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Linux tmpfs
Mark Veltzer
mark@veltzer.net
tmpfs intro
● tmpfs is a linux filesystem that stores all of your
files in RAM.
● Available in all recent linux versions.
● Is used by some distributions by default for some
folders (/tmp, /dev, /var/run, /run).
● You can mount your own folders as tmpfs, but need
root or CAP_SYS_ADMIN to do so.
● Or you can just have a per-prepared tmpfs already
defined in /etc/fstab.
tmpfs read(2) performance
● tmpfs offers some performance benefits for
read and write operations.
● Actually the read difference between tmpfs
and regular file system is not that great and is
centered around the first time you read a
certain area of a file.
● In tmpfs this, and all subsequent reads, will be
a memcpy.
tmpfs read performance (cont)
● In a regular disk based file system the first read
will be slow while subsequent reads will
probably be from the OS page cache and so
will resolve to memcpy.
● But in tmpfs you are guaranteed to get memcpy
performance on both first and all subsequent
reads.
● And you can control the maximum size of your
tmpfs filesystem.
tmpfs write performance
● Write performance in tmpfs is always just a
memcpy to the kernel (if you have enough
space that is...)
● And you don't apply any pressure on any real
disk file system that you have running in
parallel and used by other programs.
● But the downside is that you data never
reaches any persistent storage...
tmpfs disadvantages
● When your system crashes (power failure, OS
bug, hardware failure, whatever) you lose your
tmpfs data.
● This is unlike disk based file systems where
you can guarantee your data will be there by
calling fsync, fdatasync, sync, sync_file_range.
● tmpfs also lacks some advanced file system
features (compression and others).
When should you use tmpfs?
● You should use tmpfs for faster access to data
which is not critical.
● Much like a memory cache.
● Actually it's better to use some in process
memory cache which will be more effective
because accessing it does not require system
calls.
● On the other hand you could mmap tmpfs files.
Using tmpfs
● First make a folder:
mkdir -p /mnt/tmp
● Then mount it:
mount -t tmpfs -o size=20m tmpfs /mnt/tmp
References
● http://www.thegeekstuff.com/2008/11/overview-of-ra
● http://www.linux-mag.com/id/7689/
● http://en.wikipedia.org/wiki/Tmpfs
● http://www.ibm.com/developerworks/library/l-fs3/ind
● http://fixunix.com/linux/1717-tmpfs-swap.html
References
● http://www.thegeekstuff.com/2008/11/overview-of-ra
● http://www.linux-mag.com/id/7689/
● http://en.wikipedia.org/wiki/Tmpfs
● http://www.ibm.com/developerworks/library/l-fs3/ind
● http://fixunix.com/linux/1717-tmpfs-swap.html

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Linux tmpfs

  • 2. tmpfs intro ● tmpfs is a linux filesystem that stores all of your files in RAM. ● Available in all recent linux versions. ● Is used by some distributions by default for some folders (/tmp, /dev, /var/run, /run). ● You can mount your own folders as tmpfs, but need root or CAP_SYS_ADMIN to do so. ● Or you can just have a per-prepared tmpfs already defined in /etc/fstab.
  • 3. tmpfs read(2) performance ● tmpfs offers some performance benefits for read and write operations. ● Actually the read difference between tmpfs and regular file system is not that great and is centered around the first time you read a certain area of a file. ● In tmpfs this, and all subsequent reads, will be a memcpy.
  • 4. tmpfs read performance (cont) ● In a regular disk based file system the first read will be slow while subsequent reads will probably be from the OS page cache and so will resolve to memcpy. ● But in tmpfs you are guaranteed to get memcpy performance on both first and all subsequent reads. ● And you can control the maximum size of your tmpfs filesystem.
  • 5. tmpfs write performance ● Write performance in tmpfs is always just a memcpy to the kernel (if you have enough space that is...) ● And you don't apply any pressure on any real disk file system that you have running in parallel and used by other programs. ● But the downside is that you data never reaches any persistent storage...
  • 6. tmpfs disadvantages ● When your system crashes (power failure, OS bug, hardware failure, whatever) you lose your tmpfs data. ● This is unlike disk based file systems where you can guarantee your data will be there by calling fsync, fdatasync, sync, sync_file_range. ● tmpfs also lacks some advanced file system features (compression and others).
  • 7. When should you use tmpfs? ● You should use tmpfs for faster access to data which is not critical. ● Much like a memory cache. ● Actually it's better to use some in process memory cache which will be more effective because accessing it does not require system calls. ● On the other hand you could mmap tmpfs files.
  • 8. Using tmpfs ● First make a folder: mkdir -p /mnt/tmp ● Then mount it: mount -t tmpfs -o size=20m tmpfs /mnt/tmp
  • 9. References ● http://www.thegeekstuff.com/2008/11/overview-of-ra ● http://www.linux-mag.com/id/7689/ ● http://en.wikipedia.org/wiki/Tmpfs ● http://www.ibm.com/developerworks/library/l-fs3/ind ● http://fixunix.com/linux/1717-tmpfs-swap.html
  • 10. References ● http://www.thegeekstuff.com/2008/11/overview-of-ra ● http://www.linux-mag.com/id/7689/ ● http://en.wikipedia.org/wiki/Tmpfs ● http://www.ibm.com/developerworks/library/l-fs3/ind ● http://fixunix.com/linux/1717-tmpfs-swap.html