Eventually Persistent Ram Disk

EPRD is an eventually persistent ramdisk.

This blockdevice can be configured in a number of ways. The first way to configure EPRD is to assign a certain amount of ram to eprd. In this mode no disk io will ever take place except when the device is configured or unconfigured. When the device is created it will simply load the data from disk in memory. When the device is remove the data will be save to disk.

You can create this type of disk backed ramdisk like this: insmod ./eprd.ko ./eprd_setup -f /data/persramdisk.img -s 1G -c Now device /dev/eprda will be created.

To unload the device simply type : rmmod eprd or eprd_setup -d /dev/eprda

** Please note that the -c parameter means create and should only be used the first time. **

You can however also create a ramdisk that will have it's changes flushed to disk every N seconds and instead of keeping everything in ram it only allocated 512M for caching.

./eprd_setup -f /data/saverimg -s 10G -m 3 -c -p512M

In this example the data is flushed to disk every 3 seconds.

Although this is already somewhat safer there is an even more 'reliable' mode of operation.

./eprd_setup -f /data/saverimg -s 10G -m 3 -c -p512M -b

EPRD supports barriers. So when the device is configured with the -b option a sync() on the filesystem will mean a sync of the blockdevice and all 'dirty' blocks in the cache will be flushed to disk.

You can easily test the effect of barrier support when you format eprd with ext4 or xfs. In this example I will use ext4 mkfs.ext4 /dev/eprda mount /dev/eprda /mnt -obarrier=0

root@saturn:/mnt# writetest running: 1000 ops Elapsed time: 0.029147 seconds 1000 ops: 34308.85 ops per second

While after : mount -oremount,barrier=1 /mnt root@saturn:/mnt# writetest running: 1000 ops Elapsed time: 42.926026 seconds 1000 ops: 23.30 ops per second

As you can see the difference is huge.

EPRD can also be used with raw devices: ./eprd_setup -f /dev/sdc -m 3 -p512M -b

The example above creates a new eprd device that is backed by a raw device. Please note that you do not need to specify -c when a raw device is used.

tools/writetest is a small tool provided by Oracle for testing Berkeley DB performance.