Limitations

Understand the limitations when using Persistent Write Log (PWL) cache.

When using Persistent Write Log (PWL) cache, there are several limitations that should be considered.

• The underlying implementation of persistent memory (PMEM) and solid-state disks (SSD) is different, with PMEM having higher performance. At present, PMEM can provide persist on write and SSD is persist on flush or checkpoint. In future releases, these two modes will be configurable.
• When users switch frequently and open and close images repeatedly, Ceph displays poor performance. If PWL cache is enabled, the performance is worse. It is not recommended to set num_jobs in a Flexible I/O (fio) test, but instead setup multiple jobs to write different images.