Feature #3673
Updated by Loïc Dachary about 9 years ago
ceph-disk-prepare takes an option to use an external disk as a journal. It is commonly suggested that the journal is stored on an SSD and is also widely known that performance and resiliency characteristics of SSDs depends on alignment on the erase block. While sgdisk has a --set-alignment argument (defaults to 2048), there is no way to pass that via ceph-disk-prepare.
Additionally, there is currently no way to tell ceph-disk-prepare that you want to use a block device (or even a file) as the journal, as it'll attempt to partition it no matter what. That would be an easy way to workaround the above issue. It'd also be useful in other configurations, e.g. we use the SSDs as the root filesystem too and prepartition them at install time.
Note that because of #3255 (prepare triggers an activate), I couldn't even just create the symlink myself; I had to shutdown the OSD, --flush-journal just in case, change the symlink and --mkjournal again, for each OSD. That's hard and risky to automate using configuration management.