The subject sums up the scenario. I believe the answer is there is no downtime but the system might simply be slower during the rebuild. Which is fine. But if there is downtime, how long? A small blip for a short maintenance window is acceptable. Days of rebuild would require a new plan to assess how to handle.
Details: The client has an iSCSI connection going to a Synology RS2414RP+ (running DSM 5) which is a rack mounted unit with 12 bays. All bays are populated with 3TB drives since it was initially setup. And since it is a device dedicated to one large iSCSI volume, it was setup to use the max amount of space. Which has worked for months despite that space being around 1GB in size.
Now the system has occasionally run into “read only” mode which I believe is being caused by the thin provisioning of the iSCSI volume reaching the outer limits of the volume itself. Managed to bring the volume “back to life” by SSHing in & tossing an old/side-by-side backup of a PostgreSQL database which freed up about 36MB of space on the underlying volume which holds the iSCSI partition. Which was enough breathing room to get the iSCSI volume to mount again. But obviously is a problem waiting to happen.
So, since this Synology unit has the volume formatted as a Synology Hybrid RAID setup with 2 disk fault tolerance, the decision was made to replace one of the 3TB drives with a 4TB drive which should expand the volume. The drive has been inserted, is formatted & is running through the slow as molasses parity check stage.
But once parity checking is done, I will have to login to expand the volume. At this final stage, what happens from a service outage perspective? From what I have read online since the Synology DSM is is based on standard open source tools such as LVM (Logical Volume Manager), the volume will expand without having to incur an outage. But since we are talking about 27TB+ of storage, I want to doubly/triply make sure my assumption is correct.
Which is basically: When the parity checking is done, and I go into expand the volume, all services will still be up and running & the expansion will occur as a background process, thus ensuring zero downtime?