Re: Disck write latency > 50ms

John that is quite amazing. We will go to RAID 10, we have enough bays to
put in extra disks, we are to but another RAID card for this too. You
calculations are no doubt correct, can you explain how you got those results,
I just want to explain to my bosses. What does IOPS stand for?

"John Fullbright" wrote:

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 everyone
logs
in.






.

Relevant Pages

  • Re: Disck write latency > 50ms
    ... Also how can I tell how many users are connected to the Exchange 2003 ... long IO operations against a given physical disk are taking. ... criteria from "Optimizing Storage for Exchange Server 2003" to alert; ... we are to but another RAID card for this too. ...
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