[vdr] mdadm software raid5 arrays?

Alex Betis alex.betis at gmail.com
Fri Nov 13 07:59:53 CET 2009


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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>
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