Raid1 replaced with raid10?

Raid1 replaced with raid10?

[Peter Rabbitson]

Hi,

I just tried an idea I got after fiddling with raid10 and to my dismay 
it worked as I thought it will. I used two small partitions on separate 
disks to create a raid1 array. Then I did dd if=/dev/md2 of=/dev/null. I 
got only one of the disks reading. Nothing unexpected. Then I created a 
raid10 array on the same two partitions with the options -l10 -n2 -pf2. 
The same dd executed at twice the speed, reading _simultaneously_ from 
both drives. I did some bonnie++ benchmarking - same result - raid1 
reads only from a single disk raid10 from both. Write performance is 
worse (about 10% slower) with raid10, but you get twice the read speed.
In this light the obvious question is: can raid10 be used as a drop-in 
replacement for raid1 or there is a caveat with having the amount of 
disks equal the amount of chunk copies?

Peter

Raid1 replaced with raid10?

[Peter Rabbitson]

Hi,

I asked this question back in march but received no answers, so here it
goes again. Is it safe to replace raid1 with raid10 where the amount of
disks is equal to the amount of far/near/offset copies? I understand it
has the downside of not being a bit-by-bit mirror of a plain filesystem.
Are there any other caveats?

Thanks

Peter

Re: Raid1 replaced with raid10?

[Bill Davidsen]

Peter Rabbitson wrote:
> Hi,
>
> I asked this question back in march but received no answers, so here it
> goes again. Is it safe to replace raid1 with raid10 where the amount of
> disks is equal to the amount of far/near/offset copies? I understand it
> has the downside of not being a bit-by-bit mirror of a plain filesystem.
> Are there any other caveats?
>   
Clearly you have reduced capacity, since there's a mirror AND a CRC, 
otherwise I don't see any drawbacks. The performance should be much better.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

Re: Raid1 replaced with raid10?

[Neil Brown]

On Friday May 4, davidsen@tmr.com wrote:
> Peter Rabbitson wrote:
> > Hi,
> >
> > I asked this question back in march but received no answers, so here it
> > goes again. Is it safe to replace raid1 with raid10 where the amount of
> > disks is equal to the amount of far/near/offset copies? I understand it
> > has the downside of not being a bit-by-bit mirror of a plain filesystem.
> > Are there any other caveats?
> >   
> Clearly you have reduced capacity, since there's a mirror AND a CRC, 
> otherwise I don't see any drawbacks. The performance should be much better.

CRC ??? md/raid10 doesn't have any CRC.
What CRC are you thinking of?

To answer the original question, I assume you mean "replace" as in
"backup, create new array, then restore".
You will get different performance characteristics.  Whether they
better suit your needs or not will depend largely on your needs.

NeilBrown

Re: Raid1 replaced with raid10?

[Peter Rabbitson]

Neil Brown wrote:
> On Friday May 4, davidsen@tmr.com wrote:
>> Peter Rabbitson wrote:
>>> Hi,
>>>
>>> I asked this question back in march but received no answers, so here it
>>> goes again. Is it safe to replace raid1 with raid10 where the amount of
>>> disks is equal to the amount of far/near/offset copies? I understand it
>>> has the downside of not being a bit-by-bit mirror of a plain filesystem.
>>> Are there any other caveats?
>>>   
> 
> To answer the original question, I assume you mean "replace" as in
> "backup, create new array, then restore".
> You will get different performance characteristics.  Whether they
> better suit your needs or not will depend largely on your needs.

Hi Neil,
Yes I meant take an existing 2 drive raid1 array (non bootable data) and
put a raid10 array in its place. All my testing indicates that I get the
same write performance but nearly double the read speed (due to
interleaving I guess). It seemed to good to be true, thus I am asking
the question. Could you elaborate on your last sentence? Are there
downsides I could not think of? Thank you!

Peter

Re: Raid1 replaced with raid10?

