Sun Microsystems Sun StorEdge T3, Installation, Operation And Service Manual

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3-6 Sun StorEdge T3 Disk Tray Installation, Operation, and Service Manual • July 2000
has a data capacity of 4 x 18 = 72 Gbytes.
TABLE 3-4
shows the data blocks in a RAID
5 volume. The location of the parity drive in the stripe rotates from right to left in
order to eliminate a performance bottleneck.
3.2.3 Cache Modes
Data cache is used for the following purposes:
■
Buffering. Data that is being transferred between the host and the drives.
■
Read caching. Data for future retrieval, to reduce disk I/O as much as possible.
■
Write caching. Data that is used to build up stripes of data, eliminating the read-
modify-write overhead. Write caching improves performance for applications that
are writing to disk.
Four cache modes are supported: auto, write-behind, write-through, and none. The
default mode for the Sun StorEdge T3 disk tray is set to auto.
In the auto cache mode, if the disk tray has full redundancy available, then the
caching mode is write-behind. If any component is nonredundant, then the cache
mode is set to write-through. Read caching is always performed. This mode
provides the best performance while retaining full redundancy protection.
The write-behind cache mode forces write-behind caching to always be used. There is
the possibility of data loss in this mode if the units are not a fully redundant
configuration (partner group) and a disk tray system failure occurs.
TABLE 3-4
RAID 5 Data Blocks
1
1. Block numbers are written in hexadecimal notation.
Stripe Drive1 Drive2 Drive3 Drive4 Drive5 Drive6 Drive7 Drive8 Drive9
0 0 1 2 3 4 5 6 7 P
1 9 A B C D E F P 8
2 12 13 14 15 16 17 P 10 11
3 1B 1C 1D 1E 1F P 18 19 20
4 24 25 26 27 P 20 21 22 23
5 2D 2E 2F P 28 29 2A 2B 2C
6 36 37 P 30 31 32 33 34 35
7 3F P 38 39 3A 3B 3C 3D 3E
8 P 40 41 42 43 44 45 46 47
9 48 49 4A 4B 4C 4D 4E 4F P