Re: RAID 1 using SSD and 2 HDD

[John Robinson <john.robinson@anonymous.org.uk>]

On 29/07/2011 00:53, Xavier Brochard wrote:
> Le jeudi 28 juillet 2011 20:31:10 Doug Ledford, vous avez écrit :
>> On 07/20/2011 08:59 AM, brian.foster@emc.com wrote:
[...]
>>>> On 07/19/2011 11:32 AM, Roman Mamedov wrote:
[...]
>>>>> See http://bcache.evilpiepirate.org/
[...]
>>> https://github.com/facebook/flashcache
>>
>> Having not looked at those two, I can say that an md raid1 with two hard
>> drives and one SSD works *very* well.  It's blazing fast.  Here's how I
>> set mine up:
>>
>> SSD: three partitions, one for boot, one for /, and one for ~/repos
>> (which is where all my git/cvs/etc. checkouts reside)
>> hard disks: four partitions, one for boot, one for /, one for /home, one
>> for ~/repos
>>
>> Then I created four raid1 arrays like so:
>>
>> mdadm -C /dev/md/boot -l1 -n3 -e1.0 --bitmap=internal --name=boot
>> /dev/sda1 --write-mostly --write-behind=128 /dev/sdb1 /dev/sdc1
>> mdadm -C /dev/md/root -l1 -n3 -e1.2 --bitmap=internal --name=root
>> /dev/sda2 --write-mostly --write-behind=1024 /dev/sdb2 /dev/sdc2
>> mdadm -C /dev/md/home -l1 -n2 -e1.2 --bitmap=internal --name=home
>> /dev/sdb3 /dev/sdc3
>> mdadm -C /dev/md/repos -l1 -n3 -e1.2 --bitmap=internal --name=repos
>> /dev/sda4 --write-mostly --write-behind=1024 /dev/sdb4 /dev/sdc4
>>
>> Works for me with stellar performance.  Treats the SSD as the only
>> device that matters on the three arrays it participates in with the hard
>> drives there merely as a backing store for safety in case the SSD blows
>> chunks some day.  Obviously, if you need some other aspect of your home
>> directory to have the SSD benefit then modify to your tastes, but all my
>> scratch builds happen under ~/repos and the thing flies when compiling
>> stuff compared to how it used to be.
>
> One thing you didn't said is the respective size of the SSD and HD partitions.
> How did you determine them?

Since he's running RAID-1, the partitions on the SSD and HDDs must be 
the same size. Note that the rest of the space on the HDDs was given to 
/home and was not mirrored on the SSD.

Cheers,

John.

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