SAN Latency

gwills · 29th February 2012, 11:50 AM

Hi Guys,

Just want to find out what latency you would find acceptable in your environment.

I've been running the stats on mine

FC 15K aggregate 2ms normal spiking to 10 ms during backup / SQL optimization (high i/o)

SATA 7200 aggregate, average latency around 12ms spiking out to 30 during backup.

What kind of latency do you run in your environment ? I'm planning a SAN migration and was asked what latency figure I was looking for, I could say under 1ms but it would mean throwing lots spindles at it.

Obviously lower latency the better, I'm just trying to work out what a normal latency is ?

QuakeDude · 29th February 2012, 2:56 PM

The latency is only one factor of many. What sort of applications are you running? What do they need? Backups - how long are they taking? etc

We're running an XIV with SATA drives, which give us something like 5-8ms. However, this sucker runs all of our VM infrastructure, 40+ Citrix servers and currently pulls around the 8K IOPS during peak backup periods.

The San can do 80,000 IOPS if needed. I could have gone 2ms, but would it have made any difference? Not really. Not for our applications at least.

Schnooper · 29th February 2012, 3:47 PM

As a very general rule of thumb, above 20ms will impact user experience.

However it really depends on what the server and san is doing as well. Some apps are effected by high latency, and some arent.

gwills · 1st March 2012, 9:47 AM

Yeah I guess its all relative to the applications, All of our IO intensive apps (database) are on FC and all low priority is on SATA and it seems to work fine

I have also read too that over 20 ms is where it becomes noticeable

I just thought it would be interesting to hear what other people are experiencing in their environments or what they feel is acceptable latency limit

Jase · 1st March 2012, 11:59 AM

Quote:

Originally Posted by gwills

I just thought it would be interesting to hear what other people are experiencing in their environments or what they feel is acceptable latency limit

When I do SAN perf analysis, my typical expectations are:

FC 15K
Read Miss = 6-8ms

FC 10K or SAS
Read Miss = 10-12ms

SATA
Read Miss = 14-26ms

ANY DRIVE
Write = 1-2ms (always to array cache, no backend replication)

Note: A read miss is when the data being requested is not in the array cache and must be requested from a backend disk.

29th February 2012, 11:50 AM	#1
gwills Member Join Date: Jan 2005 Location: Melbourne Posts: 367	SAN Latency Hi Guys, Just want to find out what latency you would find acceptable in your environment. I've been running the stats on mine FC 15K aggregate 2ms normal spiking to 10 ms during backup / SQL optimization (high i/o) SATA 7200 aggregate, average latency around 12ms spiking out to 30 during backup. What kind of latency do you run in your environment ? I'm planning a SAN migration and was asked what latency figure I was looking for, I could say under 1ms but it would mean throwing lots spindles at it. Obviously lower latency the better, I'm just trying to work out what a normal latency is ?

29th February 2012, 2:56 PM	#2
QuakeDude Member Join Date: Aug 2004 Location: Melbourne Posts: 7,288	The latency is only one factor of many. What sort of applications are you running? What do they need? Backups - how long are they taking? etc We're running an XIV with SATA drives, which give us something like 5-8ms. However, this sucker runs all of our VM infrastructure, 40+ Citrix servers and currently pulls around the 8K IOPS during peak backup periods. The San can do 80,000 IOPS if needed. I could have gone 2ms, but would it have made any difference? Not really. Not for our applications at least. __________________ PSN: quakedude311

29th February 2012, 3:47 PM	#3
Schnooper Member Join Date: Apr 2007 Posts: 496	As a very general rule of thumb, above 20ms will impact user experience. However it really depends on what the server and san is doing as well. Some apps are effected by high latency, and some arent.

1st March 2012, 9:47 AM	#4
gwills Member Join Date: Jan 2005 Location: Melbourne Posts: 367	Yeah I guess its all relative to the applications, All of our IO intensive apps (database) are on FC and all low priority is on SATA and it seems to work fine I have also read too that over 20 ms is where it becomes noticeable I just thought it would be interesting to hear what other people are experiencing in their environments or what they feel is acceptable latency limit