[vdr] mdadm software raid5 arrays?

Magnus Hörlin magnus at alefors.se
Fri Nov 13 08:50:49 CET 2009


I too have only good experiences with md raid5 and have used it for years on
my vdr server. It’s not uncommon that I record 7-8 programs simultaneously,
including HD channels, and I have never had any problems with that. On the
other hand, this is just a VDR/NFS server serving two diskless VDR/XBMC
frontends so maybe it would affect OSD performance if I had it on the same
machine.

/Magnus H

 

 

  _____  

Från: vdr-bounces at linuxtv.org [mailto:vdr-bounces at linuxtv.org] För Alex
Betis
Skickat: den 13 november 2009 08:00
Till: VDR Mailing List
Ämne: Re: [vdr] mdadm software raid5 arrays?

 

In general good experience.
I don't record much, so I don't worry about speed.
There are many web pages about raid5 speed optimizations. 
The slowdown in raid5 writes mostly happen when a part of a strip (chunk of
data) has to be written,
so the driver has to read the strip, and write it back. The optimizations
talk about alignment of file system block size with raid strip size.

Since we're talking about movie recordings (huge files), then big file
system blocks will not create much waste.
Smaller strip size will probably reduce the read performance a bit, but will
increase write speed since there will be less cases where not the whole
strip has to be updated.

In one sentence, you won't know if it's slow until you'll try :)
RAID 10 will obviously give better write speed, but I'm not yet convinced
that raid 5 can't handle 4 recordings at the same time.

If we're talking about HD recording, it's about 3Gigs/hour, meaning less
than MByte per second.
Don't think there should be a problem to write 3-4 MByte/sec without any
raid.

By the way, I had a very bad experience with LVM on top of raid in latest
distros, so if you want to save some hairs on your head, don't try it :)



On Fri, Nov 13, 2009 at 1:06 AM, Simon Baxter <linuxtv at nzbaxters.com> wrote:

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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