linux-kernel - Re: RAID + LUKS + LVM performance

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Date:	Thu, 11 Mar 2010 20:07:29 +0100
From:	Matthias Schniedermeyer <ms@...d.de>
To:	david@...g.hm
Cc:	Mathias Buren <mathias.buren@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: RAID + LUKS + LVM performance

On 11.03.2010 09:51, david@...g.hm wrote:
> On Thu, 11 Mar 2010, Matthias Schniedermeyer wrote:
>
>> On 11.03.2010 13:08, Mathias Buren wrote:
>>>
>>> Hi,
>>>
>>> (please cc me as I'm not subscribed)
>>>
>>> I've a friend who's going to set up a fileserver consisting of 8x 1.5TB
>>> HDDs, an 8-port PCI-E RAID card (Areca ARC-1220 @
>>> http://www.areca.com.tw/products/pcie.htm ) etc.
>>> The plan is create a RAID5 array spanning all the disks, then create 4
>>> partitions. These 4 partitions would be encrypted using LUKS (Twofish or
>>> AES256).
>>> These 4 encrypted partition would be set up in RAID0 using Linux' software
>>> (mdadm), then LVM would be used on top of that (one big PV, one big VG and
>>> a big LV or so).
>>>
>>> The reason for this is that kcryptd is not multithreaded (afaik). By having
>>> 4 encrypted partitions, then md0 on top of them, I'm forcing 4 kcryptd
>>> processes to run on all four cpu cores whenever something is written to the
>>> disks, which should improve (encryption) performance.
>>>
>>> Is this a good way of doing it, or is there a smarter way?
>>
>> The setup you describe would only work with SSDs. HDDs would seek
>> themselves to death.
>>
>> The problem is the RAID-0 over the 4 partitions. At that point you would
>> need, instead of the 4 partitions, something that is round-robin. So
>> that the mapping of the (physical) blocks from the upper to the lower
>> would be effectivly linear/unchanged.
>>
>> AFAIK something like that is (currently) not possible.
>
> linux software raid (the md tools) support linear or striped modes for  
> raid0, so what you are looking for is available.

Nope. What i meant is:

Let say you had a block-device which has 16 blocks:
0-15
With the OPs description the blocks would be distributed like this:
Part 0: 00 01 02 03
Part 1: 04 05 06 07
Part 2: 08 09 10 11
Part 3: 12 13 14 15

What you need is a distribution like this:
Device 0: 01 05 09 13 
Device 1: 02 06 10 14
Device 2: 03 07 11 15
Device 3: 04 08 12 16

IOW:
Blocks % 4 == 0 on device 0
Blocks % 4 == 1 on device 1
Blocks % 4 == 2 on device 2
Blocks % 4 == 3 on device 3

I still other words:
You don't want a cake in exactly 4 same size parts. You want a cake in a 
million parts and then every 4th starting from the first piece in one 
set, every 4th starting from the second in the next and so on.

> however I think that defeats part of the OPs purpose, which was to try 
> and spread the I/O across all 4 partitions to be able to use multiple 
> cores for the encryption.

I think i just didn't make clear enough what i meant.




Bis denn

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