All posts tagged RAM

Linux And High Io Wait

By blogger on

When you look at the CPU activity of your computer, one of the parameters is the iowait. This value shows how much time your CPU wastes while it is waiting for I/O operations for complete. These include disk read/write operations, network, IPC, etc. Is this behavior a problem and, if so, what causes it and how to fix it? One one of the popular Unix-related forums one “genius” wrote:

The iowait “problem” is funny. It’s like when people complain that Linux is “using all my memory”. Yeah, no shit. You should be upset if you are copying files and your computer is /not/ in 100% iowait.

In reality, 100% iowait indicates that there is a problem and in most cases – a big problem that may even lead to data loss. Essentially, there is a bottleneck somewhere in the system. Maybe one of your disks is getting ready to die; or, perhaps, the NIC firmware is having problems with the latest kernel upgrade you installed. The troubleshooting process starts with the potentially more serious possibility: bad disk.

Take a quick look at /etc/messages, /etc/dmesg, /etc/boot.log and any other system log files. You are looking for disk I/O errors, failed read/write operations, bad sectors – anything that indicates a hardware problem with a disk. If you don’t find anything, look for IRQ and disk controller errors. Also look for memory errors and kernel panics. The three most likely culprits of high iowait are: bad disk, faulty memory and network problems.

If you still see nothing relevant, it is time to test your system. If possible, kick all the users off the box, shut down Web server, database and any other user application. Log in via command line and stop XDM.

Open three shell windows: run “top” in one, “iostat -x 1? in the other and “find /etc -type f -print” in the third. Make sure you can see all three windows at the same time. This is a simple test that should generate some I/O activity on the system disk. Repeat this process for other disks. If you see iowait hovering near 100%, chance are you have a problem but we don’t know what it is yet. However, now we do know that network is probably not the cause.

deathstar:/ # iostat -x 1
Linux 2.6.5-7.201-default (deathstar) 12/20/08

avg-cpu: %user %nice %sys %iowait %idle
2.83 0.42 1.45 9.11 86.20

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
hda 40.63 66.34 27.45 6.04 936.50 581.23 468.25 290.61 45.32 2.42 72.16 2.22 7.42
hdc 0.01 0.00 0.01 0.00 0.03 0.00 0.02 0.00 4.02 0.00 1.17 1.17 0.00
sda 0.09 2.32 4.15 1.33 71.56 29.23 35.78 14.62 18.37 0.65 118.49 6.39 3.51
sdb 3.47 0.00 1.90 0.00 15.32 0.01 7.66 0.01 8.08 0.74 391.31 5.68 1.08
fd0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.00 0.00 45.00 45.00 0.00

deathstar:/ # top
top – 21:28:28 up 1:22, 2 users, load average: 0.09, 0.14, 0.16
Tasks: 77 total, 1 running, 76 sleeping, 0 stopped, 0 zombie
Cpu(s): 2.8% us, 1.3% sy, 0.4% ni, 86.2% id, 9.1% wa, 0.1% hi, 0.0% si
Mem: 508644k total, 503612k used, 5032k free, 34052k buffers
Swap: 1020088k total, 458980k used, 561108k free, 16012k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 16 0 640 56 28 S 0.0 0.0 0:05.14 init
2 root 34 19 0 0 0 S 0.0 0.0 0:00.00 ksoftirqd/0
3 root 5 -10 0 0 0 S 0.0 0.0 0:00.09 events/0
4 root 5 -10 0 0 0 S 0.0 0.0 0:00.00 khelper

Next step, lets stress out your CPU but not the disks. The command below will try to create an endless zip file in /dev/null. This generates no disk activity, but loads the CPU. Continue running “top” and “iostat -x 1? in the other two windows.

cat /dev/zero | bzip2 -c > /dev/null

If you see high CPU load but low iowait, we can eliminate CPU issues, IRQ conflicts, and faulty memory. Just to be on the safe side, let’s test memory anyway:

deathstar:/ # free
total used free shared buffers cached
Mem: 508644 503504 5140 0 37036 48968
-/+ buffers/cache: 417500 91144
Swap: 1020088 516196 503892

This server has 508644Kb of RAM. Use the corresponding value for the following test:

deathstar:/ # dd if=/dev/hda2 bs=508644 of=/backups/memtest count=1050
1050+0 records in
1050+0 records out

deathstar:/ # md5sum /backups/memtest ; md5sum /backups/memtest ; md5sum /backups/memtest
04762ff36b2231aac75754ab9c1a564a /backups/memtest
04762ff36b2231aac75754ab9c1a564a /backups/memtest
04762ff36b2231aac75754ab9c1a564a /backups/memtest

The three MD5 values above should be identical. If they are not – your system has a faulty RAM chip.

