I/O is an acronym for Input/Output, which refers to the mechanism or process for the transfer of information between one system or component and another.

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56
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Redirecting stdout to a file you don't have write permission on

When you attempt to modify a file without having write permissions on it, you get an error: > touch /tmp/foo && sudo chown root /tmp/foo > echo test > /tmp/foo zsh: permission ...
38
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3answers
6k views

When is dd suitable for copying data? (or, when are read() and write() partial)

Short version: In what circumstances is dd safe to use for copying data, safe meaning that there is no risk of corruption due to a partial read or write? Long version — preamble: dd is often used to ...
19
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3answers
8k views

Determining Specific File Responsible for High I/O

This is a simple problem but the first time I've ever had to actually fix it: finding which specific files/inodes are the targets of the most I/O. I'd like to be able to get a general system overview, ...
18
votes
6answers
67k views

How can I read line by line from a variable in bash?

I have a variable which contains multiline output of a command. What's the most effecient way to read the output line by line from the variable? For example: jobs="$(jobs)" if [ "$jobs" ]; then ...
20
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3answers
5k views

Measure pipe throughput in the shell

There is a shell command that allows to measure how fast the data goes through it, so you can measure the speed of output of commands in a pipe. So instead of: $ somecommand | anothercommand you ...
2
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1answer
918 views

-bash: /usr/sbin/useradd: Input/output error — What does this mean?

Today, on my Centos server I tried to add a user but got the following error: -bash: /usr/sbin/useradd: Input/output error How do I fix it?
22
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3answers
939 views

How can I limit the output speed of stdout?

I'm running CentOS 5.7 and I have a backup utility that has the option of dumping its backup file to stdout. The backup file is rather large (multiple gigabytes). The target is an SSHFS filesystem. To ...
17
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2answers
7k views

Can I watch the progress of a `sync` operation?

I've copied a large file to a USB disk mounted on a Linux system with async. This returns to a command prompt relatively quickly, but when I type sync, of course, it all has to go to disk, and that ...
9
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1answer
1k views

What's the progress regarding improving system performance/responsiveness during high disk I/O?

Whenever there is high disk I/O, the system tends to be much slower and less responsive than usual. What's the progress on Linux kernel regarding this? Is this problem actively being worked on?
26
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12answers
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How can I tweak my Linux desktop to be more responsive?

I have the experience that Linux works fine until the physical memory is exhausted. As soon as swap space is used the performance is severely degraded and the GUI becomes unresponsive. This problem ...
25
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5answers
3k views

Why is iterating over a file twice faster than reading it into memory and computing twice?

I'm comparing the following tail -n 1000000 stdout.log | grep -c '"success": true' tail -n 1000000 stdout.log | grep -c '"success": false' with the following log=$(tail -n 1000000 stdout.log) echo ...
8
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3answers
4k views

How can I monitor disk I/O in a particular directory?

I've got a few processes with a known name that all write to files in a single directory. I'd like to log the number of disk block reads and writes over a period (not just file access) to test whether ...
4
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2answers
371 views

What is the difference between > and >> (especially as it relates to use with the cat program)?

Suppose that I have a file called temp.txt. Using the cat program, I would like to add the contents of this file to the end of myfile.txt -- creating myfile.txt if it does not exist and appending to ...
1
vote
3answers
580 views

Amazon EC2 micro instance large number of IO requests

I'm trying to understand why I have a huge number of EBS IO request on my Amazon EC2 micro instance. I launched the instance roughly 6 days ago and has racked up over 4 million IO requests so far. ...
11
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2answers
2k views

Heavy write activity on SSD nukes system performance

I've noticed that when I do heavy write applications, the whole system slows down. To test this further I ran this to do a (relatively) low-CPU, high disk activity: john -incremental > file_on_SSD ...
7
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3answers
714 views

Why is I/O uninterruptible?

What is the rationale behind making I/O uninterruptible? What would be the negative consequences if a process handling I/O were allowed to handle a signal?
6
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1answer
979 views

`unbuffer` or `stdbuf` for removing stdout buffering?

Is there a difference between unbuffer(1) and stdbuf(1)? From what I gather, unbuffer makes more than the "best effort" of calling the libc function set(X)buf at the beginning, and then letting things ...
4
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2answers
445 views

why does blktrace only write blocks of 8?

