Questions tagged [io]

IO (or I/O) is an abbreviation for input/output, most commonly referring to disk input/output.

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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 ...
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156 votes
13 answers
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Are networks now faster than disks?

This is a software design question I used to work on the following rule for speed cache memory > memory > disk > network With each step being 5-10 times the previous step (e.g. cache ...
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How to check disk I/O utilization per process?

I'm having a problem with a Linux system and I have found sysstat and sar to report huge peaks of disk I/O, average service time as well as average wait time. How could I determine which process is ...
Avada Kedavra's user avatar
49 votes
4 answers
88k views

Linux - How can I see what's waiting for disk IO

I have a server that has a really high load. Nothing is jumping out at me in terms of CPU usage, and it's not swapping. I think it's cause some processes are waiting for disk IO, and I want to see ...
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Measuring 'total bytes written' under Linux

We're quite interested in exploring the possibility of using SSD drives in a server environment. However, one thing that we need to establish is expected drive longevity. According to this article ...
badnews's user avatar
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40 votes
5 answers
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How to identify I/O bottlenecks on a Linux server?

How do you determine that the performance of your Linux server is I/O bound and, perhaps more importantly, what process or processes are casuing the problem?
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How can I monitor network I/O usage per process under Linux?

Such known tools like iftop/iptraf display network I/O per interface and per connection. Is there a way to see network I/O statistics per process?
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kworker consuming +90% IO and zero disk write

this is a standard apache web server on AWS Linux AMI + EBS. We are noticing high load average (+8) and iotop -a shows: Total DISK READ: 0.00 B/s | Total DISK WRITE: 2.37 M/s TID PRIO USER ...
user2383712's user avatar
21 votes
1 answer
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Determining which process is causing heavy disk I/O?

I've seen this question: How to identify heavy write to disk? And I've used dstat and atop before...but they don't seem to pinpoint what process is causing disk I/O. For example, from dstat: dstat ...
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21 votes
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Is there a way to get Cache Hit/Miss ratios for block devices in Linux?

Is it possible to see in Linux how many read and write requests from user space end up causing cache hits and misses for block devices?
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jbd2/dm-0-8 consuming all I/O

What is jbd2/dm-0-8 and why It is consuming all my I/O usage and causing I/O wait? Is there anyway to disable this? Bonnie++ results: http://pastebin.com/iQCWP1qp $ sudo hdparm -t /dev/sda1 /dev/...
Super Mario's user avatar
20 votes
2 answers
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Why CPU spent time on IO(wa)?

I know wa (in top) measures the CPU time on waiting for I/O. Many articles say that. But I am confused that, based on 2 knowledge points: if a process uses a system call to read disk, the process is ...
HUA Di's user avatar
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19 votes
2 answers
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What could explain unknown I/O on new 16TB HDD on a Debian server?

I recently obtained a new 16TB HDD for my Debian server. I first created a partition (gpt) on it via my Ubuntu-like desktop, formatted it (ext4), and rsync'ed old data on it. The disk is now usable, ...
b4too's user avatar
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17 votes
2 answers
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Total I/O cost of a process

I know that iotop lets me measure the disk bandwidth used by any or all processes in real time. iotop -a measures the accumulated disk bandwidth, which is closer to what I want. However, when the ...
user308485's user avatar
14 votes
3 answers
2k views

Virtual machines and I/O heavy workload, is it ever sane?

I have seen on numerous virtualization services (Azure) and products (vmware,kvm,hyperv) I/O and system stalls under heavy I/O workload. My questions are: Is it ever sane to use a Virtualized ...
bryan hunt's user avatar
14 votes
4 answers
3k views

How do limit disc i/o during backup?

I have a cron that basically do a simple "tar zcf" in the night. The server has: 8 Cores - Intel(R) Xeon(R) CPU E5606 @ 2.13GHz 25GB RAM Ubuntu 12.04.2 LTS Hardware RAID 1 (LSI Logic / Symbios ...
acemtp's user avatar
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12 votes
4 answers
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linux: how to simulate hard disk latency? I want to increase iowait value without using a cpu power

Is it possible to set an arbitrary delay for I/O operations? I would like to learn how do iowait and disk %util impact the performance. There are a lot of tools to stress the disk (bonnie++ etc.) but ...
user156233's user avatar
12 votes
11 answers
17k views

What creates CPU I/O wait but no disk operations?