When you have eliminated hardware problems as possible causes of high iowait, the next step is to review firmware and drivers. You are particularly interested in disk controller firmware: unstable performance and no error messages are the signs of a firmware problem. Try really hard to remember if you made any system changes recently, especially something that required a reboot – like kernel upgrade, for example. If this is the case, roll back the upgrade or search for upgrade firmware. You should grab a copy of Sysinfo (free 30-day trial) to help you identify makes and models of your disks, controllers, etc.

While your disks and controllers may be tip-top, your may have a problem with a filesystem. Even if you see high iowait when accessing any filesystem, you should still check out the partition where /var is mounted and swap – if there is a problem, it will manifest itself regardless of what your system is doing. But here you will run into a little problem: fsck will not scan a mounted partition and you cannot unmount /var. Let’s say these are your partitions:

deathstar:/ # more /etc/fstab
/dev/hda2 / reiserfs acl,user_xattr 1 1
/dev/hda1 swap swap pri=42 0 0

You need to fsck /dev/hda2 because this is where your /var is mounted. Download KNOPPIX or Ubuntu LiveCD, boot from CD (without installing) and “fsck /dev/hda2? from there. If everything looks clean, shut down your system, take the CD out and boot normally. The next step is to check out swap. If you just run fsck on the swap partition, it will fail:

deathstar:/ # fsck /dev/hda1
fsck 1.34 (25-Jul-2003)
fsck: fsck.swap: not found
fsck: Error 2 while executing fsck.swap for /dev/hda1

You need to disable swap on /dev/hda1 before you can scan it. Before you can do this, you need to add another swap area: you cannot run without any swap space. So, to add swap on the fly, create a swap file (1Gb in this example):

deathstar:/ # dd if=/dev/zero of=/swapfile bs=1024 count=1048576
1048576+0 records in
1048576+0 records out

deathstar:/ # chmod 600 /swapfile

deathstar:/ # ls -lash /swapfile
1.1G -rw——- 1 root root 1.0G Dec 20 22:48 /swapfile

Now you can set up and activate the new swap file:

deathstar:/ # mkswap /swapfile
Setting up swapspace version 1, size = 1073737 kB
deathstar:/ # free
total used free shared buffers cached
Mem: 508644 500996 7648 0 38912 147332
-/+ buffers/cache: 314752 193892
Swap: 1020088 521784 498304
deathstar:/ # swapon /swapfile
deathstar:/ # free
total used free shared buffers cached
Mem: 508644 502232 6412 0 39400 147392
-/+ buffers/cache: 315440 193204
Swap: 2068656 521784 1546872

Now we need to deactivate the original swap partition. This operation may take a couple minutes to complete:

deathstar:/ # swapoff /dev/hda1
deathstar:/ # free
total used free shared buffers cached
Mem: 508644 501624 7020 0 31712 10416
-/+ buffers/cache: 459496 49148
Swap: 1048568 167032 881536

The next step is to create a standard filesystem on the old swap partition, so that fsck has something to scan:

deathstar:/ # mke2fs -c /dev/hda1
mke2fs 1.34 (25-Jul-2003)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
127744 inodes, 255024 blocks
12751 blocks (5.00%) reserved for the super user
First data block=0
8 block groups
32768 blocks per group, 32768 fragments per group
15968 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376
Checking for bad blocks (read-only test): done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

The previous operation already ran fsck and so, if you see no errors, you can now re-activate your original swap space and remove the temporary swap you created:

deathstar:/ # mkswap /dev/hda1
Setting up swapspace version 1, size = 1044574 kB
deathstar:/ # swapon /dev/hda1
deathstar:/ # swapoff /swapfile
deathstar:/ # rm /swapfile
deathstar:/ # free
total used free shared buffers cached
Mem: 508644 503172 5472 0 33668 9256
-/+ buffers/cache: 460248 48396
Swap: 1020088 156300 863788

