RAID6とは
定義、構成、データ復旧のオプション

RAID 6
RAID 6の構成はどのようなものなのか？

RAID 6は、パリティとデータストライプを使用する点で、RAID 5と似ています。違いは、RAID 5がパリティを1つ持つのに対し、RAID 6はパリティストライプを2つ持っていることです。これにより、RAID 6アレイは1台だけでなく、2台のドライブ故障にも耐えることができます。ほとんどのRAID 6構成の最初のパリティストライプに含まれるデータは、他のストライプからのデータのXORであり、2番目のパリティストライプは、通常、独自のアルゴリズムです。

RAID 6アレイでは、2台のディスク分のデータをパリティに割り当てる必要があります。つまり、RAID 6アレイは、同じサイズのRAID 10よりも、ドライブ２台分のスペースがパリティに割り当てられるだけなので、実装コストはそれよりも安いです。また、RAID 6はRAID 1よりも柔軟性があり、ボリュームサイズも大きくすることが可能です。

上の例では、Dell PowerEdgeサーバーに見られるような5ドライブのRAID 6アレイを使用しています。最初のストライプのドライブ4にある最初のパリティブロック（パリティ1）は、データ1（ドライブ1）、データ2（ドライブ2）、データ3（ドライブ3）という名前のブロックからのデータのXORです。

RAID 6
What does a RAID 6 configuration look like?

RAID 6 is similar to RAID 5 in that both arrays use parity and data striping. The difference is where RAID 5 has one instance of parity, RAID 6 has two parity stripes. This allows a RAID 6 array to withstand two drive failures rather than just one. The data contained in the first parity stripe in most RAID 6 configurations is an XOR of the data from the other stripes, the second parity stripe is typically a proprietary algorithm.

RAID 6 arrays will need to dedicate two disks worth of data to parity. This means a RAID 6 array is still cheaper to implement than a RAID 10 of the same size as only two drives worth of space is allocated to parity. Raid 6 also allows more flexibility and greater volume sizes than a RAID 1.

In the example above, we have a five drive RAID 6 array like you would see in a Dell PowerEdge server. The first parity block (Parity 1) found on Drive 4 for the first stripe, is the XOR of the data from the blocks named Data 1 (Drive 1), Data 2 (Drive 2), and Data 3 (Drive 3).

The second parity block (Parity 2) found on Drive 5 for the first stripe, is a combination of the data from Data1, Data2, Data 3 and can include Parity 1 depending on the manufacturer and controller. These parity calculations are repeated across all the data stripes using different drive combinations.

RAID6アレイにおけるパリティの役割について

パリティは数学的な機能であり、失われたデータの再構築を可能にするため、さらなる保護を提供します。すべてのデータストライプの一部としてパリティのブロックを持つことで、ドライブの1つまたは複数が故障またはオフラインになった場合でも、システムを再構築することができます。RAIDコントローラやRAIDソフトウェアは、パリティを使用することで、失われたデータセグメントを事実上再構築することができます。

下の例では、ドライブ2が故障したことがわかります。

ドライブを失うと、アレイはデグレードモードに移行します。デグレードモードでは、RAIDコントローラはデータストライプとパリティを必要に応じて組み合わせ、オペレーティングシステムに良好なデータを表示します。この例では、コントローラは最初のストライプでデータ1、データ3、パリティ1を組み合わせて、データ2の欠落データを置き換えます。2番目のストライプでは、データ5を置き換えるために、データ4、データ6、およびパリティ1が使用されます。3番目と4番目のストライプでは、すべてのデータドライブが存在するため、パリティは必要ありません。

1つのストライプに2つのパリティブロックがあるため、RAID 6では2台のドライブに障害が発生しても大丈夫です。下の例では、ドライブ2とドライブ4が故障していることがわかります。

2台のドライブを失った場合、コントローラはデータストライプとパリティ1およびパリティ2を組み合わせて、欠落したデータを再作成するために使用します。この例では、コントローラは最初のストライプにデータ1、データ3、パリティ2を組み合わせて、データ2の欠落データを置き換えます。2番目のストライプでは、データ5を置き換えるために、データ4、データ6、およびパリティ1が使用されます。3番目のストライプでは、データ7、データ9、パリティ2がデータ8を置き換えるために使用されます。

What does parity do in a RAID 6 array?

Parity is a mathematical feature that provides additional protection because it allows for the reconstruction of lost data. Having a block of parity as part of every data stripe allows the system to rebuild in the event one or more of the drives fails or goes offline. The RAID controller or RAID software can virtually rebuild any missing data segment by using parity.

