Storage EVA4000

[Cleber Zorzi]

Hello,

I have a storage EVA and some Windows servers in the same SAN.

I created a VDISK-01 and presented for server-1 (MSSQL Database server). Can I to presente the same VDISK-01 for server-02 (Backup server)?

These servers are not in cluster.

Thanks,

Cleber

[Uwe Zessin · Accepted solution]

The EVA technology allows that, but as soon as the backup server mounts the disk, too, volume corruption has started!

A plain Windows server does not have a shared file system implementation and expects exclusive access to a volume. When the backup server mounts a volume it sees that it is "open" (has not been dismounted) and starts rollback operations to make it 'consistent'. Then it 're-opens' the volume for its own use.
But the database server is not aware of these changes.

[Cleber Zorzi]

Hello,

Can I to present the volume read only?

Thanks,

Cleber

[Uwe Zessin]

Yes, but it is a characteristic of the whole virtual disk. You cannot present a single virtual disk for one server read-write and for another read-only.

And even if it were possible, there are two other problems:
Windows usually requires read-write access to a disk in order to mount an NTFS volume (the 'open' thing). I have not checked recent versions, but Windows 2000 even hang during boots when a data volume was read-only.

The other problem is that the database server continues to write to the disk. It can even make changes to the file system's meta data.
The backup server does not pick up all or only part of those changes, because it does not expect somebody else to make changes. So it will see inconsistencies.

That is one of the reasons why storage arrays (including the EVA) provide clones or snapshots. Those are copies or 'frozen views' of the data and changes by the database server are not visible on the clone/snapshot.

[Cleber Zorzi]

And if operacional system was Linux, I could share the volume? One with read-write and other with read-only.

Thanks

Cleber

[Uwe Zessin]

The same limitations will apply:

while Linux can mount a filesystem read-only, the problem is that the writer will make changes that the reader does not expect. Read-only will only ensure that the backup server does not actively cause corruptions, but the backup server can see very inconsistent data (due to writer's changes) which could even result in system crashes.

The workarounds would be the same:
- use a shared filesystem (ext2/3 is not)
- make use of controller-based clones/snapshots

[Cleber Zorzi]

Thanks!

Cleber