[mythtv-users] Software RAID and ionice

[David Rees]

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