[mythtv-users] Software RAID and ionice

Richard Freeman r-mythtv at thefreemanclan.net
Tue May 22 23:28:22 UTC 2007


David Rees wrote:
> 
> No, it doesn't ignore ionice settings, but some raid setups can
> actually perform worse than a single disk, especially raid5 setups.
> 
> I'd bet that you are using ext3 for your filesystem. xfs tends to
> perform better for the types of workloads that mythtv presents (except
> for large file deletes which jfs performs very well).
> 

But - the RAID5 only needs to perform well enough to handle mythtv 
recording when it is otherwise idle, correct?  After all, if it is using 
ionice all other processes should block on disk access when mythbackend 
is accessing the disk (since it is in the realtime class, and nothing 
else is).

If ionice is working correctly it shouldn't be possible for an 
idle-class process to cause any significant delay at all to a 
realtime-class process.  That is how things worked on a regular block 
device - I could build two large applications at the same time on gentoo 
and not have IOBOUND issues on mythtv.  Ditto for large file copy 
operations.  But with my RAID I get tons of IOBOUND errors if I copy a 
large file onto the RAID at the same time.

The raid5 works just fine for mythbackend when the system isn't busy 
writing to the same disk.

As far as xfs goes - I agree that it tends to perform better, but the 
only thing that concerns me about xfs is that it wipes out any file that 
is open if it isn't cleanly unmounted.  I used to use xfs and migrated 
to ext3 for precisely that reason.


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