RAMFS vs TMPFS on Linux

Using ramfs or tmpfs you can allocate part of the physical memory to be used as a partition. You can mount this partition and start writing and reading files like a hard disk partition. Since you’ll be reading and writing to the RAM, it will be faster.

When a vital process becomes drastically slow because of disk writes, you can choose either ramfs or tmpfs file systems for writing files to the RAM.


Both tmpfs and ramfs mount will give you the power of fast reading and writing files from and to the primary memory. When you test this on a small file, you may not see a huge difference. You’ll notice the difference only when you write large amount of data to a file with some other processing overhead such as network.

1. How to mount Tmpfs

# mkdir -p /mnt/tmp  # mount -t tmpfs -o size=20m tmpfs /mnt/tmp

The last line in the following df -k shows the above mounted /mnt/tmp tmpfs file system.

# df -k Filesystem      1K-blocks  Used     Available Use%  Mounted on /dev/sda2       32705400   5002488  26041576  17%   / /dev/sda1       194442     18567    165836    11%   /boot tmpfs           517320     0        517320    0%    /dev/shm tmpfs           20480      0        20480     0%    /mnt/tmp

2. How to mount Ramfs

# mkdir -p /mnt/ram  # mount -t ramfs -o size=20m ramfs /mnt/ram

The last line in the following mount command shows the above mounted /mnt/ram ramfs file system.

# mount /dev/sda2 on / type ext3 (rw) proc on /proc type proc (rw) sysfs on /sys type sysfs (rw) devpts on /dev/pts type devpts (rw,gid=5,mode=620) /dev/sda1 on /boot type ext3 (rw) tmpfs on /dev/shm type tmpfs (rw) none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw) sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw) fusectl on /sys/fs/fuse/connections type fusectl (rw) tmpfs on /mnt/tmp type tmpfs (rw,size=20m) ramfs on /mnt/ram type ramfs (rw,size=20m)

You can mount ramfs and tmpfs during boot time by adding an entry to the /etc/fstab.

3. Ramfs vs Tmpfs

Primarily both ramfs and tmpfs does the same thing with few minor differences.

• Ramfs will grow dynamically. So, you need control the process that writes the data to make sure ramfs doesn’t go above the available RAM size in the system. Let us say you have 2GB of RAM on your system and created a 1 GB ramfs and mounted as /tmp/ram. When the total size of the /tmp/ram crosses 1GB, you can still write data to it. System will not stop you from writing data more than 1GB. However, when it goes above total RAM size of 2GB, the system may hang, as there is no place in the RAM to keep the data.
• Tmpfs will not grow dynamically. It would not allow you to write more than the size you’ve specified while mounting the tmpfs. So, you don’t need to worry about controlling the process that writes the data to make sure tmpfs doesn’t go above the specified limit. It may give errors similar to “No space left on device”.
• Tmpfs uses swap.
• Ramfs does not use swap.

4. Disadvantages of Ramfs and Tmpfs

Since both ramfs and tmpfs is writing to the system RAM, it would get deleted once the system gets rebooted, or crashed. So, you should write a process to pick up the data from ramfs/tmpfs to disk in periodic intervals. You can also write a process to write down the data from ramfs/tmpfs to disk while the system is shutting down. But, this will not help you in the time of system crash.

Table: Comparison of ramfs and tmpfs

Experimentation	Tmpfs	Ramfs
Fill maximum space and continue writing	Will display error	Will continue writing
Fixed Size	Yes	No
Uses Swap	Yes	No
Volatile Storage	Yes	Yes

If you want your process to write faster, opting for tmpfs is a better choice with precautions about the system crash.

CentOS: find out what network ports are in use

To get a quick idea of what ports you have open on your local box, you can use nmap.

