Re: mdadm raid1 read performance

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On 06/05/11 23:05, Leslie Rhorer wrote:


-----Original Message-----
From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-
owner@xxxxxxxxxxxxxxx] On Behalf Of David Brown
Sent: Thursday, May 05, 2011 2:27 AM
To: linux-raid@xxxxxxxxxxxxxxx
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.

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