RAID 1 Volume Read and Write Policies
Solaris Volume Manager enables different read and write policies to be configured for a mirror. Properly set read and write policies can improve performance for a given configuration.
Table 9-1 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 mirror. |
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 mirror 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 policy should be used only when the device or devices that comprise the first submirror are substantially faster than those of the second submirror. |
Table 9-2 Mirror Write Policies
Write Policy | Description |
|---|---|
Parallel (Default) | A write to a 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. |
RAID 1 Volume (Mirror) Resynchronization
RAID 1 volume (mirror) resynchronization is the process of copying data from one submirror to another after submirror failures, system crashes, when a submirror has been taken offline and brought back online, or after the addition of a new submirror.
While the resynchronization takes place, the mirror remains readable and writable by users.
A mirror resynchronization ensures proper mirror operation by maintaining all submirrors with identical data, with the exception of writes in progress.
Note - A mirror resynchronization is mandatory, and cannot be omitted. You do not need to manually initiate a mirror resynchronization. This process occurs automatically.
Full Resynchronization
When a new submirror is attached (added) to a mirror, all the data from another submirror in the mirror is automatically written to the newly attached submirror. Once the mirror resynchronization is done, the new submirror is readable. A submirror remains attached to a mirror until it is explicitly detached.
If the system crashes while a resynchronization is in progress, the resynchronization is restarted when the system finishes rebooting.
Optimized Resynchronization
During a reboot following a system failure, or when a submirror that was offline is brought back online, Solaris Volume Manager performs an optimized mirror resynchronization. The metadisk driver tracks submirror regions and knows which submirror regions might be out-of-sync after a failure. An optimized mirror resynchronization is performed only on the out-of-sync regions. You can specify the order in which mirrors are resynchronized during reboot, and you can omit a mirror resynchronization by setting submirror pass numbers to 0 (zero). (See "Pass Number" for information.)
Caution - A pass number of 0 (zero) should only be used on mirrors that
are mounted as read-only.
Partial Resynchronization
Following a replacement of a slice within a submirror, Solaris Volume Manager performs a partial mirror resynchronization of data. Solaris Volume Manager copies the data from the remaining good slices of another submirror to the replaced slice.
Pass Number
The pass number, a number in the range 0-9, determines the order in which a particular mirror is resynchronized during a system reboot. The default pass number is 1. Smaller pass numbers are resynchronized first. If 0 is used, the mirror resynchronization is skipped. A pass number of 0 should be used only for mirrors that are mounted as read-only. Mirrors with the same pass number are resynchronized at the same time.
Background Information for RAID 1 Volumes
Unmirroring - The Enhanced Storage tool within the Solaris Management Console does not support unmirroring root (/), /opt, /usr, or swap, or any other file system that cannot be unmounted while the system is running. Instead, use the command-line procedure for these file systems.
Attaching - You can attach a submirror to a mirror without interrupting service. You attach submirrors to mirrors to create two-way and three-way mirrors.
Detach vs. Offline - When you place a submirror offline, you prevent the mirror from reading from and writing to the submirror, but you preserve the submirror's logical association to the mirror. While the submirror is offline, Solaris Volume Manager keeps track of all writes to the mirror and they are written to the submirror when it is brought back online. By performing an optimized resynchronization, Solaris Volume Manager only has to resynchronize data that has changed, not the entire submirror. When you detach a submirror, you sever its logical association to the mirror. Typically, you place a submirror offline to perform maintenance. You detach a submirror to remove it.
Background Information for Creating RAID 1 Volumes
Before you create a mirror, create the RAID 0 (stripe or concatenation) volumes that will make up the mirror.
Any file system including root (/), swap, and /usr, or any application such as a database, can use a mirror.
Caution - When you create a mirror for an existing file system, be sure
that the initial submirror contains the existing file system.When creating a mirror, first create a one-way mirror, then attach a second submirror. This strategy starts a resynchronization operation and ensures that data is not corrupted.
You can create a one-way mirror for a future two-way or three-way mirror.
You can create up to a three-way mirror. However, two-way mirrors usually provide sufficient data redundancy for most applications, and are less expensive in terms of disk drive costs. A three-way mirror enables you to take a submirror offline and perform a backup while maintaining a two-way mirror for continued data redundancy.
Use components of identical size when creating submirrors. Using components of different sizes leaves wasted space in the mirror.
Adding additional state database replicas before you create a mirror can improve the mirror's performance. As a general rule, add two additional replicas for each mirror you add to the system. Solaris Volume Manager uses these additional replicas to store the dirty region log (DRL), used to provide optimized resynchronization. By providing adequate numbers of replicas to prevent contention or using replicas on the same disks or controllers as the mirror they log, you will improve overall performance.
Background Information for Changing RAID 1 Volume Options
You can change a mirror's pass number, and its read and write policies.
Mirror options can be changed while the mirror is running.
How Booting Into Single-User Mode Affects RAID 1 Volumes
If a system with mirrors for root (/), /usr, and swap, the so-called "boot" file systems, is booted into single-user mode (by using the boot -s command), these mirrors and possibly all mirrors on the system will appear in the "Needing Maintenance" state when viewed with the metastat command. Furthermore, if writes occur to these slices, the metastat command shows an increase in dirty regions on the mirrors.
Though this situation appears to be potentially dangerous, there is no need for concern. The metasync -r command, which normally occurs during boot to resynchronize mirrors, is interrupted when the system is booted into single-user mode. Once the system is rebooted, the metasync -r command will run and resynchronize all mirrors.
If this is a concern, run the metasync -r command manually.
Scenario--RAID 1 Volumes (Mirrors)
RAID 1 volumes provide a means of constructing redundant volumes, in which a partial or complete failure of one of the underlying RAID 0 volumes does not cause data loss or interruption of access to the file systems. The following example, drawing on the sample system explained in Chapter 4, Configuring and Using Solaris Volume Manager (Scenario), describes how RAID 1 volumes can provide redundant storage.
As described in "Interlace Values for Stripes", the sample system has two RAID 0 volumes, each of which is approximately 27Gbytes in size and spans three disks. By creating a RAID 1 volume to mirror these two RAID 0 volumes, a fully redundant storage space can provide resilient data storage.
Within this RAID 1 volume, the failure of either of the disk controllers will not interrupt access to the volume. Similarly, failure of up to three individual disks might be tolerated without access interruption.
To provide additional protection against problems that could interrupt access, use hot spares, as described in Chapter 15, Hot Spare Pools (Overview) and specifically in "How Hot Spares Work".