# Support Criteria

If your stores are file-based, there are several options available for implementing shared storage, using a combination of hardware and software. EMS requires that your storage solution guarantees the following listed criteria.

Note that these criteria are inherently satisfied by database store implementations.

Warning: Always consult your shared storage vendor and your operating system vendor to ascertain that the storage solution you select satisfies all four criteria.

Criterion | Description
--- | ---
Write Order | The storage solution must write data blocks to shared storage in the same order as they occur in the data buffer. (Solutions that write data blocks in any other order (for example, to enhance disk efficiency) do not satisfy this requirement.)
Synchronous Write Persistence | Upon return from a synchronous write call, the storage solution guarantees that all the data have been written to durable, persistent storage.
Distributed File Locking | The EMS servers must be able to request and obtain an exclusive lock on the shared storage. The storage solution must not assign the locks to two servers simultaneously. (See Software Options.) EMS servers use this lock to determine the primary server.
Unique Write Ownership | The EMS server process that has the file lock must be the only server process that can write to the file. Once the system transfers the lock to another server, pending writes queued by the previous owner must fail.

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