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Maintaining Units > Background Tasks > About Initialization

About Initialization
This topic is organized into the following:
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Overview of Initialization
For 3ware SATA RAID controllers, initialize means to put the redundant data on the drives of redundant units into a known state so that data can be recovered in the event of a disk drive failure. For RAID 1 and RAID 10, initialization copies the data from the lower port to the higher port. For RAID 5, RAID 6, and RAID 50, initialization calculates the RAID 5 parity and writes it to disk (background initialization). This is sometimes referred to as background initialization or resynching, and does not erase user data.
Some RAID levels must be initialized for best performance. (For specifics, see Initialization of Different RAID Types.) When these units are created in the BIOS (through 3BM), initialization begins immediately. This type of initialization is referred to as foreground initialization, because it is the primary task being performed on the system, before the operating system has loaded. This process erases existing data, by writing zeroes to all of the drives in the unit. You can elect to cancel foreground initialization, put the units into service, and have initialization run in the background, instead.
Foreground Versus Background Initialization
Initialization makes parity information valid. Foreground initialization does this by simply writing zeroes to all the drives so that they all have the same values, overwriting any existing data in the process. In contrast, background initialization uses an algorithm to resynch the parity information on the drives and does not rewrite existing data.
A foreground initialization is run from the BIOS using 3BM. It clears all existing data from the drives. Foreground initialization can take several hours, depending upon the size of the unit. After foreground initialization completes, you can start the operating system and units will perform at peak efficiency.
If immediate access to the unit is important, you can stop foreground initialization by pressing Esc and booting to the operating system. Background initialization will then start automatically within about 10 minutes. Background initialization (resynching) does not write zeroes to the drives or harm your data. You can partition, format, and use the unit safely while it is initializing. The unit is fully fault-tolerant while the initialization takes place. That is, if the unit degrades before the initialization is complete, the data will remain intact.
When initializing is done after booting to the operating system, the process of initializing takes longer than it does if initialization is done by writing zeroes to the unit in the BIOS. Consequently, it will be a longer period of time until the performance of the unit is fully optimal.
Although you can use the unit while it is being initialized in the background, initialization does slow I/O performance until completed. You can adjust how much initialization will slow performance by setting the rate at which it occurs. (See Setting Background Task Rate.) You can also postpone initialization until a scheduled time. (See Scheduling Background Tasks).
 
Note: Units that do not need to be immediately initialized for full performance will be automatically initialized using background initialization when they are verified for the first time. (Verification requires that the units have been previously initialized.) This will not affect the data on the drives, and the units will perform normally, although performance will be slowed until the initialization and verification are completed.
Initialization of Different RAID Types
Information about initialization for each of the different RAID types is described below and summarized in Table: Initialization Requirements for Different RAID Configurations.
Initialization of RAID 0 Units
RAID 0 units do not need to be initialized and cannot be initialized. RAID 0 units are immediately available for use with full performance when created.
Initialization of RAID 5, RAID 6, and RAID 50 Units
RAID 5 and RAID 6 units will be automatically initialized the first time they are verified.
Regardless of the size, all 9000-series RAID 5, RAID 6, and RAID 50 units are fully fault tolerant upon creation. These configurations use a specialized scheme for writing to the unit, which does not have to be valid to provide fault tolerance.
Performance of RAID 6, RAID 5 units with 5 or more disks, and RAID 50 units with 2 subunits of 5 or 6 disks will improve after the unit has been initialized. For these configurations, initialization begins automatically after you create them. If you create them in the 3BM utility, zeroes are written to all unit members. If you create them through 3DM, RAID 5 parity is calculated and written to disk, keeping any data in the unit intact.
RAID 5 units with 3 or 4 disks do not need to be initialized to have full performance upon creation. Similarly, RAID 50 units with a grouping of 3 or 4 do not need to be initialized, however RAID 50 with a grouping of 6 do need to be initialized for full performance.
 
For RAID 5 with more 5 or more drives, it is strongly recommended that you initialize the unit before using it. Initializing such a unit is critical to insuring data integrity on the unit.
For RAID 5 with 3 or 4 drives, initialization before use is not required. However, initialization is required before a unit can be verified. Consequently, if you attempt to verify a RAID 5 with 3 or 4 drives that has not yet been initialized, you will see a message that the array has not been initialized, and initialization will begin. This is considered part of normal operation of the unit.
Initialization of RAID 1 and RAID 10 Units
RAID 1 and RAID 10 units do not need to be initialized when they are created to be fault tolerant and are immediately available for use with full performance when created.
Initialization of RAID 1 or RAID 10 units will take place automatically the first time the unit is verified.
Initialization of a RAID 1 unit results in data from one disk (the disk on the lower port number) being copied to the other disk. In RAID 10 units, data from one half of the unit is copied to the other half.
After the initialization, subsequent verifies to a RAID 1 or RAID 10 unit check for data consistency by comparing the data from one drive (or set of drives) to the other drive (or set of drives).
Initialization Requirements for Different RAID Configurations
 
Initialization Required
for Highest
Performance?
Background Initialization After Power Failure
The 3ware controller detects and handles power failures, using a mechanism that ensures that redundant units have consistent data and parity. When a redundant unit is unexpectedly shutdown, there is a possibility some data and parity may be inconsistent. If a unit or sub-unit of a redundant unit is detected to have been shutdown uncleanly, the unit or sub-unit will change its mode to either ‘Initializing’ or ‘Verifying.’
When the initialization is complete, the unit is guaranteed to be redundant again. The initialization does not erase user data.

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