I have CPU I/O wait steady around 50%, but when I run iostat 1 it shows little to no disk activity. What causes wait without iops? NOTE: There no NFS or FUSE filesystems here, but it is using Xen ...
Jason Cohen's user avatar
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12 votes
2 answers
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How does Linux handle concurrent disk IO?

When a Linux server is serving many concurrent requests to read many different files, does it: Seek to File_1, read the entire file, then seek to File_2, read the entire file, then seek to File_3, ...
Continuation's user avatar
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how to do I/O sniffing

For sizing purposes I need to understand what requirements an application has to the I/O subsystem. I would love to do what I call I/O sniffing and obtain a list of events from the block layer like ...
Thorsten Staerk's user avatar
11 votes
2 answers
2k views

How do you monitor the performance of a character device (e.g. tape drive)?

There are many ways to monitor block device performance: dstat and iostat, heck, even sar will give you data on block device I/O rates. Unfortunately, I don't know of any good spur-of-the-moment ...
Slartibartfast's user avatar
10 votes
6 answers
14k views

How to identify heavy write to disk?

I have this problem with server running CakePHP application. The server is insanely slow, I first thought that it's application problem, but then I found constant 5-6MB/s write to disk. What is the ...
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What does it mean when Linux has no I/O scheduler

I have some virtual machines running Ubuntu cloud-based image 14.04-1 LTS version. I wanted to see the IO performance of different IO schedulers on the VM so I went to /sys/block/<drive>/queue/...
Ha Son Hai's user avatar
10 votes
3 answers
13k views

Is there a way to delete 100GB file on Linux without thrashing IO / load?

I have a huge log file I need to delete on a production web server. I'm worried it'll bring the system to a crawl if I rm it on Linux. Any brilliant ideas? Update: Filesystem: ext3 Partition: /var (...
objectoriented's user avatar
10 votes
2 answers
10k views

MySQL extremely slow on very simple SELECT queries

We have a simple web application running on a virtual machine that saves its data in a MySQL 5.5 database with the InnoDB engine. Everything worked fine for around three years, but suddenly it became ...
tabb's user avatar
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10 votes
2 answers
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Amazon RDS: What are IO Requests?

I have an RDS instance that is costing me a lot of money. From my account activity on amazon I see that the instance has had about 800,000,000 IO requests over the past 7 days. To give you a little ...
Abe's user avatar
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10 votes
2 answers
21k views

what is the relation between block size and IO?

I have been reading about disk recently which led me to 3 different doubts. And I am not able to link them together. Three different terms I am confused with are block size, IO and Performance. I ...
Ankit Kulkarni's user avatar
9 votes
2 answers
4k views

Logging hurts MySQL performance - but, why?

I'm quite surprised that I can't see an answer to this anywhere on the site already, nor in the MySQL documentation (section 5.2 seems to have logging otherwise well covered!) If I enable binlogs, I ...
James Green's user avatar
9 votes
2 answers
9k views

Why do sequential writes have better performance than random writes on SSDs?

An LBA (logical block addresses) is a mapping table implemented in the FTL to match between logical and physical pages/blocks in SSDs, my guess is that most SSDs (at least when they are empty) keeps ...
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1 answer
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Interpreting read, write and total IO time in /proc/diskstats

I've noticed that when I look at the output of /proc/diskstats there is a discrepancy between the total time spent reading, the total time spent writing, and the total time performing IO. For example, ...
jeromefroe's user avatar
9 votes
1 answer
793 views

Finding which process is reading from disk constantly on FreeBSD

EDIT: This ended up being a scrub process. Run zpool status -v and read the scan status for your pool clearly. I have a TrueNAS server running FreeBSD and this morning at 12 AM the disks started to be ...
Bilbo Baggins's user avatar
9 votes
1 answer
17k views

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 ...
Yolo Perdiem's user avatar
9 votes
1 answer
1k views

5.5GB written daily to 1.2GB root volume - 4 times previous levels

Problem: I recently revamped one of my servers, it was tested prior to use, and functions well, however, a few days ago, I noticed approximately 4 times the usual amount of writes to the root volume. ...
cyberx86's user avatar
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9 votes
5 answers
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High load due to I/O wait in Ubuntu 12.04 on EC2 instance

I am using Ubuntu server 12.04 , having trouble finding cause of load, I have seen change in response time of server from past week after reading Linux Troubleshooting, Part I: High Load It seems ...
Vaibhav Dhage's user avatar
8 votes
1 answer
5k views