Anothe command commonly used for analyzing system bottlenecks is vmstat. The following example runs vmstat five times at 2-second intervals:

deathstar:~ # vmstat -S M 2 5
procs ———–memory———- —swap– —–io—- –system– —-cpu—-
r b swpd free buff cache si so bi bo in cs us sy id wa
0 0 15 174 70 58 0 0 189 50 5 6 1 3 94 1
0 0 15 174 70 58 0 0 0 0 1005 35 4 0 96 0
0 1 15 174 70 58 0 0 0 258 1515 45 0 6 88 7
0 0 15 173 71 58 0 0 0 194 1083 24 0 1 83 16
0 0 15 173 71 58 0 0 0 0 1003 19 0 0 100 0

Explanation of vmstat columns:

(a) procs is the process-related fields are:

* r: The number of processes waiting for run time.
* b: The number of processes in uninterruptible sleep.

(b) memory is the memory-related fields are:

* swpd: the amount of virtual memory used.
* free: the amount of idle memory.
* buff: the amount of memory used as buffers.
* cache: the amount of memory used as cache.

(c) swap is swap-related fields are:

* si: Amount of memory swapped in from disk (/s).
* so: Amount of memory swapped to disk (/s).

(d) io is the I/O-related fields are:

* bi: Blocks received from a block device (blocks/s).
* bo: Blocks sent to a block device (blocks/s).

(e) system is the system-related fields are:

* in: The number of interrupts per second, including the clock.
* cs: The number of context switches per second.

(f) cpu is the CPU-related fields are:

These are percentages of total CPU time.

* us: Time spent running non-kernel code. (user time, including nice time)
* sy: Time spent running kernel code. (system time)
* id: Time spent idle. Prior to Linux 2.5.41, this includes IO-wait time.
* wa: Time spent waiting for IO. Prior to Linux 2.5.41, shown as zero.

If you failed to identify the cause of the iowait problem, you should consider the possibility that there is no problem: perhaps your system is handling extra load and running short on resources. Take a look at the running processes and see what’s eating up memory. Perhaps you upgraded an application and now it is using more RAM, which leads to high swapping, which leads to high disk activity, which leads to high iowait.

The solutions are simple:

1. Install more RAM
2. Move swap to another disk or – even better – move it to another disk on a separate controller.
3. Move user applications to another disk/controller and specify default log locations outside of the system disk.

How to choose the right Linux VPS plan

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Linux VPS plans are an excellent choice for website owners or administrators who wish to benefit from the flexibility and infrastructure of private servers, but the cost effectiveness of shared hosting. A profitable combination between shared hosting and dedicated servers, Linux VPS hosting allow users to run all popular distributions, from Ubuntu and CentOS to Fedora and Debian. Since virtual private servers have become such a popular choice these days, there are now plenty of providers available and countless possible plans for users to select from, which is why it is important to research your options properly before committing to a plan, as well as to a certain provider. Fortunately, the Internet comes as a great help, as there are numerous resources available on the subject and you can read invaluable information with regard to Linux virtual private servers online. An informed decision is the best decision, but there are also particular factors that need to be taken into account when choosing your hosting plan.

No matter how promising an ad or deal may appear, before choosing or subscribing to a Linux VPS hosting plan from any provider, you need to accurately assess your needs. If you are running a static website, then your memory needs and storage use shouldn’t be as high as if you were running, let’s say, a database driven website, in which case you should also consider a higher value of premium traffic. There are also different packages and plans for WordPress blogs and Magento systems, in accordance to particular necessities of this type of websites, so if you have this kind of site look for providers that offer special packaging for it. As far as RAM memory goes, you will be able to find virtual private servers hosting plans that vary from 256MB to 8192MB, so there is no lack of alternatives. You just need, as mentioned above, to assess your requirements and needs accurately, in order to make the most suitable choice. Traffic amount also varies significantly and you will be able to find plans that offer 250GB of premium traffic and others that give you 4TB of premium traffic.

Apart from the type of website you are running and your traffic needs, another factor that you should take into consideration when choosing the right Linux VPS hosting plan is value for money. Since there now are so many providers that offer this service, the competition between them is fierce and with a thorough research, you will be able to find a reasonable offer for what you need. Truthfully, you might come across attractive packages that provide great storage space, high amounts of premium traffic, proper CPU power and excellent RAM size, but the asking price may be way above the market value or simply above what you can afford. Take your time and compare providers and plans as well and you will certainly find a hosting plan that gives you best value for money.