In the example below, we see that Drive 2 failed.

Upon losing a drive, the array will go into a degraded mode. In degraded mode, the RAID controller will combine the data stripes with parity as needed to present good data to the operating system. In our example, the controller will combine Data 1, Data 3 and Parity 1 for the first stripe to replace the missing data in Data 2. In the second stripe, Data 4, Data 6 and Parity 1 are used to replace Data 5. In the third and fourth stripes, no parity is needed as all the data drives are present.

With two parity blocks per stripe, RAID 6 allows for two drives to fail. In the example below, we see that Drives 2 and 4 have failed.

Upon losing two drives, the controller will use the data stripes combined with Parity 1 and Parity 2 to recreate the missing data. In our example, the controller will combine Data 1, Data 3 and Parity 2 for the first stripe to replace the missing data in Data 2. In the second stripe, Data 4, Data 6 and Parity 1 are used to replace Data 5. In the third stripe, Data 7, Data 9 and Parity 2 are used to replace Data 8.

What does parity do in a RAID 6 array?

Parity is a mathematical feature that provides additional protection because it allows for the reconstruction of lost data. Having a block of parity as part of every data stripe allows the system to rebuild in the event one or more of the drives fails or goes offline. The RAID controller or RAID software can virtually rebuild any missing data segment by using parity.

In the example below, we see that Drive 2 failed.

Upon losing a drive, the array will go into a degraded mode. In degraded mode, the RAID controller will combine the data stripes with parity as needed to present good data to the operating system. In our example, the controller will combine Data 1, Data 3 and Parity 1 for the first stripe to replace the missing data in Data 2. In the second stripe, Data 4, Data 6 and Parity 1 are used to replace Data 5. In the third and fourth stripes, no parity is needed as all the data drives are present.

With two parity blocks per stripe, RAID 6 allows for two drives to fail. In the example below, we see that Drives 2 and 4 have failed.

Upon losing two drives, the controller will use the data stripes combined with Parity 1 and Parity 2 to recreate the missing data. In our example, the controller will combine Data 1, Data 3 and Parity 2 for the first stripe to replace the missing data in Data 2. In the second stripe, Data 4, Data 6 and Parity 1 are used to replace Data 5. In the third stripe, Data 7, Data 9 and Parity 2 are used to replace Data 8.

RAID6アレイにおけるホットスペアの動作について

ホットスペアとは、RAID 6アレイに追加できる1つまたは複数の追加ドライブのことで、ドライブが故障した場合に早急に復旧できるようにするためのものです。上記の例では、健全なRAID 6アレイにホットスペアが1台追加されています。故障が発生してドライブが必要になるまで、ホットスペアにはデータが含まれないことに注意してください。

ホットスペアがシステムで利用可能な場合、障害発生時にコントローラーが自動的に障害ドライブからホットスペアへの欠損データの再構築を開始します。

上の例では、ドライブ 2 が故障したことを示しています。システムはホットスペアを使用し、ドライブ2からホットスペアに失われたデータをすべて再構築しました。ドライブ2をシステムから取り外し、新しいドライブ（新しいホットスペアまたは新しいドライブ2）と交換し、その上でデータを再構築することができるようになりました。

なぜホットスペアを使うのか？ドライブが故障した場合、再構築には時間が重要です。デグレードモードで動作させると、残りのドライブにさらなるストレスがかかり、迅速に修正しないとさらなる故障の原因となる可能性があります。また、同じ製造ロットのドライブに同様の欠陥がある可能性もあり、他のドライブがすぐに故障する可能性もあります。ホットスペアを1台以上用意しておくと、復旧が早くなります。

How does a Hot Spare work in a RAID 6 array?

A hot spare is one or more additional drives that can be added to a RAID 6 array to allow for fast recovery in the event of a failed drive. In the above example, we see a healthy RAID 6 array with a single hot spare added. Note that the hot spare does not contain any data until a failure occurs and the drive is needed.

If a hot spare is available to the system, the controller will automatically begin rebuilding the missing data from the failed drive to the hot spare in the event of a failure.

The example above shows Drive 2 failed. The system used the hot spare and rebuilt all the missing data from Drive 2 on to the Hot Spare. Drive 2 can now be removed from the system and replaced with a new drive, either as a new hot spare or as a new Drive 2 and the data can be rebuilt on it.