Dmesg full of I/O errors, smart ok, four disks affected

I'm working on a remote server (Dell Poweredge) that was a new install. It has four drives (2TB) and 2 SSD's (250 GB). One SSD contains the OS (RHEL7) and the four mechanical disks are eventually ...
Scu11y's user avatar
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8 votes
6 answers
2k views

Linux I/O bottleneck with data-movers

I have a 24 core machine with 94.6GiB RAM running Ubuntu server 10.04. The box is experiencing high %iowait, unlike another server we have (4 cores) running the same types and amounts of processes. ...
Benjamin's user avatar
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8 votes
3 answers
2k views

How to keep subtree removal (`rm -rf`) from starving other processes for Disk I/O?

We have a very large (multi-GB) Nginx cache directory for a busy site, which we occasionally need to clear all at once. I've solved this in the past by moving the cache folder to a new path, making a ...
David Eyk's user avatar
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8 votes
4 answers
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Flush-0:n processes causing massive bottleneck

I have a LAMP cluster that shares files via NFS and occasionally one of them will be stricken for a while when mysterious flush processes start appearing. Can anyone help me? The only way to resolve ...
Tom's user avatar
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8 votes
2 answers
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Can I run iostat on a Synology?

I have a Synology RS812RP+ which I'm using for a little bit of departmental testing. I've stuffed a bunch of SSD disks in it, and I'm curious about monitoring the disk IO. iostat doesn't come pre-...
Zorlack's user avatar
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8 votes
1 answer
3k views

using temporary files vs pipes advantages and disadvantages

Say I have a file named jobs.csv and I would like to get the top 50k jobs done by Foo I can either do: # cat jobs.csv | sort -u | head -n 50000 > /tmp/jobs.csv # cat /tmp/jobs.csv | while read ...
Tzury Bar Yochay's user avatar
8 votes
1 answer
472 views

rrdgraph generation fails on high IO load

We have a 4 core CPU production system which does a lot of cronjobs , having constant proc queue and an usual load of ~1.5. During night time we do some IO intensive stuff with postgres. We generate ...
Dennis Nolte's user avatar
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7 votes
3 answers
4k views

What program in Linux can measure I/O over time?

I'm trying to measure the total amount of disk writing and reading that is done to a particular volume by a particular process over a specified duration. I've found iotop, which can output IO every ...
Neil's user avatar
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7 votes
2 answers
4k views

Identify which process is responsible for heavy disk i/o under FreeBSD

I wonder how I can monitor which process takes out all my IO bandwith. The only command i know is gstat but it shows cumulative info. Is there any tools i can use to inspect the running processes for ...
Sergey Efimov's user avatar
7 votes
1 answer
2k views

How efficient is the tac command on large files

The taccommand (catreversed) can be used to read a file backwards, just like cat reads it rom the beginning. I wonder, how efficient this is. Does it have to read the whole file from the beginning and ...
Daniel Schneller's user avatar
7 votes
4 answers
30k views

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 ...
grs's user avatar
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7 votes
2 answers
3k views

Is a 'high' IO rate healthy?

I do a lot of work with my Linode that currently hosts a site, but every now and then (usually once every two months or so) - I'll get a warning about my IO rate being too high (Usually just higher ...
user avatar
7 votes
1 answer
5k views

Linux - KVM - very slow disk io

The I/O performance of my new server is pretty low. I Googled and tried everything I can imagine. First of all, here are the Hardware Components: AMD Opteron(tm) Processor 6344 4x Seagate ...
T-Bash's user avatar
  • 71
7 votes
2 answers
4k views

How to set i/o priority for nfs client processes?

The configuration is: A linux server and a nas box (netgear) acting as nfs server. It is easy for a single process on the linux server to use all i/o bandwidth by simply copying a file from the nfs ...
Moritz Both's user avatar
6 votes
7 answers
38k views

I/O Performance Benchmarking Linux

Just finished setting up a poor mans SAN with iSCSI and want to benchmark it. What are some good I/O performance benchmarks for Linux besides: hdparm -tT /dev/sda1 How do I get a IOPS measurement? ...
Justin's user avatar
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6 votes
3 answers
57k views

Evaluating the CPU I/O wait on Linux

Doing a top to check the io wait, I get these figures: Cpu(s): 6.7%us, 1.4%sy, 1.2%ni, 85.5%id, 5.0%wa, 0.0%hi, 0.3%si, 0.0%st Looking at these figures (%us ~= %wa), do they mean that: there ...
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