Re: Disck write latency > 50ms
- From: "John Fullbright" <fullbrij@xxxxxxxxxxx>
- Date: Wed, 22 Feb 2006 04:17:36 -0800
Dump the RAID 5 and go RAID 10. The problem is write performance on Drive
F:
Let's assume a 4 disk RAID 5 vs. a 4 disk RAID 10 array. Further, let's
assume 10K RPM SCSI drives and a 2:1 read/write ratio.
RAID 5:
Write perf = P*N'/4 = 85*3/4 = 63.75 IOPS
read perf = P*N' = 85*3 = 255 IOPS
mixed perf = 255*.66+63.75*.33 = 168.3 + 21 = 189.3 IOPS
RAID 10
write perf = P*N/2 = 85*4/2 = 170 IOPS
read perf = P*N = 85*4 = 340 IOPS
mixed perf = 340*.66 + 170*.33 = 224.4 + 56.1 = 280.5 IOPS
That's a 100% increase in write performance and a 55.5% increase in overall
performance for RAID 10 for the same number of spindles. I would never
recommend RAID 5 for exchange.
"SW" <SW@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:E072839C-D358-40B3-B67D-CA89B83840A2@xxxxxxxxxxxxxxxx
It's the "writes" that are the problem, and when this is high our users
"nag". I use MOM 2005 with the Exchange Management Pack which alerts me
when
the Physical Disck Avg. Disk sec/write is greater than 50ms for 10 minutes
(it polls every 60 seconds). Here is one I got earlier:
Severity: Warning
Status: New
Source: PhysicalDisk: Avg. Disk sec/Write: 0 F:
Name: Disk Write Latencies > 50 msec (F: Drive)
Description: High Disk Write Latencies for the past 10 minutes
PhysicalDisk: Avg. Disk sec/Write: 0 F: value = 6.41240062645155E-02.
The
average over last 10 samples is 0.064124.
Domain: DOMAIN
Agent: EXCHANGE2003
Time: 2/22/2006 09:57:00
"Neil Hobson [MVP]" wrote:
RAID5 probably isn't the best answer. Are you using MOM to monitor this
server? Is it actually causing a problem to the users? In a nutshell,
you
should look to size your server correctly regarding disk IOPS. There are
a
number of articles on doing this, such as those by Rui Silva at
msexchange.org, or Microsoft's own paper called something like Optimizing
Storage for Exchange 2003.
--
Neil Hobson
Exchange MVP
"SW" <SW@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:582F4999-F46F-4DAA-9B59-E60B3227B368@xxxxxxxxxxxxxxxx
What can we do about disk latency that goes over 5ms at various time of
the
day?
We have an Exchange 2030 Ent server (400 users), 2 DB's 80gb and 10GB.
The high latency is on our F drive, he is our setup:
c: OS, Exchange program, paging file (RAID 1 channel 0)
e: Transaction logs (RAID 1 channel 1)
f: Databases (RAID 5 on a separate RAID card)
Usually the server runs below 20ms on the Avg. Disk sec/write what we
hit
50ms at stages throughout the day, especially first thing when everyonein.
logs
.
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