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Linux Support For Your Innovative Operative System

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Linux is another type of operating system which has become increasingly popular worldwide and is growing in the number of users. Linux is an operating system which loads itself into the memory of a computer or laptop and manages the resources that are made available on your system.

It is always beneficial to have an operating system which is technologically fast and reliable and many users have found the Linux operating system to be just that. The system works by providing the resources that the computer system wants to use and helps to run the facilities on the computer or laptop more efficiently.

The efficient Linux system provides laptops or computers with a task scheduler which can be allocated to carry out a number of different tasks. These can be done while the computer is running or when the power is off. The task scheduler is normally responsible on the operating system for when you want to print a document or download a file onto a PowerPoint presentation and other additional tasks.

These additional features that the operating system can conjure up include managing the disks and network in order to create and maintain directories on the computer and request files to bring them to a specific disc on your computer. The Linux operating system also features a network manager which allows the system to control the data that is moving from the system to the network. This is particularly important when downloading from the internet or from a server online. There may be times when you may need extra assistance in your operating system if the transfer of information is slow or is not happening efficiently.

Most operating systems such as Linux are relatively fast at processing information on the computer or laptop and transferring information from one place to another. It is important to check out the various operating systems available according to your computer or laptop so you can assess the overall performance of your computer based on the type of operating system it runs on.

Many people are choosing the Linux operating system over more popular choices such as Windows due to its functionality and transferal of data which is fast and can be done quickly. There are also many businesses that use computers which run on the Linux operating system making it an investable operating system for downloading and controlling the transfer of data and information from online resources. Inevitably, as with any operating systems, you may find that a problem may arise and it is best to sort out these problems as early as you can.

There are many other features that the Linux operating system boasts such as being the memory manager for controlling the features of the systems RAM and creating a large virtual memory by using a file on the hard disc. In addition to the responsibilities of the operating system the Linux can often need additional help in tackling problems that may arise such as a virus on the operating system. It is therefore worth considering Linux Support groups or specialists that can assist you to help you fix your operating system efficiently.

Why Linux Is Better Than Windows Os

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Are you tired with your Windows operating system? Is the OS infected by viruses and there are security issues as well? Well, you are not alone; there are many Windows users who are not satisfied with this operating system. Microsoft Windows might be the most popular operating system in the market but there are a number of flaws. On the other hand, Linux, a less popular operating system, is much better in terms offering hassle-free computing experience. To get Windows OS you need to pay a hefty amount while Linux is absolutely free. In this article you will find why Linux is a better choice than Microsoft Windows.

More of the complaints we get regarding Windows PC is about virus infection. You cannot use a Windows machine without antivirus or anti-malware. Even after having these programs installed there is still the risk of getting infected. Sometimes, viruses and malware even get past the best solutions. On the other hand, with Linux, you don’t need to worry at all about this. Your PC will hardly get infected with viruses with this operating system.

Windows PC users also experience mysterious slowdowns. The reason behind this shut down could be diverse. It might be due to viruses, malware, fragmentation, or any other operating system issues. You will never experience these types of abnormal behavior from on a Linux computer. Not only the operating system but also you will hardly face mysterious application crashes with Linux.

Troubleshooting on Linux computer is very easy as you can go through the /var/log to find out what is ailing your PC. If you are not well familiar with the command line, you can install a GUI. This will allow you to scan through the log files and check for the issues and fix it. You can also run a backtrace to get all of the debugging information for the applications that you are facing problem with. Then you can fix the issues on your own or contact an Operating system support provider for Linux support.

Another most important thing is resource. To run a Windows 7 desktop your computer must have minimum 1 GB of RAM. Anything less than that makes your computer appear like it is working on slow motion. With Linux operating system you will never face this type of problem. Linux requires very less amount of hardware resources. This OS make the use of RAM to cache files until the cached files goes above the amount of RAM available on the computer. On the other hand, Windows 7 OS uses memory for cache only for the cases if there is no other demand for said memory.

These are some notable features of Linux operating system which make them stand at much better position than Windows OS in terms of safe and secure computing.

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