[mythtv-users] Software RAID and ionice

[davatar]

Bah, this is stupid. RAID5 performance is fine as long as you aren't doing 
stupidly small writes. If the flush from the Mythtv is the size of the 
stripe, then basically the performance hit is NIL, pretty much like writing 
to a RAID0.

Someone needs to fix the Mythtv file writer, period. Don't blame your disks 
for asking them to do something dumb. Theoretically a single disk is plenty 
for almost any Mythtv system, but again stupid flushing algorithms will 
bring any disk subsystem to it's knees.


----- Original Message ----- 
From: "David Rees" <drees76 at gmail.com>
To: "Discussion about mythtv" <mythtv-users at mythtv.org>
Sent: Thursday, May 24, 2007 2:58 PM
Subject: Re: [mythtv-users] Software RAID and ionice


On 5/24/07, Richard Freeman <r-mythtv at thefreemanclan.net> wrote:
> Ok, here is the output of bonnie++.  This was run as a normal user at
> without any (io)nicing of any kind:

It's really hard to read the data since it wrapped, but let me see if
I picked this out right:


> Here are the stats for a regular ext3 partition directly on a 7200RPM
> ATA100 drive (no lvm2/raid/etc):

Random create: 23846/sec

> And here is the data for the video partition running on top of lvm2
> running on top of raid5 on 3x 150MB/s SATA 7200RPM drives:

Random create: 1408/sec

That is what I mean by RAID5 having horrible small write performance.
While all other performance numbers are similar (except for RAID5
seq-create which is much faster, not sure why it is 10x faster than
the single disk, and RAID5 random reads which is ~3x faster as
expected), small write performance such really, really sucks.

> The stats are fairly comparable - block writes are a hair slower, block
> reads are a lot faster - about what you'd expect.  In any case, the
> speed should be WAY more than needed to stream video at 1-2Mbps.

Yes, but only if you ignore any small random writes which will hold up
all access to the array. It is very common for IO bandwith to drop to
1MB/s or less under high random loads.

-Dave
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