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Striped Organization
Data Integrity
The final problem with disks is that they are unreliable. Although reliability has increased tremendously over the last few years, disk drives are still the most likely core component of a server to fail. When they do, the results can be catastrophic and replacing a failed disk drive and restoring data can result in server downtime.
One approach to this problem is _mirroring_, or `RAID-1`, which keeps two copies of the data on different physical hardware. Any write to the volume writes to both disks; a read can be satisfied from either, so if one drive fails, the data is still available on the other drive.
Mirroring has two problems:
It requires twice as much disk storage as a non-redundant solution.
Writes must be performed to both drives, so they take up twice the bandwidth of a non-mirrored volume. Reads do not suffer from a performance penalty and can even be faster.
An alternative solution is _parity_, implemented in `RAID` levels 2, 3, 4 and 5. Of these, `RAID-5` is the most interesting. As implemented in [.filename]#vinum#, it is a variant on a striped organization which dedicates one block of each stripe to parity one of the other blocks. As implemented by [.filename]#vinum#, a `RAID-5` plex is similar to a striped plex, except that it implements `RAID-5` by including a parity block in each stripe. As required by `RAID-5`, the location of this parity block changes from one stripe to the next. The numbers in the data blocks indicate the relative block numbers.
`RAID`-5 Organization
Compared to mirroring, `RAID-5` has the advantage of requiring significantly less storage space. Read access is similar to that of striped organizations, but write access is significantly slower, approximately 25% of the read performance. If one drive fails, the array can continue to operate in degraded mode where a read from one of the remaining accessible drives continues normally, but a read from the failed drive is recalculated from the corresponding block from all the remaining drives.
[.filename]#vinum# Objects
In order to address these problems, [.filename]#vinum# implements a four-level hierarchy of objects:
The most visible object is the virtual disk, called a _volume_. Volumes have essentially the same properties as a UNIX(R) disk drive, though there are some minor differences. For one, they have no size limitations.
Volumes are composed of _plexes_, each of which represent the total address space of a volume. This level in the hierarchy provides redundancy. Think of plexes as individual disks in a mirrored array, each containing the same data.
Since [.filename]#vinum# exists within the UNIX(R) disk storage framework, it would be possible to use UNIX(R) partitions as the building block for multi-disk plexes. In fact, this turns out to be too inflexible as UNIX(R) disks can have only a limited number of partitions. Instead, [.filename]#vinum# subdivides a single UNIX(R) partition, the _drive_, into contiguous areas called _subdisks_, which are used as building blocks for plexes.
Subdisks reside on [.filename]#vinum#_drives_, currently UNIX(R) partitions. [.filename]#vinum# drives can contain any number of subdisks. With the exception of a small area at the beginning of the drive, which is used for storing configuration and state information, the entire drive is available for data storage.
The following sections describe the way these objects provide the functionality required of [.filename]#vinum#.
Volume Size Considerations