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RamDisc FAQs...
Q. What is the difference between disk buffer cache and a ramdisk?
A. A copy of the data retrieved from a disk drive is stored in a disk
buffer cache (a portion of main memory). If the same data is needed
again and it is still in the disk buffer cache then the O/S will attempt to
retrieve the data from the cache instead of getting it from the disk
drive (a slower access device). Data that has not been accessed for
a certain time period will be flushed out of the buffer cache. The
time period depends on much memory was assigned for the cache and if
any applications need extra memory and the amount of disk activity.
There is no guarantee that the data will still be in the cache when
it is needed again. By using RamDisc, the disk drive is not accessed
to retrieve data even if the data is not in the buffer cache because
it is still (and guaranteed to remain) in main memory until you decide
to deallocate the memory from the ramdisc.
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Q. Is there a way to view the performance differences before ordering?
A. Yes, we offer a live online demonstration comparing one of Seagate's
fastest disk drives to our RamDisc product: RamDisc Comparison Demo
There are benchmark reports posted on our website comparing RamDisc
to various disk drives and RAID arrays. We offer free demo licenses
if you wanted to try out our product for yourself on your machine.
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Q. Does the ramdisk swap out to disk when running other applications?
A. No, swapping to the disk drive would defeat the purpose of a ramdisk
filesystem since the swap space is really stored on the disk drive.
During the creation of a ramdisk, the memory used is locked from the
OS or other applications so no swapping occurs.
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Q. Why can't I use all of the lockable memory reported from dmesg?
A. The memory used for the ramdisk gets allocated early in the bootup
process. There are many programs started afterwards that still need
some lockable memory in order to execute. The RamDisc configuration
utility reserves some memory for the functionality of other programs.
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Q. Why do I have to reboot when creating new LVM Ram Volume Groups?
A. Simply that there is no guarantee that the memory required to build
a new ramdisc will be currently available at all times. However, the
memory may be deallocated from the ramdisc without a reboot when it
is no longer needed.
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Q. Why is the LAN id used for the license not the primary LAN (lan0)?
A. If your system has multiple LAN cards, the license may not be using
the onboard or first installed LAN device. The RamDisc utility uses
OS calls to get a MAC address. Whatever device appears first is the
one that we use for our license. That is why the licreq.exe program
has to be executed instead of the lanscan command.
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Q. Can I get a demo copy of your RamDisc software?
A. Yes, all you need to do is download the latest ramplus-revxxx.tar
file and the licreq.exe onto your hp-ux system. Execute licreq.exe
and send the output back to us with how many days for a demo. If you
need more time, you can obtain an extension license online. The only
difference between the demo and runtime programs is the expiration
date in the license code. All you need to do is change the license
code should you decide to purchase RamDisc.
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Q. How much does your RamDisc software cost?
A. The cost is based upon the maximum amount of memory that you may use
with the software. The RamDisc Plus price list may be found on our
RamDisc Plus software page.
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Q. What type of test should I do to prove the speed of the ramdisk?
A. Try not to use commands like tar that have built-in blocking factors
(input and output buffers). The dd command works well but try some
different block sizes (bs=) to see if there is a difference. Use of
the date command before and after instead of the timex command will
only give you approximate times. Here are some examples:
Create a 256MB file on the ramdisk in 0.87 seconds:
timex dd if=/dev/zero of=/ramdisc/ddtest.img bs=262144 count=1000
Here we send ddtest.img to /dev/null using two different commands:
timex tar cbf 20 /dev/null /ramdisc/ddtest.img (used 5.45 seconds)
timex cat /ramdisc/ddtest.img > /dev/null (only used 0.53 seconds)
A disk drive may report the same results as a ramdisk if using the
tar command with the method shown above.
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Q. How do I recover RamDisc volume groups after running a vgscan command?
A. The man page for the vgscan command states: This command should be run
only in the event of a catastrophic error such as the deletion of the
/etc/lvmtab file or the mismatch of names of the physical volumes in
the /etc/lvmtab file to the actual physical volume path configuration.
This command will re-construct /etc/lvmtab by searching throughout the
typical disk I/O of the system and find physical volumes that belong
to each volume group. However, the vgscan command is not aware of the
RamDisc device belonging to RamDisc volume groups and does not include
them in the newly created /etc/lvmtab file. Based on how your RamDisc
is configured, please review the instructions for recovery from vgscan
Please contact RamDisc technical support if you have any questions or
concerns before attempting any scripted command or file edit listed in
the instructions. Upgrade your current RamDisc software to revision
4.03 or greater. The support contract allows an online license upgrade
from release 3 to 4 using our website online support utilities.
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Q. How do I configure ServiceGuard to ingore RamDisc volume groups?
A. There are a couple of ServiceGuard commands that will attempt to scan
all LVM volume groups configured (even those that are not included in
the cluster) while performing their tasks. The command may result with
an error indicating a problem with /dev/vr0X (the X matching a RamDisc
volume group) or /dev/dsk/crtXd0 (RamDisc device files). This type of
error occurs when the command attempts to gather physical disk device
LVM data on a RamDisc device file. Please review the known workarounds
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Q. How do I modify Dynamic Root Disks to ingore RamDisc volume groups?
A. There are several options of the drd command that will attempt to scan
all LVM volume groups configured (even the non-Root disk volume groups)
while performing their tasks. The drd command may result with an error
indicating a problem while "Reading Current System Information". This
type of error occurs when the command attempts to gather physical disk
info using a RamDisc device file. Please review the known workarounds
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Q. Are revision updates required in order to remain on annual support?
A. There are no mandates to upgrade your software to the current revision
of RamDisc during the annual renewal of support. We do understand some
companies have policies or procedures in place to document any changes
in software (including revsion upgrade verifications) so if a customer
is satisfied with the revsion of RamDisc they have in place, so are we.
However, if a customer loads an Operating System patch (for example)
then encounters a RamDisc issue that has already been indentified and
resolved in a newer revsion of RamDisc, the current revision should be
installed to correct the issue. The current revision can be found here.
