¿Qué es RAID 6?
Definición, Configuración y Opciones de Recuperación de Datos

RAID 6
¿Cómo es una Configuración de RAID 6?

RAID 6 es similar al RAID 5 ya que ambas matrices utilizan paridad y división de datos. La diferencia es que el RAID 5 tiene una instancia de paridad, RAID 6 tiene dos franjas de paridad. Esto permite que una matriz RAID 6 resista fallos de dos unidades en lugar de sólo una. Los datos contenidos en la primera franja de paridad en la mayoría de las configuraciones de RAID 6 son un XOR de los datos de las otras franjas, la segunda franja de paridad suele ser un algoritmo propietario.

Las matrices RAID 6 deberán dedicar dos discos de datos a la paridad. Esto significa que una matriz RAID 6 sigue siendo más barata de implementar que una RAID 10 del mismo tamaño, ya que solo se asignan dos unidades de espacio a la paridad. RAID 6 también permite más flexibilidad y mayores tamaños de volumen que RAID 1.

En el ejemplo anterior, tenemos una matriz RAID 6 de cinco unidades como la que vería en un servidor Dell PowerEdge. El primer bloque de paridad (Paridad 1) que se encuentra en la Unidad 4 para la primera franja, es el XOR de los datos de los bloques denominados Datos 1 (Unidad 1), Datos 2 (Unidad 2) y Datos 3 (Unidad 3).

El segundo bloque de paridad (Paridad 2) que se encuentra en la Unidad 5 para la primera franja, es una combinación de los datos de Datos 1, Datos 2, Datos 3 y puede incluir la Paridad 1 según el fabricante y el controlador. Estos cálculos de paridad se repiten en todas las franjas de datos utilizando diferentes combinaciones de unidades.

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.

¿Qué hace la paridad en una matriz RAID 6?

La paridad es una característica matemática que brinda protección adicional porque permite la reconstrucción de datos perdidos. Tener un bloque de paridad como parte de cada franja de datos permite que el sistema se reconstruya en caso de que una o más de las unidades fallen o se desconecten. El controlador RAID o el software RAID pueden prácticamente reconstruir cualquier segmento de datos que falte mediante el uso de la paridad.

En el siguiente ejemplo, vemos que la unidad 2 falló.

Al perder una unidad, la matriz entrará en un modo degradado. En el modo degradado, el controlador RAID combinará las franjas de datos con paridad según sea necesario para presentar buenos datos al sistema operativo. En nuestro ejemplo, el controlador combinará Datos 1, Datos 3 y Paridad 1 en la primera franja para reemplazar los datos que faltan en los Datos 2. En la segunda franja, los Datos 4, los Datos 6 y la Paridad 1 se utilizan para reemplazar los Datos 5. En las franjas tercera y cuarta, no se necesita paridad ya que todas las unidades de datos están presentes.

Con dos bloques de paridad por banda, RAID 6 permite que dos unidades fallen. En el siguiente ejemplo, vemos que las unidades 2 y 4 fallaron.

Al perder dos unidades, el controlador usará las franjas de datos combinadas con Paridad 1 y Paridad 2 para recrear los datos que faltan. En nuestro ejemplo, el controlador combinará Datos 1, Datos 3 y Paridad 2 en la primera franja para reemplazar los datos que faltan en los Datos 2. En la segunda franja, los Datos 4, los Datos 6 y la Paridad 1 se utilizan para reemplazar los Datos 5. En la tercera franja, Datos 7, Datos 9 y Paridad 2 se utilizan para reemplazar Datos 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.

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.

¿Cómo funciona un repuesto dinámico en una matriz RAID 6?

Un repuesto dinámico es una o más unidades adicionales que se pueden agregar a una matriz RAID 6 para permitir una recuperación rápida en caso de que falle una unidad. En el ejemplo anterior, vemos una matriz RAID 6 saludable con un solo repuesto activo agregado. Tenga en cuenta que el repuesto dinámico no contiene ningún dato hasta que se produce un error y se necesita la unidad.

Si hay un repuesto dinámico disponible para el sistema, el controlador comenzará automáticamente a reconstruir los datos faltantes de la unidad fallida en el repuesto dinámico en caso de fallo.

El ejemplo anterior muestra que la unidad 2 falló. El sistema usó el repuesto dinámico y reconstruyó todos los datos faltantes de la unidad 2 en el repuesto dinámico. La unidad 2 ahora se puede quitar del sistema y reemplazar con una nueva unidad, ya sea como un nuevo repuesto dinámico o como una nueva unidad 2 y los datos se pueden reconstruir en ella.

