[vdr] mdadm software raid5 arrays?

Simon Baxter linuxtv at nzbaxters.com
Fri Nov 13 00:06:21 CET 2009


Thanks Alex.  I think I've decided to go RAID 1+0 rather than RAID 5 as
I'm worried about the write speed.

I often record 3 or 4 channels at once and do see some "slow down" on OSD
responsiveness during this.

What's your experience with RAID5?


----- Original Message -----
From: Alex Betis
To: VDR Mailing List
Sent: Friday, November 13, 2009 1:03 AM
Subject: Re: [vdr] mdadm software raid5 arrays?


Simon,

Pay attention that /boot can be installed only on a single disk or RAID-1
where every disk can actually work as a stand alone disk.

I personally decided to use RAID-5 on 3 disks with RAID-1 on 3xsmall
partitions for /boot and RAID-5 on the rest.
RAID-5 also allows easier expansion in the future.



On Tue, Nov 10, 2009 at 8:48 PM, Simon Baxter <linuxtv at nzbaxters.com> wrote:

Thanks - very useful!

So what I'll probably do is as follows...
* My system has 4x SATA ports on the motherboard, to which I'll connect my
4x 1.5TB drives.
* Currently 1 drive is in use with ~30G for / /boot and swap and ~1.4TB
for /media
* I'll create /dev/md2, using mdadm, in RAID1 across 2 ~1.4TB partitions
on 2 drives
* move all active recordings (~400G) to /dev/md2
* split /dev/md2 and create a raid 1+0 (/dev/md1) using 4x partitions of
~1.4TB across 4 drives

At this point I have preserved all my data, and created a raid1+0 for
recordings and media.

I should now use the remaining ~100G on each drive for raid protection for
(root) / and /boot.  I've read lots on the web on this, but what's your
recommendation?  RAID1 mirror across 2 of the disks for / (/dev/md0) and
install grub (/boot) on both so either will boot?




On Tue, Nov 10, 2009 at 09:46:52PM +1300, Simon Baxter wrote:

What about a simple raid 1 mirror set?



Ok.. short comparison, using a single disk as baseline.

using 2 disks
raid0: (striping)
++   double read throughput,
++   double write throughput,
--   half the reliability (read: only use with good backup!)

raid1: (mirroring)
++   double read throughput.
o    same write throughput
++   double the reliability


using 3 disks:

raid0: striping
+++  tripple read performance
+++  tripple write performance
---  third of reliability

raid1: mirroring
+++  tripple read performance
o    same write throughput
+++  tripple reliability

raid5: (distributed parity)
+++  tripple read performance
-    lower write performance (not due to the second write but due
    to the necessary reads)
+    sustains failure of any one drive in the set

using 4 disks:

raid1+0:
++++ four times the read performance
++   double write performance
++   double reliability


please note: these are approximations and depending on your hardware
they may be off by quite a bit.

cheers
-henrik






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