SQL Server System Table's Data should be more durable
12/7/2012 11:51:13 AM
Errors we are running into:
SQL Server detected a logical consistency-based I/O error: incorrect checksum (expected: 0x212ae472; actual: 0x6fe2546c). It occurred during a read of page (3:1747008) in database ID 5 at offset 0x00000355080000 in file 'D:\SQLDATA\DBSales.ndf'. Additional messages in the SQL Server error log or system event log may provide more detail. This is a severe error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.
One of the strong design principle per-used with Hadoop\HDFS is the concept of "replication factor". By default it is set to 3:
For the common case, when the replication factor is three, HDFS’s placement policy is to put one replica on one node in the local rack, another on a different node in the local rack, and the last on a different node in a different rack. This policy cuts the inter-rack write traffic which generally improves write performance. The chance of rack failure is far less than that of node failure; this policy does not impact data reliability and availability guarantees. However, it does reduce the aggregate network bandwidth used when reading data since a block is placed in only two unique racks rather than three. With this policy, the replicas of a file do not evenly distribute across the racks. One third of replicas are on one node, two thirds of replicas are on one rack, and the other third are evenly distributed across the remaining racks. This policy improves write performance without compromising data reliability or read performance.
In Microsoft SQL Server identical design principles are currently in-place:
a) Database Clustering (Unique Disk Sub-Systems)
b) Database Mirroring (Shared Disk Sub-system)
c) Replication (Unique Disk Sub-Systems)
d) Log Shipping (Unique Disk Sub-Systems)
e) DB Backup
Database Mirroring is really pushing the envelope and is able to repair bad problems, by utilizing data from partner nodes.
On Mirrored Systems we have strong impediments that toughen the task of using "DBCC CHECKDB" when trying to repair the passive nodes of DB Mirrored pair. One can not place the partner mode is single-user nor access the partner DB.