compression option is provided to the densync, it is not handled properly which ends up transferring zero's over the wire due to a known issue documented internally.

Resolution

None at the moment.