Re: nfs performance problem

[Benjamin LaHaise <bcrl@redhat.com>]

You're wrong.  Reads (not sequential, but scattered) on a 1+0 setup will be 
faster as the raid1 driver optimizes requests somewhat, plus 2 (or more for 
larger raid1s of disks) drives will be able to service requests for the same 
stripe offset on different disks.  There is no way a raid5 can service two 
requests for the same stripe offset at different offsets in the array.  And 
even for a read-heavy workload, there are still writes to update metadata 
(and journal unless you've got a separate journal device), and the impact 
of those is that *all* reads to the array have to suffer from seeks when 
even the smallest write is active.

On a side note, make sure you have the filesystem mounted with the noatime 
flag if you can afford losing atimes.

		-ben

On Tue, Nov 05, 2002 at 01:46:04PM -0700, Matt Heaton wrote:
> Cachefs will help quite a lot in my opinion because it doesn't just store
> the files in RAM,
> it uses the hard drive.  So if you have an NFS client with an extra 5 gig
> that you can
> designate as cache then reads to the NFS server will go down DRAMATICALLY as
> it will hit local cache on the NFS clients drive.
> 
> I agree raid 1+0 should be much faster for writes and a little for read, but
> RAID 5 still
> reads from all drives simultaneously (Has to read parity in too I know), but
> can read
> all 7 drives at once instead of only 4 drives at once in a raid 1+0
> configuration with 8 drives
> in the array.  I have never used 1+0 so I am only talking about physical
> drive layout rather
> than any personal experience.  Are my assumptions correct that raid 5 does
> in fact read
> from all drives at the same time?  If so, reading might be a LITTLE faster
> on raid 1+0 than
> raid 5, but it shouldn't be HUGE.  When I contacted 3ware, they basically
> said the same thing.
> I do agree that writes are MUCH faster on 1+0 than raid 5.
> 
> Any thoughts?
> 
> L8r...
> 
> Matt
> 
> 
> > On Tue, Nov 05, 2002 at 01:22:25PM -0700, Matt Heaton wrote:
> > > each NFS server.  So even though our throughput of only 1.5 MB isn't
> high.
> > > The number of files per second is
> > > actually quite high, and causes things to slow down because of seek time
> > > issues.  PLEASE GIVE US CACHEFS SOMEONE??
> >
> > How is cachefs going to help?  The kernel is already trying to cache data
> > as much as possible.  Once you're trying to serve more data than you have
> > RAM, this are naturally going to degreate quite significantly as the
> system
> > becomes seek bound.
> >
> > > Does anyone have experience with IDE Raid arrays that get over 250 tps
> in
> > > iostat that work fine?  I would
> > > be VERY VERY VERY interested to find out.
> >
> > Use raid1+0 and you'll be much happier, as read requests will be balanced
> > over multiple drives (mirroring means the same data can be read from all
> > of the mirrors).  Additionally, you'll have much lower CPU utilization
> > and writes won't cause all disks in the array to seek for strip updates.
> > Read the archives for the past couple of weeks for another example of the
> > performance increase when going from raid5 to raid1+0.
> >
> > -ben
> >
> >
> 

-- 
"Do you seek knowledge in time travel?"


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