RAID 1 volumes have names like other volumes (d0, d1, and so forth). For more information on volume naming, see Volume Conventions. Each submirror (which is also a volume) has a unique device name.
A RAID 1 volume is made of one or more stripes or concatenations. The stripes or concatenations within a RAID 1 volume are called submirrors. (A RAID 1 volume cannot be made of RAID 5 volumes.)
A RAID 1 volume can consist of up to three (3) submirrors. (Practically, creating a two-way mirror is usually sufficient. A third submirror enables you to make online backups without losing data redundancy while one submirror is offline for the backup.)
Submirrors are distinguished from other volumes in that normally they can only be accessed by the RAID 1 volume. The submirror is accessible only through the RAID 1 volume when you attach it to the RAID 1 volume.
If you take a submirror "offline," the RAID 1 volume stops reading and writing to the submirror. At this point, you could access the submirror itself, for example, to perform a backup. However, the submirror is in a read-only state. While a submirror is offline, Enhanced Storage keeps track of all writes to the RAID 1 volume. When the submirror is brought back online, only the portions of the RAID 1 volume that were written (resync regions) are resynced. Submirrors can also be taken offline to troubleshoot or repair physical devices which have errors.
Submirrors have names like other volumes (d0, d1, and so forth). For more information on volume naming, see Volume Conventions.
Submirrors can be attached or detached from a RAID 1 volume at any time. To do so, at least one submirror must remain attached at all times. You can force a submirror to be detached using the -f option to the metadetach(1M) command. Enhanced Storage always "forces" a RAID 1 volume detach, so there is no extra option. Normally, you create a RAID 1 volume with only a single submirror. Then you attach a second submirror after creating the RAID 1 volume.
The following options are available to optimize RAID 1 volume performance:
RAID 1 (mirror) read policy
RAID 1 (mirror) write policy
The order in which RAID 1 (mirror) volumes are resynced (pass number)
You can define RAID 1 (mirror) volume options when you initially create the RAID 1 volume, or after a RAID 1 (mirror) volume has been set up. For tasks related to changing these options, refer toSolaris Volume Manager Administration Guide.
Enhanced Storage enables different read and write policies to be configured for a RAID 1 (mirror) volume. Properly set read and write policies can improve performance for a given configuration.
Table 2-1. RAID 1 (Mirror) Read Policies
Read Policy |
Description |
---|---|
Round Robin (Default) |
Attempts to balance the load across the submirrors. All reads are made in a round-robin order (one after another) from all submirrors in a RAID 1 volume. |
Geometric |
Enables reads to be divided among submirrors on the basis of a logical disk block address. For instance, with a two-way submirror, the disk space on the RAID 1 volume is divided into two equally-sized logical address ranges. Reads from one submirror are restricted to one half of the logical range, and reads from the other submirror are restricted to the other half. The geometric read policy effectively reduces the seek time necessary for reads. The performance gained by this mode depends on the system I/O load and the access patterns of the applications. |
First |
Directs all reads to the first submirror. This should be used only when the device(s) comprising the first submirror are substantially faster than those of the second submirror. |
Table 2-2. RAID 1 (Mirror) Write Policies
Write Policy |
Description |
---|---|
Parallel (Default) |
A write to a RAID 1 volume (mirror) is replicated and dispatched to all of the submirrors simultaneously. |
Serial |
Performs writes to submirrors serially (that is, the first submirror write completes before the second is started). The serial option specifies that writes to one submirror must complete before the next submirror write is initiated. The serial option is provided in case a submirror becomes unreadable, for example, due to a power failure. |
Enhanced Storage cannot guarantee that a RAID 1 volume will be able to tolerate multiple slice failures and continue operating. However, depending on the RAID 1 volume's configuration, in many instances Enhanced Storage can handle a multiple-slice failure scenario. As long as multiple slice failures within a RAID 1 volume do not contain the same logical blocks, the RAID 1 volume continues to operate. (The submirrors must also be identically constructed.)
RAID 1 volume d1 consists of two stripes (submirrors), each of which consists of three identical physical disks and the same interlace value. A failure of three disks, A, B, and F can be tolerated because the entire logical block range of the RAID 1 volume is still contained on at least one good disk.
If, however, disks A and D fail, a portion of the RAID 1 volume's data is no longer available on any disk and access to these logical blocks will fail.
When a portion of a RAID 1 volume's data is unavailable due to multiple slice errors, access to portions of the RAID 1 volume where data is still available will succeed. Under this situation, the RAID 1 volume acts like a single disk that has developed bad blocks; the damaged portions are unavailable, but the rest is available.