Megaraid with brakes on?

[BSmith at lyrix.com]

Well, you're asking the system to cache ~30GB of data using just the 
onboard RAID controller's memory and the available "free" system RAM. 
Since the system doesn't have 30GB of memory (I'm guessing), then the job 
of getting data falls back to the disks themselves.  The system is 
configured with one large RAID 5 array, so any IO request makes the heads 
on all 3 disks move to get the data.  For.  Each.  File.   Having ~10 
million files @ 3K each is just about the worst possible case for a RAID 5 
configuration.   Better to split the IO across many disks (either multiple 
RAID 1 or no RAID at all) so you have multiple drive heads independently 
accessing different files - but of course this requires that the http 
server and configuration know about each drive's filesystem since they 
would be separate instead of one giant filesystem like it is now. 
Upgrading to 15K rpm drives would also help a little.

- Brian





"Martin Sarsale" <martin at malditainternet.com>
Sent by: linux-poweredge-bounces at dell.com
01/03/2007 03:41 PM
 
        To:     linux-poweredge at dell.com
        cc: 
        Subject:        Megaraid with brakes on?


Dear all:

We have a Dell  poweredge 2850 server, running as webserver and we're
not satisfied with the speed of our RAID hardware. We're not sure if
we're expecting too much from it or if there's something which is not
working good.

We're using RedHat 4 ES, with the ext3 FS. Our RAID controller is Dell
PowerEdge Expandable RAID controller 4 (rev 06) (according to lspci)
and we have setup a RAID level 5 with 3 SCSI disks, Seagate Barracuda
10k 73gb

We're using the machine for serving a few million 3k files, sizing up
~30gb via HTTP, using the lighttpd webserver.

The average io-wait for a whole day is ~18.2%, and we've some iostat
output that is pretty scaring (util is 98.80%).

I'm attaching the output of lspci -vvvv, the output of 5 repetitions
of iostat and the output from dmesg.

I could run some bonnie / iozone tests, if you need that.

Thanks in advance!

[snipped log output]
-- 

Martin Sarsale - martin at malditainternet.com

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