Starting Share Files with NFS
What kind of data can we keep on NFS? The simplest answer is any kind, including the whole root system for diskless machines. In order to plan your system, you have to take into account that all files can be divided into two categories: shareable and unshareable data. The latter must be kept on a particular machine. For instance, device (and lock) files are not shareable. The size of some directories is stable for some and unstable for others.
Assuming we have a small system, we show /home as typical shareable data that should be located on the server host. On medium and large systems, it is useful to subdivide user home directories by using such names as /home/students, etc. The size of the $HOME directory is not static, of course, but to limit disk space for many people, you may set up a quota system on the servers.
What about another directories? Some of them have a static size, some change permanently. Shareable static directories include /usr and /opt, while unshareable static directories include /etc and /boot. Static variable directories include /var/mail and /var/spool/news, whereas unshareable variable directories include /var/run and /var/lock.
Many people do not put /usr/bin and /bin files on NFS because of speed, but this is not very critical. We believe an ordinary user does not notice any difference in calling a program from an NFS directory vs. from a local one. Therefore, if disk space is critical, you may mount mostly everything via NFS, importing files from the server machine while keeping space on client machines for other files, such as MP3s or two OSes.
Although you may keep everything on the server, the standard solution is to keep only the $HOME directories and documentations there, including man pages and other documentation. Sharing by means of NFS software that is used rarely and located typically under /opt and /usr/local, is also a very good idea. Two subdirectories should be shared via NFS: /usr/doc and /usr/share.
Now, how do we use the mount parameters in these cases? As discussed previously, the recommended options for $HOME are intr and hard. In this case, you can be sure the data will not be lost. For /usr/doc and similar subdirectories, however, this may not be the optimal choice. If you access man pages while the Net is overloaded, or even if the NFS server is down, your machine will wait until it is able to re-establish contact with your NFS server. In this case, using the soft option is better; the disconnect message will appear on the screen and the operation will be canceled. This option may be useful for noncritical data of any kind, including news and FTP directories. However, the standard choice is not to access /var/mail and /var/news via NFS; use special protocols such as imap, pop and nntp.
If you are concerned about the speed of the NFS connection, you also can play with the rsize and wsize options. The default size is set by the kernel and may or may not fit. For instance, either your Ethernet card or the kernel is not able to handle large blocks, something that usually happens with old hardware and old kernels. If you have a newer card, a larger size might help you reach better performance. In some cases you can increase the speed by five or more times, but you have to do some experiments with your network.
The easiest way of calculating the speed is to create a file located on NFS volume and find the time of creating. This is just an experimental file that may hold any kind of data, including zero sequence, so we will create it by means of the dd command, using /dev/zero as the source file. Command time calculates time of creating.
How large should the temporary file be? The rule is to have two or three times the size of the memory on the server machine. If you have 2MB of RAM there, the file should be 4MB. Sometimes it is hard to find a place for a such file, but in many cases 256MB on your NFS disk machine should be fine. To calculate the free area on your disk, use the command df. Then, assuming you are working on a client machine, unmount it and mount again, put the magic value 1,024 for rsize and wsize parameters. Then execute the dd command with time:
time dd if=/dev/zero of=/home/tempo bs=16k count=16384
We use /dev/zero as the source for /home/tempo in order to avoid calculating the time of reading the source. This command sends 16,384 16KB blocks of null bytes, creating a file with a total size of 16,000 bytes × 16,384/1024 kilobytes = 256 kilobytes or 256MB. Write the time down. Then calculate the time of reading, sending the file to /dev/null:
time dd if=/home/tempo of=/dev/null bs=16kRecord the time of reading, and then delete the temporary file:
rm /home/tempo verbatimRepeat the describing procedure at least three times and calculate the average reading/writing time. Unmount your client host:
umount /home verbatimMake sure you have NFS unmounted; it complains if there are errors and keeps silent if not. Then mount it again, using the new value 2,048 for rsize and wsize, and repeat the procedure above, again and again, up to 32,768.
Observe the result. The faster the transferring, the shorter the time. Find your optimal value and put it into /ets/fstab. Keep the paper with the calculations in a safe place.
There are also a few tips to help you make this list shorter. If you have a new network card and are sure your NFS server runs version 3, you may start from the value 32,768 and go down. This value should not be tested with NFS server version 2, however, since it only works with older kernels and some Solaris and SunOS machines.
We found the best result for kernel 2.2.19 to be value 4,096, so you may start from it and try the nearest, 2,048 and 8,192. For the new kernel, this value (in our case) was 8,192, but very often kernel 2.2.19 worked better in comparison with 2.4.6. It strongly depends on your hardware, so be very careful if you want to switch your NFS server into new kernel release.
The optimal value for client machines that use NFS over TCP/IP is 1,024 or less. This value also works if you have a router between a client and the NFS server.
Free DevOps eBooks, Videos, and more!
Regardless of where you are in your DevOps process, Linux Journal can help!
We offer here the DEFINITIVE DevOps for Dummies, a mobile Application Development Primer, and advice & help from the expert sources like:
- Linux Journal
- Android Candy: Google Keep
- Handling the workloads of the Future
- Readers' Choice Awards 2014
- How Can We Get Business to Care about Freedom, Openness and Interoperability?
- Days Between Dates?
- Synchronize Your Life with ownCloud
- diff -u: What's New in Kernel Development
- Computing without a Computer
- December 2014 Issue of Linux Journal: Readers' Choice