[mythtv-users] Software RAID and ionice

[Matthew Daubenspeck]

On Tue, May 22, 2007 at 04:50:48PM -0700, David Rees wrote:
> Which indicates a bottleneck somewhere. ext3 is notorious for blocking
> writes when the journal is flushing which kills performance. Did you
> create the filesystem with the stride option? How big are the stripes
> in your raid array? How many disks and how fast are they?
> 
> As I said before, for some workloads which involve small writes, RAID5
> can be significantly slower than a single disk. When ext3 flushes the
> journal to disk and it is writing out inode information, a lot of
> small writes scattered all over the disk will end up blocking all
> writes to the array for some period of time. ionice or not, your
> application is going to get stuck if it is waiting for data to be
> written to disk. jfs also suffers from this from some degree. xfs is
> much better about laying out data on the disk and doesn't suffer from
> this.
> 
> This is one reason I wish mythtv didn't sync all writes to disk and
> instead let the OS handle it on it's own since then the OS could queue
> up the write and and write out the data when it can.

What kind of machine hardware are we talking about here? I have a fairly
modest system (1.8Ghz, AMD64 processor) running a software raid5 system
with no issues whatsoever. I am using a standard SATA 3.0 4 port card
with only 3 drives, and haven't noticed any issues even when recording 4
programs and watching another simultaneously. It uses ext3 as well.
Could your bottleneck be somewhere else?
-- 
  Matthew Daubenspeck
  http://oddprocess.org

Gentoo Linux x86_64 Dual Core AMD Opteron(tm) Processor 165
20:36:28 up 1 day, 7:23, 1 user, load average: 0.03, 0.01, 0.00