Striping Trade-offs

• Striping cannot be used to encapsulate existing file systems.

• Striping performs well for large sequential I/O and for uneven I/O distributions.

• Striping uses more CPU cycles than concatenation, but the trade-off is usually worth it.

• Striping does not provide any redundancy of data.

To summarize the trade-offs: Striping delivers good performance, particularly for large sequential I/O and for uneven I/O distributions, but it does not provide any redundancy of data.

Write intensive applications: Because of the read-modify-write nature of RAID5, metadevices with greater than about 20 percent writes should probably not be RAID5. If data protection is required, consider mirroring.

RAID5 writes will never be as fast as mirrored writes, which in turn will never be as fast as unprotected writes. The NVRAM cache on the SPARCstorage Array closes the gap between RAID5 and mirrored configurations.

Full Stripe Writes: RAID5 read performance is always good (unless the metadevice has suffered a disk failure and is operating in degraded mode), but write performance suffers because of the read-modify-write nature of RAID5.

In particular, when writes are less than a full stripe width or don't align with a stripe, multiple I/Os (a read-modify-write sequence) are required. First, the old data and parity are read into buffers. Next, the parity is modified (XOR's are performed between data and parity to calculate the new parity--first the old data is logically subtracted from the parity and then the new data is logically added to the parity), and the new parity and data are stored to a log. Finally, the new parity and new data are written to the data stripe units.

Full stripe width writes have the advantage of not requiring the read-modify-write sequence, and thus performance is not degraded as much. With full stripe writes, all new data stripes are XORed together to generate parity, and the new data and parity are stored to a log. Then, the new parity and new data are written to the data stripe units in a single write.

Full stripe writes are used when the I/O request aligns with the stripe and the I/O size exactly matches:

interlace_size * (num_of_columns - 1)

For example, if a RAID5 configuration is striped over 4 columns, in any one stripe, 3 chunks are used to store data, and 1 chunk is used to store the corresponding parity. In this example, full stripe writes are used when the I/O request starts at the beginning of the stripe and the I/O size is equal to: stripe_unit_size * 3. For example, if the stripe unit size is 16 Kbyte, full stripe writes would be used for aligned I/O requests of size 48 Kbyte.

Performance in degraded mode: When a slice of a RAID5 metadevice fails, the parity is used to reconstruct the data; this requires reading from every column of the RAID5 metadevice. The more slices assigned to the RAID5 metadevice, the longer read and write operations (including resyncing the RAID5 metadevice) will take when I/O maps to the failed device.