Re: What is a Block Device?

JFS (Journaled File System) is AIX's native filesystem, like HP's HFS and DEC's AdvFS.
JFS is especially designed to offer recovery capabilities in case of a AIX system crash.
The term journaled refers to the fact that it keeps a log of recent operations on the filesystem so that is can redo the last changes and keep the filesystem consistent, roughly similar to the redo log in Oracle. Raw/block devices can be implemented with the Logical Volume Manager (LVM) in AIX.
JFS, like any other UNIX filesystem, has a big overhead over raw and block devices, in that in you must keep the datafiles in UNIX files, which must use the underlying consistency, indexing, buffering and journaling characteristics of the filesystem. Therefore, theoretically speaking, its performance would be slower.

You should consider the tradeoff:

raw/block devices: relatively faster
filesystem: relatively slower, much easier management (beacause you are dealing with regular OS files).

I have read some articles, and it seem a trend, saying that becasue of the overwhelming improvements on HD technology, the difference in speed between raw/block devices and filesystem does not make up for the trouble of managing awkward LVM. And it makes sense to me !

 So I'd consider using filesystem insetead of Raw/block revices.

regards,

Andre

Doug Cowles escreveu na mensagem <385C3C05.4860A348_at_nospambigfoot.com>...
>How does that differ from a JFS?
>
>- Dc
>
>Andre Whittick Nasser wrote:
>
>> Hi !
>>
>> Well, the difference between a block and a raw(or character) device is
that
>> a block device uses OS buffer cache and is more specific to physical
devices
>> that work with blocks (like disks). Raw devices bypass any kind of
buffer.
>> If you go to /dev and list the pseudofiles with "ls -l | more", you'll
find
>> a "c" before raw devices and a"b" before block devices.
>> At first, it would have a better performance because of the caching
effect.
>> And if the bock size is big, you'll feel the "spatial locality" working.
>> That is, in one read, you will bring along more data that is physically
>> close to that you originally wanted. And according to statistics, this
other
>> data will very likely be used in the near future.
>> Its maintenace difficulties are the same.
>>
>> Chakravarthy KM Nalamotu escreveu na mensagem
>> <83e1ja$au0$1_at_news.hawaii.edu>...
>> >Hello Oracle Users,
>> >According to Oracle8 tuning manual a block device gives
>> >better performance than a raw device and a UNIX file
>> >system when large block sizes are used.
>> >I know what a raw device is and what a UNIX file system is.
>> >What is a Block Device? How does it work? Does it have
>> >maintenance difficulties similar to raw device?
>> >Thank you for your responses.
>> > Kittu.
>> >
>
>
>
Received on Sun Dec 19 1999 - 08:57:57 CST