Re: RAID 1 using SSD and 2 HDD

[Doug Ledford <dledford@redhat.com>]

On 07/19/2011 02:27 PM, Roberto Spadim wrote:
> no this not work very fast...
> use bcache  or other similar ssd+harddisk cache solution (facebook
> have one solution i don´t remember the name)
> mixing ssd and hdd are a big problem, since write speed is the slowest
> (maybe ssd maybe hdd) and read speed isn´t very well since read in ssd
> can be non sequencial and at hdd sequencial is prefered, the read
> balance algorithm is tuned to use the near position device, and
> sometimes ssd isn´t used, or less used or more used...
> a bigger speed can be done using a cache solution (ssd+hdd or ssd+raid1(hdd))
> that was my test results... maybe someone have others results

You needed to use write-mostly, that would have drastically altered your 
results.  With write-mostly and write-behind enabled on the hard drives, 
all reads go to the SSD instead of the hard drives, and writes complete 
as soon as the SSD says it is done and the writes to the hard drives 
happen in the background.

>
> 2011/7/19 Mike Power<mpower@dodtsair.com>:
>> Is it possible to implement a RAID 1 array using two equal size HDD and one
>> smaller and faster SSD.  The idea being that the resulting RAID would have
>> the same size of the HDD while picking up the speed benefits of the SSD.
>>   You can do something similar today by just buying two hybrid drives and
>> putting them in an array.  By purchasing a dedicated SSD drive for that
>> purpose you gain the ability of controlling the size of the SSD portion.  I
>> was hoping the RAID array could use the SSD more as a cache then a redundant
>> storage.
>>
>> Mike Power
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>
>

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