Why use Hot Spares? When a drive fails, time is of the essence in rebuilding. Running in degraded mode puts additional stress on the remaining drives and can cause additional failures if not corrected quickly. It is also possible that drives from a similar manufacturing lot will have similar defects, so it is possible that others may fail soon. Having one or more hot spares available allows for quicker recovery times.

How does a Hot Spare work in a RAID 6 array?

A hot spare is one or more additional drives that can be added to a RAID 6 array to allow for fast recovery in the event of a failed drive. In the above example, we see a healthy RAID 6 array with a single hot spare added. Note that the hot spare does not contain any data until a failure occurs and the drive is needed.

If a hot spare is available to the system, the controller will automatically begin rebuilding the missing data from the failed drive to the hot spare in the event of a failure.

The example above shows Drive 2 failed. The system used the hot spare and rebuilt all the missing data from Drive 2 on to the Hot Spare. Drive 2 can now be removed from the system and replaced with a new drive, either as a new hot spare or as a new Drive 2 and the data can be rebuilt on it.

Why use Hot Spares? When a drive fails, time is of the essence in rebuilding. Running in degraded mode puts additional stress on the remaining drives and can cause additional failures if not corrected quickly. It is also possible that drives from a similar manufacturing lot will have similar defects, so it is possible that others may fail soon. Having one or more hot spares available allows for quicker recovery times.

RAID 6アレイの故障の原因について

RAID6アレイが故障する原因はいくつかあります。ここでは、Ontrackで目にする主な原因をいくつか紹介します。

RAID 6のデータ復旧は可能なのか？

RAID6アレイからのデータ復旧は可能です。RAID 6アレイのデータ復旧は複雑で困難な場合がありますが、一般的には成功することが多いです。最大の問題は、2番目のパリティブロックを作成するために使用される独自のアルゴリズムで、各メーカーによって実装が異なるため、これをサポートするツールを研究・開発するためのカスタム開発が必要になることがよくあります。

データ消失の原因はいくつかあり、それぞれに対する復旧作業も異なります。以下に いくつかの例を挙げます。

故障ドライブ1台の場合のデータ復旧

RAID 5アレイと同様に、アレイの1つのドライブが故障した場合、パリティを使用して不足するデータを再構築することができます。このシナリオでは、Ontrackは通常、データの100%を復旧することができます。機能しないアレイがある場合、アレイのすべてのドライブがクリーンルームでイメージを抽出されます（可能であれば故障したドライブを含む）。その後、これらのイメージを使用してアレイを仮想的に再構築します。RAIDが組み立てられたら、ファイルシステムやボリュームに破損がないか解析し、仮想的に修復してデータを抽出します。失われたデータストライプはパリティから再構築できるため、故障したドライブは必要ないことが多い。

故障した2台のドライブでデータ復旧

RAID 6は、1台を除くすべてのドライブが機能する必要があるRAID 5アレイとは異なり、最大2台のディスクが故障してもデータに影響がないように設計されています。複数の故障したドライブから復旧するプロセスは、1台のドライブ故障と同様です。 機能しないアレイを受け取り、クリーンルーム内で故障したドライブを含むドライブをイメージ抽出します。ドライブ上のデータが最新であれば、アレイの完全復旧のためには、故障したドライブは必要ない場合があります。その後、これらのイメージを使用して、アレイを仮想的に再構築します。

上記の例では、第1ストライプのData 1、Data 3、Parity 2がData 2の再構築に使用されます。データ4、パリティ1、データ6は、2番目のストライプのデータ5を再構築するために使用されます。データ7、パリティ2、データ9は、第3ストライプのデータ8を再構築するために使用される。

RAIDアレイが事実上再構築されると、ファイルシステムやボリュームが破損していないか解析されます。ファイルシステムの破損に加え、エンジニアは一貫性のないデータや古いデータも探します。これは、ドライブ故障の間に時間が空いて、ドライブの1つが劣化している場合に発生します。データ復旧エンジニアは、ボリュームを仮想的に修復し、良好なファイルデータを抽出できるように、この種の損傷を認識する経験が必要です。

複数の故障したドライブからのデータ復旧

RAID6アレイの場合、2台以上のドライブ故障があっても完全復旧が可能です。

上記の例では、RAID 6アレイで、5つのディスクの一部の領域に損傷があります。1つのストライプに2つ以上の故障ブロックがない場合、失われたデータを再構築することが可能です。Ontrackは、各ドライブの可能な限り多くの部分のイメージを抽出します。

その後、これらのイメージを使用してアレイが仮想的に再構築されます。上記の例では、ストライプ1のデータ1、データ3、パリティ2がデータ2の再構築に使用されています。データ4、データ5、データ6はすべて無傷であるため、ストライプ2にはパリティは必要ありません。データ7、パリティ2、データ8は、3番目のストライプのデータ9を再構築するために使用されます。

RAIDアレイが仮想的に再構築されると、ファイルシステムまたはボリュームが破損していないかスキャンされます。復旧可能なデータは、仮想的に再構築されたアレイから新しいメディアに抽出され、本番に戻されます。

Is data recovery from RAID 6 possible?

