Re: What is the difference between loopback device and ramdisk device

[loody <miloody@xxxxxxxxx>]

2009/3/4 Peter Teoh <htmldeveloper@xxxxxxxxx>:
> all the question u asked are answered by yourself.   :=).
>
> but i do have some questions (MinChan or anyone else can help?):
>
> a.   after mounting the ramdisk or loopback file as devices, and
> configuring it as say, ext2 filesystem, all I/O processing go through
> the same processing sequence:   syscalls->vfs layer->ext2 FS
> layer->pagecache layer->block layer->block devices->loopback
> (drivers/block/loop.c) or ramdisk (drivers/block/brd.c) layer,
> correct?
>
> Now the problem with that is "double buffering" or duplicated caching
> -> one at the pagecache layer, and another at the ramdisk level.
>
> (don't forget another duplication of caching between userspace memory
> and pagecache).
>
> Or is Linux Kernel much smarter than that?
>
> b.   At the "block layer" processing as listed in sequence above, it
> will be subjected to I/O scheduling.   Correct?   But since ramdisk
> are not subjected to any I/O latency problems, should this read over
> write prioritizing (or batching of read/write) as done by I/O
> scheduler be unnecessary?
>
> On Tue, Mar 3, 2009 at 4:48 PM, loody <miloody@xxxxxxxxx> wrote:
>> Dear all:

>> 2. The only differences between them is loopdevice uses file as
>> virtual device and ramdisk uses memory as virtual device
>>   (if it is also right, why we first use dd if=file_name
>> of=/dev/loop, then we mount -o loop /dev/loop /mnt.
>>    It seems we copy the file content to a virtual memory device,
>> /dev/loop, then mount it.
>>    So we still use a block of ram to simulation the file operation, right?)
>> thanks for your help,
>> miloody
Hi:
Thanks for your help first.

If my above assumptions are correct.
from 2 above, could i say loopback is also a ramdisk device.
In ramdisk, we "dd if=/dev/null of=/dev/ram0"
in loopdevice, we "dd if=file_name of=/dev/loop"

it seems the only difference is loopback device copy the content of
file to /dev/loop, but ramdisk re-makes a ext2 in /dev/ram0.

appreciate your help,
miloody

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