¿Por qué utilizar repuestos dinámicos? Cuando falla una unidad, el tiempo es esencial en la reconstrucción. La ejecución en modo degradado ejerce una presión adicional sobre las unidades restantes y puede causar fallos adicionales si no se corrige rápidamente. También es posible que las unidades de un lote de fabricación similar tengan defectos similares, por lo que es posible que otras fallen pronto. Tener uno o más repuestos dinámicos disponibles permite tiempos de recuperación más rápidos.

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.

¿Qué causa los fallos en las matrices RAID 6?

Hay varias razones por las que una matriz RAID 6 puede fallar. Estas son varias de las principales causas que vemos en Ontrack.

¿Es posible la recuperación de datos de RAID 6?

La recuperación de datos es posible desde una matriz RAID 6. Si bien la recuperación de datos puede ser compleja y más desafiante con una matriz RAID 6, generalmente finaliza con éxito. El mayor desafío suele ser el algoritmo patentado que se utiliza para crear el segundo bloque de paridad, ya que cada fabricante lo implementa de manera diferente y, a menudo, se necesita un desarrollo personalizado para investigar y desarrollar herramientas que lo respalden.

Hay varias razones para la pérdida de datos y el esfuerzo de recuperación para cada una de ellas es diferente. A continuación, se muestran algunos ejemplos:

Recuperación de datos con una unidad fallida

Al igual que una matriz RAID 5, si una unidad falla en una matriz, se puede usar la paridad para reconstruir los datos que faltan. En este escenario, Ontrack normalmente puede recuperar el 100 % de los datos. Al recibir una matriz que no funciona, se crean imágenes de todas las unidades de la matriz en la sala limpia (incluida la unidad fallida, si es posible). Luego, la matriz se reconstruye virtualmente utilizando esas imágenes. Una vez que se ensambla el RAID, el sistema de archivos o el volumen se escanea en busca de corrupción, se repara virtualmente y se extraen los datos. La unidad fallida a menudo no se necesita, ya que las bandas de datos que faltan se pueden reconstruir a partir de la paridad.

Recuperación de datos con dos unidades fallidas

A diferencia de la matriz RAID 5, donde se necesitan todas las unidades excepto una para que funcione, RAID 6 está diseñado para permitir el fallo de hasta dos discos sin ningún impacto en los datos. El proceso de recuperación de varias unidades fallidas es similar al fallo de una sola unidad. Al recibir una matriz no funcional, se crean imágenes de las unidades en la sala limpia, incluidas las unidades fallidas. Si los datos de las unidades están actualizados, es posible que no se necesiten las unidades fallidas para obtener una recuperación completa de la matriz. Luego, la matriz se reconstruye virtualmente utilizando esas imágenes.

En el ejemplo anterior, los datos 1, los datos 3 y la paridad 2 de la franja uno se usan para reconstruir los datos 2. Los datos 4, la paridad 1 y los datos 6 se usan para reconstruir los datos 5 en la segunda franja. Los Datos 7, Paridad 2 y Datos 9 se utilizan para reconstruir los Datos 8 en la tercera franja.

Una vez que la matriz RAID se vuelve a ensamblar virtualmente, el sistema de archivos o el volumen se analiza en busca de corrupción. Además de la corrupción del sistema de archivos, los ingenieros también buscan datos que no son consistentes o están desactualizados. Esto ocurre cuando hay un lapso de tiempo entre los fallos de la unidad y una de las unidades se degrada. Los ingenieros de recuperación de datos necesitan experiencia en el reconocimiento de este tipo de daño para que puedan reparar virtualmente el volumen y extraer buenos datos de archivo.

Recuperación de datos de varias unidades fallidas

Es posible obtener una recuperación completa de una matriz RAID 6 incluso si hay más de dos fallos en la unidad.

En el ejemplo anterior, tenemos una matriz RAID 6 con daños en algunas áreas de los cinco discos. Si no hay más de dos bloques fallidos por franja, es posible reconstruir los datos que faltan. Ontrack generará la mayor cantidad posible de imágenes de cada unidad.

Luego, la matriz se reconstruye virtualmente utilizando esas imágenes. En el ejemplo anterior, los Datos 1, Datos 3 y Paridad 2 de la franja uno se usa para reconstruir los Datos 2. No se necesita paridad para la franja 2 ya que los Datos 4, Datos 5 y Datos 6 están todos intactos. Los Datos 7, Paridad 2 y Datos 8 se utilizan para reconstruir los Datos 9 en la tercera franja.

Una vez que la matriz RAID se vuelve a ensamblar virtualmente, el sistema de archivos o el volumen se analiza en busca de corrupción. Los datos recuperables se extraen de la matriz virtualmente reconstruida a nuevos medios para volver a ponerlos en producción.

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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