[Neil Brown]

On Monday May 7, rabbit@rabbit.us wrote:
> Neil Brown wrote:
> > On Friday May 4, davidsen@tmr.com wrote:
> >> Peter Rabbitson wrote:
> >>> Hi,
> >>>
> >>> I asked this question back in march but received no answers, so here it
> >>> goes again. Is it safe to replace raid1 with raid10 where the amount of
> >>> disks is equal to the amount of far/near/offset copies? I understand it
> >>> has the downside of not being a bit-by-bit mirror of a plain filesystem.
> >>> Are there any other caveats?
> >>>   
> > 
> > To answer the original question, I assume you mean "replace" as in
> > "backup, create new array, then restore".
> > You will get different performance characteristics.  Whether they
> > better suit your needs or not will depend largely on your needs.
> 
> Hi Neil,
> Yes I meant take an existing 2 drive raid1 array (non bootable data) and
> put a raid10 array in its place. All my testing indicates that I get the
> same write performance but nearly double the read speed (due to
> interleaving I guess). It seemed to good to be true, thus I am asking
> the question. Could you elaborate on your last sentence? Are there
> downsides I could not think of? Thank you!

I would have thought that you need "far" or "offset" to improve read
performance, and they tend to hurt write performance (though I haven't
really measured "offset" much).

What layout are you using?

NeilBrown

Re: Raid1 replaced with raid10?

[Peter Rabbitson]

Neil Brown wrote:
> On Monday May 7, rabbit@rabbit.us wrote:
>> Neil Brown wrote:
>>> On Friday May 4, davidsen@tmr.com wrote:
>>>> Peter Rabbitson wrote:
>>>>> Hi,
>>>>>
>>>>> I asked this question back in march but received no answers, so here it
>>>>> goes again. Is it safe to replace raid1 with raid10 where the amount of
>>>>> disks is equal to the amount of far/near/offset copies? I understand it
>>>>> has the downside of not being a bit-by-bit mirror of a plain filesystem.
>>>>> Are there any other caveats?
>>>>>   
>>> To answer the original question, I assume you mean "replace" as in
>>> "backup, create new array, then restore".
>>> You will get different performance characteristics.  Whether they
>>> better suit your needs or not will depend largely on your needs.
>> Hi Neil,
>> Yes I meant take an existing 2 drive raid1 array (non bootable data) and
>> put a raid10 array in its place. All my testing indicates that I get the
>> same write performance but nearly double the read speed (due to
>> interleaving I guess). It seemed to good to be true, thus I am asking
>> the question. Could you elaborate on your last sentence? Are there
>> downsides I could not think of? Thank you!
> 
> I would have thought that you need "far" or "offset" to improve read
> performance, and they tend to hurt write performance (though I haven't
> really measured "offset" much).
> 
> What layout are you using?
> 

Correct, I am using 'far' layout. The interleaving of the 'offset'
layout does not work too good for sequential reads, but far really
shines. Yes write performance is hurt by about 10%. Compared to 190%
gain in reads I can live with it.

Re: Raid1 replaced with raid10?

[Bill Davidsen]

Neil Brown wrote:
> On Friday May 4, davidsen@tmr.com wrote:
>   
>> Peter Rabbitson wrote:
>>     
>>> Hi,
>>>
>>> I asked this question back in march but received no answers, so here it
>>> goes again. Is it safe to replace raid1 with raid10 where the amount of
>>> disks is equal to the amount of far/near/offset copies? I understand it
>>> has the downside of not being a bit-by-bit mirror of a plain filesystem.
>>> Are there any other caveats?
>>>   
>>>       
>> Clearly you have reduced capacity, since there's a mirror AND a CRC, 
>> otherwise I don't see any drawbacks. The performance should be much better.
>>     
>
> CRC ??? md/raid10 doesn't have any CRC.
> What CRC are you thinking of?
>   
There is an entire paragraph missing here, no doubt a finger check due 
to trying to post and get out to lunch. I was rambling on about raid6, 
mirroring 1+5, and other things which never made it. The post can be 
scrubbed, I was just rambling on at length about various other solutions 
possible.

Not worth a repost, since I was way over answering his question...
> To answer the original question, I assume you mean "replace" as in
> "backup, create new array, then restore".
> You will get different performance characteristics.  Whether they
> better suit your needs or not will depend largely on your needs.
>
> NeilBrown
>
>   


-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

Re: Raid1 replaced with raid10?

[Peter Rabbitson]

Bill Davidsen wrote:
> Not worth a repost, since I was way over answering his question...

Erm... and now you made me curios :) Please share your thoughts if it is
not too much trouble. Thank you for your time.

Peter