In the last article we reviewed the most basic RAID levels. In this article, we take things to the next level and discuss what is called "Nested RAID Levels". These concepts can provide both performance and redundancy for a data-rich world. We then look at RAID-01 and RAID-10; two of the most common Nested RAID configurations.
This particular example was taken from wikipedia. There are a total of six drives that are all the same size (120GB). This RAID-01 array has two sets of three drives each. Each set of three drives is built with RAID-0 (the lowest level) and then they are combined using RAID-1 (mirroring).
The capacity of a RAID-01 array are the fairly simple to compute. The capacity is computed as,
Capacity = (n/2) * min(disk sizes)
where n is the number of disks in the array and min(disk sizes) is the minimum common capacity across the drives (this indicates that you can use drives of different sizes). This equation also means that RAID-01 is not very capacity efficient since you lose half of the drives to gain better data redundancy (mirroring).
Examining the RAID layout for data redundancy and failures is very enlightening. In particular what happens when a drive is lost? Let’s assume drive 2 is lost (second drive from the left). Immediately the RAID-0 group that it belongs to fails. However, since we have a mirror of the RAID-0 group, we don’t lose any data (always a good thing). We can even tolerate losing disks 1 and 3 while disk 2 is down without losing any data because they are part of the first RAID-0 group. However, after losing disk 2, we cannot tolerate losing any disk in the second RAID-0 group (disk 4, disk 5, disk 6). So the redundancy of RAID-01 is one disk (worst case) and n/2 in the best case (but not likely to happen).
Table 1 below is a quick summary of RAID-01 with a few highlights.
Table 1 – RAID-01 Highlights
| Raid Level |
Pros |
Cons |
Storage Efficiency |
Minimum Number of disks |
| RAID-01 |
- Very good performance (read and write) with the exact level of performance depending upon how many drives in a RAID-0 stripe. RAID-1 results in very good read performance.
- Reasonable data redundancy (can tolerate the loss of any one disk)
- Number of disks lost with no loss of data access varies from 1 to n/2. However, the n/2 is unlikely to happen.
|
- Very poor storage efficiency (50%)
- With Nested RAID levels, more data redundancy is desired beyond the loss of one disk.
- All disks have to be used in rebuild (more on that later in article).
|
50% assuming the drives are the same size |
4 (Note: you have to use an even number of disks) |
RAID 1+0 (RAID-10)
RAID-10 is one of the most popular Nested RAID configurations and builds the configuration in the opposite sequence of RAID-01. In Figure 3 below, a minimum configuration of four disks is shown.
Figure 3: RAID-10 layout with Four Drives
For RAID-10 the configuration begins with two groups of RAID-1 pairs (we’re assuming that RAID-1 is done with only two disks). Then RAID-0 is used to combine the RAID-1 groups. Compared to RAID-01 we are using a data redundant standard RAID level first (at the lowest level), and then using a performance standard RAID level at the highest level.
The data flow for RAID-10 is fairly simple to follow. The data in chunk “A” is first striped across the two RAID-1 pairs using RAID-0 (the highest RAID level). Then each RAID pair mirrors their particular block across the two disks (RAID-1). So mirrored data blocks are closer to one another in RAID-10 than in RAID-01.
To compare directly to RAID-01 let’s look at the same six-drive configuration as Figure 2. Figure 4 below shows the RAID-10 configuration using the same six drives.
Figure 4: RAID-10 layout with Six Drives
In this case there are three pairs of RAID-1 disks that are combined with RAID-0. Comparing to RAID-01, RAID-10 first stripes the data using RAID-0 and then uses RAID-1 for data redundancy. RAID-01 does the opposite – data is mirrored first and then stripped.
The capacity of a RAID-10 array is the fairly simple to compute and is exactly the same as RAID-01. The capacity is computed as,
Capacity = (n/2) * min(disk sizes)
where n is the number of disks in the array and min(disk sizes) is the minimum common capacity across the drives (this indicates that you can use drives of different sizes). This equation also means that RAID-10 is not very capacity efficient since you lose half of the drives to gain better data redundancy (mirroring).
But what happens in RAID-10 if you lose a disk? For example, let’s again assume we lose disk 2 (second disk from the left). You can now tolerate the loss of another disk as long as it is not disk 1. You can actually lose one disk in each RAID-1 pair without losing data. However, if you lose both disks in a RAID-1 pair the RAID-0 configuration is lost. So, just like RAID-01, you can lose one disk with out losing access to the data. But you can lose up to n/2 disks without losing access to the data, IF they are correct disks (very unlikely to happen). So just like RAID-01, the redundancy of RAID-10 is one disk (worst case) and n/2 in the best case (but not likely to happen).
Table 2 below is a quick summary of RAID-10 with a few highlights.
Table 2 – RAID-10 Highlights
| Raid Level |
Pros |
Cons |
Storage Efficiency |
Minimum Number of disks |
| RAID-10 |
- Very good performance (read and write) with the exact level of performance depending upon how many pairs in a RAID-0 stripe. RAID-1 results in very good read performance.
- Reasonable data redundancy (can tolerate the loss of one disk)
- Number of disks lost with no loss of data access varies from 1 to n/2. However, the n/2 is unlikely to happen.
- Only one disk is involved in rebuild (more on that later in article).
|
- Very poor storage efficiency
- With Nested RAID levels, more data redundancy is desired beyond the loss of one disk.
|
50% assuming the drives are the same size |
4 (Note: you have to use an even number of disks) |
Comments on "Intro to Nested-RAID: RAID-01 and RAID-10"
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