More actions. Disk Queue Length. This should always be less than 1. As this average queue length number increases, disk contention increases. Com recommends that the Average Disk Queue Length to be less than 2. I havent been at my current position that long, so I am playing more catchup than anything right now.
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The spikes are quite harsh, and quite often. Machine is 2 3. Especially with SAN's these days the queue counters often don't mean much. You must be logged in to reply to this topic. Login to reply.
November 29, at am Looking at PerfMon now, the "Maximum" value is Please let me know what you think Thanks in advance, Graham. Another question that just came to me, could disk defragmentation cause IO problems, like this?
As we are in the process of buying DiskKeeper. Thanks, Graham. November 30, at am Hope this helps. Well the array consists of 3 x Seeing spikes continuously, with values from 1 to So basically I need to keep it under 6? Thanks for the response! December 1, at am They are really helpful to identify IO bottlenecks. January 6, at am Thanks for all the responses, most helpful.
Viewing 7 posts - 1 through 7 of 7 total.The goal for this presentation is to show you how to go about tuning SQL Server queries and the steps you should take to improve performance. We experience regular slowdowns on our SQL Server databases. The following performance counters can be setup to check disk performance.
The PhysicalDisk Object: Avg. Disk Queue Length counter shows you the average number of read and write requests that were queued on the selected physical disk. The higher the number the more disk operations are waiting. It requires attention if this value frequently exceeds a value of 2 during peak usage of SQL Server. If you have multiple drives you should take this number and divide by the number of drives in the array to see if the number is above 2. It is good up to 10 ms, but it is still acceptable if less than 20 ms.
Any higher value needs further investigation. The rule of thumb for this value is that it should be below 50 percent. It should be less than 85 percent of the disk capacity since the disk access time increases exponentially beyond this value. You can determine the disk capacity by gradually increasing the load on the system.
You should look for the point where the throughput is constant, but the latency increases.
Avg Disk Queue Length
You can query the sys. I would recommend that you check the disk fragmentation and the configuration of your disks used by the SQL Server instance. Fragmentation of files on NTFS can cause significant reductions in performance.
Disks should be defragmented regularly and a defragmentation policy and plan should be put in place. Research shows that in some cases a SAN can actually perform worse with defragmentation enabled thus SANs need to be treated on a case-by-case basis.
As a general rule, you should have log files on a physical disk that is separate from the data files for better performance. The database data and log files should each be placed on their own dedicated disk packs. To ensure optimal performance, I recommend that the database log file be placed on two internal disks configured as RAID 1. Ad hoc access should be disallowed.
Write caching should be enabled where possible and you should make sure the cache is protected from power failures and other possible failures. Thanks for the response. If yes, then when should I start considering index defragmentation on SANs if should be? Thanks in advance. First of all seems like reading it a bit late : But still wanted to ask the details of your statement.
Are you talking about the fragmentation of indexes on multiple DISKs arrays the storage? If this is the case then your statement is very confusing and could mislead others. Please make me understand this. View all my tips. Become a paid author. Recommended Reading. Back To Top I have collected Physical disk performance counter around 1 Hour by using system perfmon tool.
Disk Queue Length - 15 second interval values not reached more than Hi Tibor, Thanks for the response. Kind Regards, Usman.
Regards, Tibor.The disk has had a consistently high value for the Current Queue Length counter over multiple consecutive samples. Current Disk Queue Length is the number of requests outstanding on the disk at the time the performance data is collected.
Either the disk has recently experienced a significant increase in activity, and this spike has resulted in exceeding the threshold, or disk utilization has been steadily increasing over time and has finally reached a point of going over the threshold. The other possibility is that some portion of the underlying disks or the disk subsystem is malfunctioning or misconfigured, impairing the performance of the disk. Based on the findings from further investigation, resolutions may vary and could include one of the following:.
Skip to main content. Contents Exit focus mode. Causes Either the disk has recently experienced a significant increase in activity, and this spike has resulted in exceeding the threshold, or disk utilization has been steadily increasing over time and has finally reached a point of going over the threshold. Resolutions To further investigate the issue consider the following: Review the System event log on the system, to see if there are any error indicating problems with the disk or the storage subsystem.
Review the history of current queue length for this disk using either a monitoring metric chart in Azure Monitor or a Log Analytics log search. This will help in determining if the issue has started recently or if the activity has been steadily increasing over a longer period of time. Based on the findings from further investigation, resolutions may vary and could include one of the following: Address any issues or misconfigurations with the storage subsystem. Upgrade the drives or storage subsystem to handle the increased load.
If the increased load is acceptable then the threshold of the monitor can be changed to be less restrictive. Likewise the number of consecutive samples can be increased to force the health criteria to only change state when utilization is sustained over longer periods of time. Is this page helpful?
How to Identify IO Bottlenecks in MS SQL Server
Disk Queue Length" pegging the server. What would cause something like this? I do have one table in particular that has tons of inserts. Very few reads, but many inserts per second isn't unusual at all. So, anyway, it was a combination of a whole bunch of different things, mostly related to SQL, but not exclusively so Will was correct there.
I'd love to split the answer between everyone, as they had portions of it right, but what can you do Wait for a moment where the disk queue depth is high and find all currently running queries scripts available on the web. It is likely that some expensive query is running. All of these are designed to drive the IO system with all might. If SQL Server needs to expand it's files data, logthat could be a potential cause too if you have a slow or fragmented disk.
Windows Server and up has Resource Monitor, which allows you to see what operations are occurring on the disk. The disc subsystem just being too pathetic to handle the load. Check whether anything special goes on during those times. If not - maybe jsut get a cheap SSD. Sign up to join this community. The best answers are voted up and rise to the top. Asked 8 years, 5 months ago.