I want to understand the I/O pattern a database is writing to disk to decide how many disks to use for best performance. To analyse the I/O pattern I want to use blktrace and I have to grok it first. ...
4
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0answers
160 views

Repeatably generate a read error for testing? [duplicate]

I'm trying to test the error handling in some software, in particular what happens when an error occurs reading from a file or pipe. Is there a simple way to send a certain amount of data to stdout ...
9
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4answers
3k views

Special File that causes I/O error

I want to automatically test if a piece of software reacts as expected if an essential SQLite DB file fails to be read (causing an I/O error). Exactly that happened some days ago at a client. We ...
7
votes
2answers
949 views

Reduce bad block retry / wait times in Ubuntu

How can I reduce the IO wait time and retry times so that the OS doesn't continually try to write to a failing drive? I have an system that I use to make copies of demo content that gets loaned out ...
7
votes
3answers
912 views

Harddrive I/O per Pid

is there any way to see the disk I/O per pid/process ? Monitorix shows me the following and correct me if I´m wrong, but it looks like that the harddrive is slowing down the whole system: UPDATE #1 ...
5
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2answers
2k views

iotop but for particular disk?

Is there a tool like iotop but for monitoring IO on specific volumes? (Or perhaps I missed a way to filter iotop?) If not is there any way to get at this information via (say)/proc? (dstat etc. ...
5
votes
5answers
474 views

performance monitoring

Is there some performace monitoring tool which would run in background gathering info about all system activity? Somethimes my system (Arch linux, 32 bit) slows down terribly and the top utility ...
2
votes
1answer
52 views

pipe speed benchmark with machine readable output

I'm looking for a tool similar to pv which is producing machine readable output. And i cant believe that nobody did it bevore... I looked at the manpages of pv, pipemeter, dd and cpipe but all these ...
11
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1answer
553 views

How do I go about tracking down IO usage in Linux and other *nix OS'?

Occasionally, something on my system starts hammering the disk. I would like to log data that would let me know about I/O usage per process, then I could go back to it later and see what keeps doing ...
6
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2answers
437 views

Mysterious write IO

I am just reviewing the disk IO of a XEN VM server. I stumbled across a DomU that has constant write traffic. To narrow that traffic further down I logged in to the DomU (CentOS 5, 64 Bit) and ...
5
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2answers
2k views

100.0%sy with poor disk performance

I have RHEL 5.4 kernel 2.6.18-164.el5 and I am experiencing very bad disk performance that happens randomly during working with Oracle. Nothing appears in the logs. When it happens, I see one CPU ...
3
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1answer
579 views

How race conditions affect reads and writes (that happen at the same time)

Let's say I open a file a for reading. What if an application, let's call it aWriter writes to this file in random intervals. Are there any possibilities of me receiving improper file contents if I ...
2
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2answers
72 views

How do I check which files are written by an application?

On my system, sar reports an average write to disk about 5MB/s. Using dstat I found that it's not so permanently, but there are peaks to 30-50MB/s, and then are sleeps. Using iotop i found that most ...
2
votes
1answer
126 views

Relationship between IO scheduler and cpu/process scheduler?

The concepts of IO scheduler and CPU scheduler confuse me. Below are my understanding: Linux uses CFS scheduler + nice values by default to schedule processes. Each process has an IO queue. There ...
2
votes
3answers
210 views

Low level disk i/o on linux

Can a user level program do page level or block level I/O on SSDs? I've looked at disk devices, but am unsure if they provide this as they work only if the partition has a file system. UPDATE #1 I ...
1
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1answer
92 views

How to decrease the time the processes would wait (blocked) for disk reads (only to discover an error)?

Waiting too long when encountering an unreadable sector on a disk is really very annoying. (Especially if you are trying to read as much data as can be read from a disk with bad sectors, and do it ...
1
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2answers
2k views

How to know recently updated files

If I run iostat -x 1 I saw ocassionally large 5MB to 10MB writes. What files are being written? I want to check the recently created files with size over 5MB for example. How would I do so?
1
vote
1answer
311 views

Does symlinking from Drive A to Drive B affect I/O on Drive A?

I was wondering if IO through a symbolic link affects IO on the host drive even if it points to a seperate drive? My setup is, /dev/sdb2 1.8T 443G 1.3T 26% /home1 /dev/sda3 1.8T 1.7T ...
1
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1answer
307 views

performance on Linux system when daisy-chaining multiple USB hard-drives on USB hubs?

What would the I/O access be like as I plug many USB drives into a Linux system with only 2 USB 2.0 ports and 1 USB 3.0 port? I expect I will have to have multiple levels of fan-out by plugging hubs ...
1
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1answer
151 views

What Linux tool will tell me if all the mounted disks share roughly the same bandwidth?

What Linux tool will tell me if all the mounted disks share roughly the same bandwidth? Can I do something like iostat for each of my /dev/sd* devices?
0
votes
0answers
217 views

Is swapping the cause of high IO on my box?

I'm running a LAMP stack on an Amazon EC2 micro instance with 616 MB of memory. This seems sufficient to be able to run a few small websites with Apache, MySQL and PHP. I have one Wordpress site ...
0
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1answer
454 views

Does linux have something like supercache?

I installed http://www.superspeed.com/desktop/supercache.php for windows and the result is awesome. Every program I run is faster. Basically it uses the memory to cache data. Simple idea. I also use ...