hi, when creating of files or direcotries, what are the reasons when
to use filesystems or raw device?

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In article <c06e4d68.0408090334.73b78871@posting.google.com>,
yls177@hotmail.com (yls177) wrote:

> hi, when creating of files or direcotries, what are the reasons when
> to use filesystems or raw device?

You can create files and directories on a raw device?  This is news to
me.  I thought, by definition, that raw disk devices had no filesystem
structure on them.

Typically, you use a raw device for a DBMS so that the additional
filesystem layer with it's caching and overhead are removed from the
performance equation.  In practice, backing up and restoring such
devices is difficult without a 3rd-party backup tool.  dd is about it on
vanilla Solaris.  Everything else I know of requires a filesystem
structure.  As a former sysadmin, I hated raw partitions and gladly the
DBA agreed with me.

--
DeeDee, don't press that button!  DeeDee!  NO!  Dee...

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yls177 wrote:
>
> hi, when creating of files or direcotries, what are the reasons when
> to use filesystems or raw device?

Filesystems contain files or directories.  Raw devices do not.  If
you wish to create a directrory inside of a raw device, a filesystem
is the only way.

So it has something to do with what's possible and impossible at
that layer of the onion.

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dfreybur@yahoo.com (Doug Freyburger) wrote in message news:<7960d3ee.0408091100.4376b1a4@pos
ting.google.com>...
> yls177 wrote: 
>
> Filesystems contain files or directories.  Raw devices do not.  If
> you wish to create a directrory inside of a raw device, a filesystem
> is the only way.
>
> So it has something to do with what's possible and impossible at
> that layer of the onion.

filesystems are a logical thing. they resides on the disks itself and
their access is through the file system buffer cache, and they uses
block devices.

for raw devices, they bypasses the file system's buffer cache and
gives you "direct" access to the device.

i search the web and find the below
"There are two ways to access a device (hard drive, floppy, CD-ROM,
what
have you).  One is the standard method, which uses block devices.  The
other method for accessing these devices is via the raw device
interface.  You're accessing the same hardware, but the raw device
bypasses the file system's buffer cache and gives you "direct" access
to
the device.  This can be very convenient for particular purposes, but
you need to be careful if you ever intend to mix block and raw
accesses
to the same device."

1) filesystem is in the block device category?
2) when will we decide which category(block or raw) to use?

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yls177 wrote:
> Doug Freyburger wrote:
> 
>
> filesystems are a logical thing. they resides on the disks itself and
> their access is through the file system buffer cache, and they uses
> block devices.
>
> for raw devices, they bypasses the file system's buffer cache and
> gives you "direct" access to the device.

Realistically there's no difference between raw devices and block
devices like that.  It is a historical distinction no longer used.

> 1) filesystem is in the block device category?

Irrelevant block vs raw.  Filesystem is formatting method.  Raw
or block is unformatted method.  Filesystems live inside of
logical volumes, block devices, raw devices, etc.

Filesystem is inside of block device.  Layer of the onion not
a question of categories.

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dfreybur@yahoo.com (Doug Freyburger) wrote in message news:<7960d3ee.0408200847.2c551ca6@pos
ting.google.com>...
> yls177 wrote: 
>
> Realistically there's no difference between raw devices and block
> devices like that.  It is a historical distinction no longer used.
> 
>
> Irrelevant block vs raw.  Filesystem is formatting method.  Raw
> or block is unformatted method.  Filesystems live inside of
> logical volumes, block devices, raw devices, etc.
>
> Filesystem is inside of block device.  Layer of the onion not
> a question of categories.



hi, thanks for replying. So the difference between raw (or block) and
filesystem is the format method. Filesystem is format method while the
other is unformat.

Could u please elaborate more about this?

Thanks

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yls177 wrote:
> Doug Freyburger wrote:
> 
...[vbcol=seagreen] 
>
> So the difference between raw (or block) and
> filesystem is the format method. Filesystem is format method while the
> other is unformat.

No, the primary difference is what layer of the onion.  Devices are
at a layer that doesn't address the content inside the device.
Filesystems are at the layer of the onion that does include the
content so filesystems are the format of the content.

In layered modular design of Unix is important to understand.  Learn
what layers contain what function and you will be able to understand
the difference.  Fail to learn the layered nature of Unix and you
will continue asking the question in a way that doesn't lead to a
sensible answer.  Layer after layer, Unix peels like an onion.
Learn what is in each layer.

In fact, the 7-layer ISO networking model is expressed like it is
a Unix system.  Starting at the hardware and moving out to the user
interface.  Your filesystem or raw device is rather like asking
layer 2 or 3 in the ISO networking model.

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