Hot Spare in Softraid?

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Hot Spare in Softraid?

Federico Giannici
Is it possible to set a "Hot Spare" chunk for a RAID1 Softraid?
 From the "bioctl" man page seems that this functionality is available
for "RAID controllers" only.
Is it correct?

Thanks.

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Re: Hot Spare in Softraid?

noah pugsley-3
On Sat, Aug 12, 2017 at 10:55 AM, Federico Giannici
<[hidden email]> wrote:
> Is it possible to set a "Hot Spare" chunk for a RAID1 Softraid?
> From the "bioctl" man page seems that this functionality is available for
> "RAID controllers" only.
> Is it correct?
>
> Thanks.
>

I don't know about that, but from softraid(4) I know that:

"RAID 1
A mirroring discipline. It copies data across more than one chunk to
provide for data loss. Read performance is increased, though at the
cost of write speed. Unlike traditional RAID 1, softraid supports the
use of more than two chunks in a RAID 1 setup."

So, why not a 3 disk mirror?

From bioctl(8):

"Configure softraid0 with 4 special devices (/dev/sd2e, /dev/sd3e,
/dev/sd4e, /dev/sd5e) and a RAID level of 1:

# bioctl -c 1 -l /dev/sd2e,/dev/sd3e,/dev/sd4e,/dev/sd5e softraid0"

-N

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Re: Hot Spare in Softraid?

Federico Giannici
On 08/12/17 20:48, noah pugsley wrote:

> On Sat, Aug 12, 2017 at 10:55 AM, Federico Giannici
> <[hidden email]> wrote:
>> Is it possible to set a "Hot Spare" chunk for a RAID1 Softraid?
>> From the "bioctl" man page seems that this functionality is available for
>> "RAID controllers" only.
>> Is it correct?
>>
>> Thanks.
>>
>
> I don't know about that, but from softraid(4) I know that:
>
> "RAID 1
> A mirroring discipline. It copies data across more than one chunk to
> provide for data loss. Read performance is increased, though at the
> cost of write speed. Unlike traditional RAID 1, softraid supports the
> use of more than two chunks in a RAID 1 setup."
>
> So, why not a 3 disk mirror?

Good point, but now I have two more questions:

1) What about the "cost of write speed"? Will writing times increase
further with another disk? Is it negligible?

2) What happens when one of the three disk goes bad? Is it signaled in
any way? The softraid goes "degraded" or remains "Online" (I suppose the
latter)?

Thanks



> From bioctl(8):
>
> "Configure softraid0 with 4 special devices (/dev/sd2e, /dev/sd3e,
> /dev/sd4e, /dev/sd5e) and a RAID level of 1:
>
> # bioctl -c 1 -l /dev/sd2e,/dev/sd3e,/dev/sd4e,/dev/sd5e softraid0"
>
> -N
>

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Re: Hot Spare in Softraid?

Nick Holland
On 08/12/17 15:02, Federico Giannici wrote:
> On 08/12/17 20:48, noah pugsley wrote:
>> On Sat, Aug 12, 2017 at 10:55 AM, Federico Giannici
>> <[hidden email]> wrote:
>>> Is it possible to set a "Hot Spare" chunk for a RAID1 Softraid?
>>> From the "bioctl" man page seems that this functionality is available for
>>> "RAID controllers" only.
>>> Is it correct?
>>>
>>> Thanks.

you can just leave a drive on-line and waiting to be added to an array.
When you have a failure, you COULD have it happen on its own with
appropriate monitoring and scripting, or you could wait until an optimal
time to rebuild.  Rebuilding is a performance impacting task, you might
want some say about when it happens.

>> I don't know about that, but from softraid(4) I know that:
>>
>> "RAID 1
>> A mirroring discipline. It copies data across more than one chunk to
>> provide for data loss. Read performance is increased, though at the
>> cost of write speed. Unlike traditional RAID 1, softraid supports the
>> use of more than two chunks in a RAID 1 setup."
>>
>> So, why not a 3 disk mirror?
>
> Good point, but now I have two more questions:
>
> 1) What about the "cost of write speed"? Will writing times increase
> further with another disk? Is it negligible?

depends on your needs (and hw).  Test, don't speculate.  Yes, writes
will be slower.  Reads MAY be faster.  But the performance loss may be
trivial compared to the difficulty when you have a multiple disk failure.

I've often wished hw raid controllers accepted three disk RAID1 configs.

> 2) What happens when one of the three disk goes bad? Is it signaled in
> any way? The softraid goes "degraded" or remains "Online" (I suppose the
> latter)?

Don't ask these questions of others, their answers may or may not apply
to you, and YOU need to find them out yourself the easy way...before you
find out the hard way.

Practice RAID response and recovery.

Nick.

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Re: Hot Spare in Softraid?

kasak
In reply to this post by Federico Giannici


12.08.2017 22:02, Federico Giannici пишет:

> On 08/12/17 20:48, noah pugsley wrote:
>> On Sat, Aug 12, 2017 at 10:55 AM, Federico Giannici
>> <[hidden email]> wrote:
>>> Is it possible to set a "Hot Spare" chunk for a RAID1 Softraid?
>>> From the "bioctl" man page seems that this functionality is
>>> available for
>>> "RAID controllers" only.
>>> Is it correct?
>>>
>>> Thanks.
>>>
>>
>> I don't know about that, but from softraid(4) I know that:
>>
>> "RAID 1
>> A mirroring discipline. It copies data across more than one chunk to
>> provide for data loss. Read performance is increased, though at the
>> cost of write speed. Unlike traditional RAID 1, softraid supports the
>> use of more than two chunks in a RAID 1 setup."
>>
>> So, why not a 3 disk mirror?
>
> Good point, but now I have two more questions:
>
> 1) What about the "cost of write speed"? Will writing times increase
> further with another disk? Is it negligible?
>
> 2) What happens when one of the three disk goes bad? Is it signaled in
> any way? The softraid goes "degraded" or remains "Online" (I suppose
> the latter)?
>
> Thanks
>
>
>
>> From bioctl(8):
>>
>> "Configure softraid0 with 4 special devices (/dev/sd2e, /dev/sd3e,
>> /dev/sd4e, /dev/sd5e) and a RAID level of 1:
>>
>> # bioctl -c 1 -l /dev/sd2e,/dev/sd3e,/dev/sd4e,/dev/sd5e softraid0"
>>
>> -N
>>
>
You can use sensorsd to monitor raid status. Like this:

/etc/sensorsd.conf:
hw.sensors.softraid0.drive0:command=echo "Raid state: %t %2" | mail -s
"Sensor %t changed" -r [hidden email] [hidden email]

sysctl hw.sensors output:
hw.sensors.softraid0.drive0=online (sd4), OK

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