Overlay ramdisk on filesystem?

Overlay ramdisk on filesystem?

[Timothy Miller]

I have a feeling that this may be a bit too off-topic, but I'm doing 
some Linux and hardware performance tests, and some of the tests will 
put the hardware into an unstable state which could get memory errors 
which could cause filesystem corruption.

I would like to know how I could overlay a RAM disk over a read-only 
filesystem so that all new files and modified files end up in the RAM 
disk, but old files are read from the disk.  This way, when I reboot, 
the disk reverts back.

Suggestions?

Thanks.

Re: Overlay ramdisk on filesystem?

[Florian Weimer]

Timothy Miller <miller@techsource.com> writes:

> I have a feeling that this may be a bit too off-topic, but I'm doing
> some Linux and hardware performance tests, and some of the tests will
> put the hardware into an unstable state which could get memory errors
> which could cause filesystem corruption.

In the presence of memory errors, all bets are off anyway.

> I would like to know how I could overlay a RAM disk over a read-only
> filesystem so that all new files and modified files end up in the RAM
> disk, but old files are read from the disk.  This way, when I reboot,
> the disk reverts back.

You might have to tweak the underlying file system, too.  IIRC, ext2
avoids to reallocate freshly deallocated blocks, to prevent
fragmentation.  This will waste a lot of RAM on the long run.

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Re: Overlay ramdisk on filesystem?

[Erik Tews]

Am Do, den 15.04.2004 schrieb Timothy Miller um 23:08:
> I have a feeling that this may be a bit too off-topic, but I'm doing 
> some Linux and hardware performance tests, and some of the tests will 
> put the hardware into an unstable state which could get memory errors 
> which could cause filesystem corruption.
> 
> I would like to know how I could overlay a RAM disk over a read-only 
> filesystem so that all new files and modified files end up in the RAM 
> disk, but old files are read from the disk.  This way, when I reboot, 
> the disk reverts back.

This could be possible using a ramdisk, a filesystem on a disk (which
could be a read only block device like a cdrom too), device-mapper and
its snapshot-target. But I did not try yet.