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June 16th, 2010 12:00
changes made by host1 not seen by host2
We are configuring a MD3000i and are currently doing some test.
I have configured 2 disk groups each containing 1 virtual disk:
- Group1
- Virtual Disk1
- Group2
- Virtual Disk 2
I have 2 hosts (host1 and host2) both part of the same host group (host group1), i have mapped the virtual drives to the host group so that both hosts would have access to the drives. The hosts are both windows server 2008 R2 standard 64bits. They both see the virtual drives and can both use them (create files or folders, read files or folders) but changes made by host 1 (created a folder called test host1) is not visible on host 2 unless the iSCSI connection is reset (by a server reboot for instance). Is there something wrong with my configuration?
The goal is to have multiple sql servers (on virtual machines) access the same centralised databases on a single centralised hard drive.
Dev Mgr
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June 16th, 2010 12:00
SAN rule number 1:
- never share disks between multiple servers unless those servers have some kind of disk locking mechanism in place; commonly (for Windows servers) this is Microsoft Cluster Services
When a server boots, in rough steps the following happens:
- it sees a disk and looks for a partition table
- if there is no partition table (usually therefor also no signature (only happens on new (virtual) disks)), it does nothing with the disk
- if there is a partition table, the OS reads the partition table to find out where the partition(s) start and finish
- now that it knows where the partition is (are), it checks the partition for a file allocation table (index that keeps track of which piece of which file is stored where)
- now the OS can use the disk
So, if 2 servers were to share the same disk, without knowing about each other, and one server writes a file to some free space on the disk, the other server doesn't know about this change (it has no reason to re-read the file allocation table as as far as it's concerned, it's the only one using this disk), so when the 2nd server wants to do a write, it could write it's data to the same disk space. This will cause data corruption at one point or another.
Someone may have some more detail about how a partition is mounted, but in broad lines that info should be correct.