Data recovery is possible from a RAID 6 array. While data recovery can be complex and more challenging with a RAID 6 array, it generally ends successfully. The biggest challenge is often the proprietary algorithm used to create the second parity block as each manufacturer implements this differently, and custom development is often needed to research and develop tools to support this.

There are several reasons for data loss and the recovery effort for each of them is different. A few examples are below:

Data Recovery with one failed drive

Like a RAID 5 array, if one drive fails in an array, parity can be used to rebuild the missing data. In this scenario, Ontrack is usually able to recover 100% of the data. Upon receipt of a non-functional array, all of the drives from the array are imaged in the clean room (including the failed drive if possible). Then the array is virtually rebuilt using those images. Once the RAID is assembled, the file system or volume is scanned for corruption, virtually repaired and the data extracted. The failed drive is often not needed as any missing data stripes can be rebuilt from parity.

Data Recovery from two failed drives

Unlike the RAID 5 array where all but one of the drives are needed for it to function, RAID 6 is designed to allow for the failure of up to two disks without any impact on the data. The process to recover from multiple failed drives is similar to a single drive failure.  Upon receipt of a non-functional array, the drives are imaged in the clean room, including the failed drives. If the data on the drives is up to date, the failed drives may not be needed to get a full recovery of the array. Then the array is virtually rebuilt using those images. 

In the above example, Data 1, Data 3 and Parity 2 from stripe one is used to rebuild Data 2. Data 4, Parity 1 and Data 6 are used to rebuild Data 5 in the second stripe. Data 7, Parity 2 and Data 9 are used to rebuild Data 8 in the third stripe.

Once the RAID array is virtually reassembled, the file system or volume is scanned for corruption. In addition to file system corruption, engineers are also looking for data that is not consistent or out of date. This occurs when there is a gap of time between drive failures and one of the drives is degraded. Data recovery engineers need experience in recognizing this type of damage so they can virtually repair the volume and extract good file data. 

Data Recovery from multiple failed drives

It is possible to get a full recovery from a RAID 6 array even if there are more than two drive failures.

In the example above, we have a RAID 6 array with damage on some areas of all five disks. If there are no more than two failed blocks per stripe, it is possible to rebuild the missing data. Ontrack will image as much of each drive as possible.

Then the array is virtually rebuilt using those images. In the above example, Data 1, Data 3 and Parity 2 from stripe one is used to rebuild Data 2. No parity is needed for stripe 2 as Data 4,  Data 5 and Data 6 are all intact. Data 7, Parity 2 and Data 8 are used to rebuild Data 9 in the third stripe.

Once the RAID array is virtually reassembled, the file system or volume is scanned for corruption. Recoverable data is extracted from the virtually rebuilt array to new media to be put back into production.

Is data recovery from RAID 6 possible?

Data recovery is possible from a RAID 6 array. While data recovery can be complex and more challenging with a RAID 6 array, it generally ends successfully. The biggest challenge is often the proprietary algorithm used to create the second parity block as each manufacturer implements this differently, and custom development is often needed to research and develop tools to support this.

There are several reasons for data loss and the recovery effort for each of them is different. A few examples are below:

Data Recovery with one failed drive

Like a RAID 5 array, if one drive fails in an array, parity can be used to rebuild the missing data. In this scenario, Ontrack is usually able to recover 100% of the data. Upon receipt of a non-functional array, all of the drives from the array are imaged in the clean room (including the failed drive if possible). Then the array is virtually rebuilt using those images. Once the RAID is assembled, the file system or volume is scanned for corruption, virtually repaired and the data extracted. The failed drive is often not needed as any missing data stripes can be rebuilt from parity.