Active 9 months ago. Viewed 38k times. Anyone got any thoughts on what might cause that huge queue length? In Windows, the "average queue length" is roughly analogous to what Unixes would call "iowait.I need a link to a Microsoft website of how "Avg.
Disk Queue Length" is figured out as an "Average" as the name implies or an expanded definition beyond the explanation inside perfmon. Data points are gathered at set intervals in perfmon - so a little confused on the average portion. The average Queue length value calculated in this fashion includes both IRPs queued for service and actually in service We recently had a customer reporting values like 2.
SQL Server disk performance metrics – Part 2 – other important disk performance measures
That's a busy disk! The interpretation of this Counter is the average number of disk requests that are active and queued — the average Queue Length. Or is his explanation totally wrong? Measures the average time of each data transfer, regardless of the number of bytes read or written.
Shows the total time of the read or write, from the moment it leaves the Diskperf. A high value for this counter might mean that the system is retrying requests due to lengthy queuing or, less commonly, disk failures.
Disk Queue Length: Tracks the number of requests that are queued and waiting for a disk during the sample interval, as well as requests in service. As a result, this might overstate activity. If more than two requests are continuously waiting on a single-disk system, the disk might be a bottleneck.
To analyze queue length data further, use Avg. Disk Read Queue Length and Avg. Disk Write Queue Length. Does this answer your question? Sort of Last week, I spent 30 minutes telling my supervisor that performance monitor does not average its staticstics together - it only records the data from a point in time. Then he challenged me: "What about all those average counters?
The definition of average is at least two data points. Disk Queue Length" where it states: "Tracks the number of requests that are queued and waiting for a disk during the sample interval" - what does that mean? It states "Sample interval" - are those the two data points? For instance, if at it is a value of 1, and the actual reading is a value of 2, does it instead average the and entry and write in 1. Or does "Average" mean, at the sample time say pmit is looking at all data trying to enter the hard disk, and some of that data coming in has a queue length of 1 and some have a queue length of two - and at that data point sample interval it will take all that and average those incoming requests to the hard drive and put that down as the value at pm?Please note that this document is a translation from English, and may have been machine-translated.
It is possible that updates have been made to the original version after this document was translated and published. Veritas does not guarantee the accuracy regarding the completeness of the translation.
You may also refer to the English Version of this knowledge base article for up-to-date information. Support Knowledge base Article: Last Published: Ratings: 2 2. Product s : Enterprise Vault.SQL Server Performance Tuning with Wait Statistics (PageLatch - PageIOLatch - Latch)
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Article Languages. Translated Content Please note that this document is a translation from English, and may have been machine-translated.On the one hand, I know that what you'd like to see is disk read and write queues that average less than one. In overnight processing, over a two-hour period the average-average read queue length was 9, the max-average read queue length was I suspect there are complicating factors in and around the SAN, but Is it just that we assume a higher capacity so we allow a bit of queuing, or am I missing something?
Yes it gets much more complex for a SAN and I wouldn't focus on the queueing as much as the latency counters when dealing with a SAN implementation because you have a number of other factors to take into effect such as HBA configuration, fabric speed, caching, and some others discussed below.
Why does the number of disks in the array matter? That's a great question, and it actually gets more complex than just dividning the queue length by the number of disks, but you asked so here goes. RAID Arrays as you already know take lots of disks and make them work together as if they were a single disk, providing redundancy and performance improvements.
One of the items configured at array creation is the stripe size, generally 8K, 16K, 32K, 64K, K or K stripes are used in RAID arrays, with 64, and being the most common from what I have seen. If you are writing 8K data pages sequentially to tempdb during a sort operation for building an index, 16 IO's would occur to Disk1 in the RAID array before switching to Disk2 in the stripe, which would then service 16 IO's continuing around the array. In this case if you have a 10 queued IO's the odds are, they are all on the first disk.
SQL data access is primarily random in nature so your IO load is generally spread across the array. This is why partition alignment is important to performance, it aligns the partition with the start of a RAID stripe, so that IO requests don't span disks in the array up to the stripe size.
Under the correct circumstances, a single disk in the array may be a temporary "hot spot" due to the way the data happened to be striped and it could have higher queued commands, but it is next to impossible to tell this without a good controller and associated 3rd party monitoring tools.
My example used a eight disk RAID I meant to add that any kind of performance monitoring has to be done from a holistic approach. Single counters shouldn't be used in isolation to determine if a problem exists. On a SAN, your queue length can be high, but if your latency counters are on the money, you probably don't have a problem.
Every system of counters has more than one data point that can be used in making a big picture decision as to whether or not a problem exists. If the Free Pages counter falls below tolerances but PLE and BCHR are both good, its probably not an issue that there was a significant drop in Free Pages, it could just be a big query hitting the system.
The storage engines between the two products are fairly similar from the information I have on Exchange. There is a missing piece of information in Xiao's post. Windows doesn't know that a physical disk is a RAID Array, or the number of disks that back it, that information has to be queried from the controller using third party software in most cases.
A queue length of would still be suspect of a bottleneck point though, especially when coupled with IO Stalls in the virtual file stats. The reason it would spike queued requests during the data load is that writes incurr performance penalties in RAID 5 for the parity calculation, but if your primary usage is reading data, this wouldn't necessarily be a problem since you get the extra stripes for reading data.
Based on the value of the Avg. The disk access time increases exponentially beyond 85 percent capacity. If not, seems one more reason to go to RAID10, and a pretty big reason.