Re: mdadm raid1 read performance

[David Brown <david.brown@hesbynett.no>]

On 06/05/11 23:05, Leslie Rhorer wrote:
>
>
>> -----Original Message-----
>> From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
>> owner@vger.kernel.org] On Behalf Of David Brown
>> Sent: Thursday, May 05, 2011 2:27 AM
>> To: linux-raid@vger.kernel.org
>> Subject: Re: mdadm raid1 read performance
>>
>> On 05/05/2011 02:40, Liam Kurmos wrote:
>>> Cheers Roberto,
>>>
>>> I've got the gist of the far layout from looking at wikipedia. There
>>> is some clever stuff going on that i had never considered.
>>> i'm going for f2 for my system drive.
>>>
>>> Liam
>>>
>>
>> For general use, raid10,f2 is often the best choice.  The only
>> disadvantage is if you have applications that make a lot of synchronised
>> writes, as writes take longer (everything must be written twice, and
>> because the data is spread out there is more head movement).  For most
>> writes this doesn't matter - the OS caches the writes, and the app
>> continues on its way, so the writes are done when the disks are not
>> otherwise used.  But if you have synchronous writes, so that the app
>> will wait for the write to complete, it will be slower (compared to
>> raid10,n2 or raid10,o2).
>>
>> The other problem with raid10 layout is booting - bootloaders don't much
>> like it.  The very latest version of grub, IIRC, can boot from raid10 -
>> but it can be awkward.  There are lots of how-tos around the web for
>> booting when you have raid, but by far the easiest is to divide your
>> disks into partitions:
>>
>> sdX1 = 1GB
>> sdX2 = xGB
>> sdX3 = yGB
>>
>> Put all your sdX1 partitions together as raid1 with metadata layout
>> 0.90, format as ext3 and use it as /boot.  Any bootloader will work fine
>> with that (don't forget to install grub on each disk's MBR).
>>
>> Put your sdX2 partitions together as raid10,f2 for swap.
>>
>> Put the sdX3 partitions together as raid10,f2 for everything else.  The
>> most flexible choice is to use LVM here and make logical partitions for
>> /, /home, /usr, etc.  But you can also partition up the md device in
>> distinct fixed partitions for /, /home, etc. if you want.
>
> 	I agree, except that I like to have separate physical devices for
> booting and raw disks for the data.  My servers each have a pair of 500G
> hard drives partitioned into three sections.  First, /dev/sdX1 is a small
> partition which contains only /boot, it is read-only, and can be mounted at
> boot time, or not.  As you say, it has a 0.90 superblock, although I chose
> an ext2 file system.  Next, /dev/sdX2 uses about half the disk and is
> mounted at /.  Finally, I use the rest of the disk, /dev/sdX3, as swap
> space.  I chose all three to be RAID1.
>
> 	The data drives are all>= 1Tb, unpartitioned, and assembled into
> RAID6 arrays of 10 or more members, each.
>

When you need enough data space to have separate disks like this, then 
it is a good plan to separate the OS from the data disks.  These days 
I'd put the OS on a raid1 pair of SSD disks - even small and cheap 40GB 
drives are fine for the OS.  The speed of such drives is such that it 
won't make any difference if you use raid1 or raid10,far, especially 
since almost all files are small (raid0 striping only helps for big files).

> 	These systems use so little swap space and so rarely, I'm not sure I
> see any benefit to RAID10,f2 for them.  Is there?
>

Obviously when you use swap rarely, it makes little difference how it is 
laid out on the disk.  And since it is small, there is no difference 
between the speed of the outer and inner tracks (for HD's - for SSD's 
there is obviously no difference), so you don't gain there.  raid10,f2 
will still be better than raid1 for larger reads from swap - but I think 
you would have a hard time trying to spot that effect in the real world.