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Questions tagged [iowait]

69 questions
250
votes
7 answers

Can anyone explain precisely what IOWait is?

As much as I have read about iowait, it is still mystery to me. I know it's the time spent by the CPU waiting for a IO operations to complete, but what kind of IO operations precisely? What I am also not sure, is why it so important? Can't the CPU…
Peter Krumins
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4 answers

Does iowait include time waiting for network calls?

The proc(5) manpage describes iowait as "time waiting for IO to complete". This was mostly explained in an earlier question. My question is: while waiting in blocking IO, does this include waiting on blocking network IO, or only local IO?
Alex J
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1 answer

Why higher await time for DM multipath device than underlying device?

We have a CentOS 6.4 based server attached to Hitachi HNAS 3080 storage and observed the kernel remount the filesystem in read-only mode: May 16 07:31:03 GNS3-SRV-CMP-001 kernel: [1259725.675814] EXT3-fs (dm-1): error: remounting filesystem…
pdp
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1 answer

what percentage of iowait is considered to be high?

when iowait is considered to be high ? iostat -x avg-cpu: %user %nice %system %iowait %steal %idle 2,89 0,01 5,45 49,83 0,00 41,83 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz …
Gabriel Sousa
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9
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1 answer

Windows "iowait" CPU utilization reporting

Does Windows account for and report "iowait" in the same manner as Linux? That is, processes enter "uninterruptible sleep" and their time spent is subtracted from "CPU free"? If so, which Perfmon counters would be appropriate to detect "cpu busy due…
Yolo Perdiem
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8
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1 answer

high load average, high wait, dmesg raid error messages (debian nfs server)

Debian 6 on HP proliant (2 CPU) with raid (2*1.5T RAID1 + 2*2T RAID1 joined RAID0 to make 3.5T) running mainly nfs & imapd (plus samba for windows share & local www for previewing web pages); with local ubuntu desktop client mounting $HOME, laptops…
John Stumbles
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7
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4 answers

What is causing the unusual high load average?

I noticed on Tuesday night of last week, the load average went up sharply and it seemed abnormal since the traffic is small. Usually, the numbers usually average around .40 or lower and my server stuff (mysql, php and apache) are optimized. I…
James
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4 answers

How to measure IOwait per device?

I have a server, which exports home directories over NFS. They are on software RAID1 (/dev/sdb and /dev/sdc) and the OS is on /dev/sda. I noticed that my %iowait as reported by top and sar are relatively high (compare to the rest of the servers).…
grs
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4 answers

What would cause IO Wait on a SAN?

I understand IO wait when I see it on a server, it means that the CPU is blocked while I wait for the IO to catch up [source]. I am trying to understand why a SAN stats would show a high IO wait - does this indicate that the SAN CPU is blocked by…
Robert MacLean
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2 answers

Finding the root cause of 100% iowait in Linux

I have a system that is a few years old that is running Fedora 7 (I know, it's old too). Now I have PS3 Media Server running (and some other things, like Vmware and Plone) and I noticed that I was getting unexpected interruptions in the streamed…
Niels Basjes
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5
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2 answers

Strange Recurrent Excessive I/O Wait

I know quite well that I/O wait has been discussed multiple times on this site, but all the other topics seem to cover constant I/O latency, while the I/O problem we need to solve on our server occurs at irregular (short) intervals, but is…
Chris
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4 answers

High iowait on Amazon EC2 MySQL instance with EBS volume

We have a MySQL server running on an Amazon EC2 c1.medium instance relying on a single EBS volume using the ext3 filesystem for storage. This MySQL server is queried ~500/ps by several applications running on some web servers, which are also at the…
Luis Fernando Alen
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4
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2 answers

is CPU actually occupied during iowait (%wa in top) on Linux / EC2?

On an 8-way Amazon EC2 instance (running Linux 2.6.21) with 8 EBS volumes and a lot of disk traffic, we see high %wa in top (30-40%), and high load average (8-9). My understanding is that processes waiting on I/O from the EBS volumes are counted in…
user27045
4
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2 answers

Can increased disk throughput increase CPU utilization?

We recently upgraded the storage in our data center and migrated all our production systems to it. We noticed that in one machine (an HP_UX IA-64 Oracle database server for SAP), the CPU utilization started spiking up while the IO wait% was…
varun
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How can i view the disk latency of a ZFS pool, or of the disks in it?

I'm using FreeBSD. I would like to write a munin plugin to log it, but i don't understand how to get this data.
cedivad
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