I too have only good experiences with md raid5 and have used it for years on my vdr server. Its 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
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Från: vdr-bounces@linuxtv.org [mailto:vdr-bounces@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@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@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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