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Tmpfs: Mounting a RAM filesystem on Solaris

Introduction

TmpFS (Temporary File System) is the generic name given to any temporary Unix file system. Any file created in such a filesystem disappears when the system is shut down.

The default implementation of tmpfs in Linux 2.6.x kernels is based on ramfs, which uses the caching mechanism to optimize memory management. It is also available on Solaris 10.

However, tmpfs additionally offers a memory size limit that is set at mount time and can be modified on-the-fly with the "remount" option for security purposes. Tmpfs also allows the system to use swap space when necessary, which provides an additional guarantee.

Unlike a RAM Disk, it allocates memory dynamically to avoid excessive usage and offers better performance due to its extreme simplicity.

Usage

Prerequisites

We will create a mount point at /media/montmpfs.

First, create the directory:

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mkdir -p /media/montmpfs

Then change the permissions on this directory so everyone can read/write/execute:

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chmod 777 /media/montmpfs

Mounting

Once the prerequisites are completed, we can mount the partition:

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mount -F tmpfs -o size=2048m  swap /media/montmpfs
  • -o size=2048m: specify the desired size. If none is defined, it will be the size of RAM + swap.
  • /media/montmpfs: mount point for tmpfs

To have this mount persist across reboots, add it to the vfstab file (/etc/vfstab):

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...
swap    -   /media/montmpfs    tmpfs   -   yes size=2048m
...

Expanding the Partition

You didn't plan enough space for your partition? We can expand it - it's not simple, but it's feasible.

WARNING: Performing this operation in production can be very risky

Retrieving Information

First, let's check the available space:

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> df -h /media/montmpfs
Filesystem             size   used  avail capacity  Mounted on
swap                   2,0G     272K   2,0G     1%    /media/montmpfs

Let's retrieve the memory address used for our tmpfs:

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> echo "::fsinfo" | mdb -k | egrep "VFSP|/media/montmpfs"
            VFSP FS              MOUNT
ffffffffbd1e10c0 tmpfs           /media/montmpfs

Here's the allocation address: ffffffffbd1e10c0.

Now, let's retrieve the address of the tm_anonmax variable so we can change its value later:

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> echo "ffffffffbd1e10c0::print vfs_t vfs_data | ::print -ta struct tmount tm_anonmax" | mdb -k
ffffffffbfb42068 ulong_t tm_anonmax = 0x80000

Here tm_anonmax (number of pages) is equal to 0x80000 (512kb) at memory address ffffffffbfb42068.

Applying the New Value

We will now change its value. Let's say we want to increase it to 3GB. First, we need to retrieve the default block size allocated for swap:

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> pagesize 
4096

So I have 4kb blocks.

If I want to change to 3GB, I need to calculate the value in hexadecimal:

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Desired size => desired size in kb / block size in kb = size in Kb = size in hexadecimal
3G => 3145728Kb / 4kb = 786432Kb = 0xC0000

Now let's use these values and apply them to the current memory address to expand it:

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> echo "ffffffffbfb42068/Z 0xC0000" | mdb -kw
0x3000f488d00:                  0x80000                 =       0xC0000

Let's verify that our changes were applied correctly:

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> echo "ffffffffbfb42068/J" | mdb -k
0x3000f488d00:             0xC0000

or

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> echo "ffffffffbfb42068::print vfs_t vfs_data | ::print struct tmount tm_anonmax" | mdb -k
tm_anonmax = 0xC0000

And check the result:

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> df -h /media/montmpfs
Filesystem             size   used  avail capacity  Mounted on
swap                   3,0G     272K   3,0G     1%    /media/montmpfs

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