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Understanding storage

The MWS stores data in a storage pool exclusive to the server and reserves some disk space for snapshots. If snapshots exhaust the available disk space, data loss will occur.

MWS storage layout

Each MWS server uses a dedicated storage pool only available to that server. Its size is twice the disk size of the given server type plus 5GB, that is, a 20GB server will actually use 45GB of storage behind the scenes. Roughly 5GB of storage is reserved for the operating system and the amount over the server's disk size is reserved for snapshots.

Snapshots

Snapshots are a means of preserving the state of the filesystem at the time the snapshot was taken. They work by rerouting and storing changes made to the filesystem since that point in time. The size of the snapshot is the amount of these changes it can hold. Snapshots on the MWS do not have a fixed size, instead they allocate disk space from the available reserved disk space in the pool. This means that an arbitrary number of snapshots can be taken, but there are limits to how much data they can store. If snapshots exhaust the available disk space, no changes can be written to them and they will need to be discarded, as there is no other way to preserve consistency of the filesystem. This is a catastrophic event resulting in the loss of any data held in the snapshots and will cause downtime and require manual recovery. The MWS tries to avoid this situation by deleting one or more automatically created snapshots taken daily to reduce disk usage. If it fails to recover sufficient amounts of storage this way it will send a warning or panic email to the site administrators, depending on the severity of the problem. We will never delete snapshots created using the panel, so it may be necessary for you to delete a snapshot to remedy the problem. See this page on using snapshots for details on how  create, use, and delete them.