[root@api1dev /usr/local/bin]nmap localhost
Starting Nmap 5.00 ( http://nmap.org ) at 2010-05-10 03:02 EDT
Interesting ports on vmlinux-testing1 (127.0.0.1):
Not shown: 991 closed ports
PORT     STATE SERVICE
21/tcp   open  ftp
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
389/tcp  open  ldap
631/tcp  open  ipp
726/tcp  open  unknown
8009/tcp open  ajp13
8080/tcp open  http-proxy

For more detailed information, try netstat:

netstat -an

You’ll get a breakdown of every socket open on your machine – useful for figuring out who’s connected and from where.

Tweaking hard disk on Linux

 
hdparm -Tt /dev/hda

/dev/hda:
Timing buffer-cache reads:   128 MB in  1.34 seconds =95.52 MB/sec
Timing buffered disk reads:  64 MB in 17.86 seconds = 3.58 MB/sec

hdparm /dev/hda

/dev/hda:
multcount    =  0 (off)
I/O support  =  0 (default 16-bit)
unmaskirq    =  0 (off)
using_dma    =  0 (off)
keepsettings =  0 (off)
nowerr       =  0 (off)
readonly     =  0 (off)
readahead    =  8 (on)
geometry     = 1870/255/63, sectors = 30043440, start = 0

1. multcount: Short for multiple sector count. This controls how many sectors are fetched from the disk in a single I/O interrupt. Almost all modern IDE drives support this. The man page claims: when this feature is enabled, it typically reduces operating system overhead for disk I/O by 30-50%. On many systems, it also provides increased data throughput of anywhere from 5% to 50%.
2. I/O support: This is a big one. This flag controls how data is passed from the PCI bus to the controller. Almost all modern controller chipsets support mode 3, or 32-bit mode w/sync. Some even support 32-bit async. Turning this on will almost certainly double your throughput (see below.)
3. unmaskirq: Turning this on will allow Linux to unmask other interrupts while processing a disk interrupt. What does that mean? It lets Linux attend to other interrupt-related tasks (i.e., network traffic) while waiting for your disk to return with the data it asked for. It should improve overall system response time, but be warned: Not all hardware configurations will be able to handle it. See the manpage.
4. using_dma: DMA can be a tricky business. If you can get your controller and drive using a DMA mode, do it. But I have seen more than one machine hang while playing with this option.

 
hdparm -X66 -d1 -u1 -m16 -c3 /dev/hda:
setting 32-bit I/O support flag to 3
setting multcount to 16
setting unmaskirq to 1 (on)
setting using_dma to 1 (on)
setting xfermode to 66 (UltraDMA mode2)
multcount    = 16 (on)
I/O support  =  3 (32-bit w/sync)
unmaskirq    =  1 (on)
using_dma    =  1 (on)

hdparm -tT /dev/hda

/dev/hda:
Timing buffer-cache reads:   128 MB in  1.43 seconds = 89.51 MB/sec
Timing buffered disk reads:  64 MB in  3.18 seconds = 20.13 MB/sec

System calls analysys tool

strace -cp 18875

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 77.61    0.019695           0    782458       read
 12.31    0.003123           0      9456        write
  3.60    0.000913           0     47291        fcntl64
  3.32    0.000843           0      9463         open
  1.34    0.000341           0     18920        close
  1.27    0.000323           0     18914        dup2
  0.55    0.000140           0      9457         rt_sigprocmask
  0.00    0.000000           0         6           fstat64
  0.00    0.000000           0        12          getdents64
------ ----------- ----------- --------- --------- ----------------
100.00    0.025378                895977           total

Change hostname in Linux

First you need to find out your hostname, you can do this with

$ hostname
localhost.localdomain
$

Edit /etc/hosts

You need to edit /etc/hosts and add a line for your host name

$ cat /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1               localhost.localdomain localhost
$

My new server IP is 72.232.196.90, i need to assign it hostname server12.hosthat.com, to do this, i have edited /etc/hosts as follows.

# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1               localhost.localdomain localhost
72.232.196.90           server12.hosthat.com server12

Edit /etc/sysconfig/network

First lets see what is in the file

$ cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=localhost.localdomain
$

To change servers hostname to server12.hosthat.com, change the file as follows.

$ cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=server12.hosthat.com
$

Now you need to reboot the server to change the hostname.