Data Recovery from two failed drives

Unlike the RAID 5 array where all but one of the drives are needed for it to function, RAID 6 is designed to allow for the failure of up to two disks without any impact on the data. The process to recover from multiple failed drives is similar to a single drive failure.  Upon receipt of a non-functional array, the drives are imaged in the clean room, including the failed drives. If the data on the drives is up to date, the failed drives may not be needed to get a full recovery of the array. Then the array is virtually rebuilt using those images. 

In the above example, Data 1, Data 3 and Parity 2 from stripe one is used to rebuild Data 2. Data 4, Parity 1 and Data 6 are used to rebuild Data 5 in the second stripe. Data 7, Parity 2 and Data 9 are used to rebuild Data 8 in the third stripe.

Once the RAID array is virtually reassembled, the file system or volume is scanned for corruption. In addition to file system corruption, engineers are also looking for data that is not consistent or out of date. This occurs when there is a gap of time between drive failures and one of the drives is degraded. Data recovery engineers need experience in recognizing this type of damage so they can virtually repair the volume and extract good file data. 

Data Recovery from multiple failed drives

It is possible to get a full recovery from a RAID 6 array even if there are more than two drive failures.

In the example above, we have a RAID 6 array with damage on some areas of all five disks. If there are no more than two failed blocks per stripe, it is possible to rebuild the missing data. Ontrack will image as much of each drive as possible.

Then the array is virtually rebuilt using those images. In the above example, Data 1, Data 3 and Parity 2 from stripe one is used to rebuild Data 2. No parity is needed for stripe 2 as Data 4,  Data 5 and Data 6 are all intact. Data 7, Parity 2 and Data 8 are used to rebuild Data 9 in the third stripe.

Once the RAID array is virtually reassembled, the file system or volume is scanned for corruption. Recoverable data is extracted from the virtually rebuilt array to new media to be put back into production.

Is data recovery from RAID 6 possible?

Data recovery is possible from a RAID 6 array. While data recovery can be complex and more challenging with a RAID 6 array, it generally ends successfully. The biggest challenge is often the proprietary algorithm used to create the second parity block as each manufacturer implements this differently, and custom development is often needed to research and develop tools to support this.

There are several reasons for data loss and the recovery effort for each of them is different. A few examples are below:

Data Recovery with one failed drive

Like a RAID 5 array, if one drive fails in an array, parity can be used to rebuild the missing data. In this scenario, Ontrack is usually able to recover 100% of the data. Upon receipt of a non-functional array, all of the drives from the array are imaged in the clean room (including the failed drive if possible). Then the array is virtually rebuilt using those images. Once the RAID is assembled, the file system or volume is scanned for corruption, virtually repaired and the data extracted. The failed drive is often not needed as any missing data stripes can be rebuilt from parity.

Data Recovery from two failed drives

Unlike the RAID 5 array where all but one of the drives are needed for it to function, RAID 6 is designed to allow for the failure of up to two disks without any impact on the data. The process to recover from multiple failed drives is similar to a single drive failure.  Upon receipt of a non-functional array, the drives are imaged in the clean room, including the failed drives. If the data on the drives is up to date, the failed drives may not be needed to get a full recovery of the array. Then the array is virtually rebuilt using those images. 

In the above example, Data 1, Data 3 and Parity 2 from stripe one is used to rebuild Data 2. Data 4, Parity 1 and Data 6 are used to rebuild Data 5 in the second stripe. Data 7, Parity 2 and Data 9 are used to rebuild Data 8 in the third stripe.

Once the RAID array is virtually reassembled, the file system or volume is scanned for corruption. In addition to file system corruption, engineers are also looking for data that is not consistent or out of date. This occurs when there is a gap of time between drive failures and one of the drives is degraded. Data recovery engineers need experience in recognizing this type of damage so they can virtually repair the volume and extract good file data. 

Data Recovery from multiple failed drives

It is possible to get a full recovery from a RAID 6 array even if there are more than two drive failures.

In the example above, we have a RAID 6 array with damage on some areas of all five disks. If there are no more than two failed blocks per stripe, it is possible to rebuild the missing data. Ontrack will image as much of each drive as possible.

Then the array is virtually rebuilt using those images. In the above example, Data 1, Data 3 and Parity 2 from stripe one is used to rebuild Data 2. No parity is needed for stripe 2 as Data 4,  Data 5 and Data 6 are all intact. Data 7, Parity 2 and Data 8 are used to rebuild Data 9 in the third stripe.

Once the RAID array is virtually reassembled, the file system or volume is scanned for corruption. Recoverable data is extracted from the virtually rebuilt array to new media to be put back into production.

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