-=PCTechTalk=- Re: More external drive questions

Don,
    A cache is a long term holding area for data.  In certain situations, a 
cache can hold various pieces of data for long stretches at a time 
(semi-permanently in the case of your Temporary Internet files folder).  A 
hard drive's cache works similarly, but it can only hold data until the 
system is powered down (powering down forces the transfer of data from the 
cache to the drive platters).  Hard drive caches are often used to retain 
previously retrieved information such as part of the drive's file allocation 
table and the metadata associated with the files in that area so that it 
won't have to re-read that info the next time you access that data.  If you 
choose to access a different part of the drive and there is not enough space 
to retain the older info plus the new, the older info will be replaced by 
the newer info.  For a more 'real world' example of this, try Moving a large 
folder (containing at least 500MB of files & subfolders) to another 
location.  If you do this after a reboot or without having recently accessed 
the folder recently, you'll first see "Preparing to Move ..." in the 
transfer window in place of the words "Moving ...".  After a while (when 
it's through caching the metadata), it will start the Move process.  When it 
has completed this initial Move, try Moving the folder back to where it was 
originally.  Since all of that metadata is already cached, it will jump 
directly to "Moving ...".

    On the other hand, a buffer is a VERY short term cache (here's where 
those millisecond speeds come in) which exists to help speed up the transfer 
of data from one device to another.  By utilizing a buffer, the originating 
device can focus on sending to the buffer at a faster speed than the drive 
platters themselves can handle.  As soon as the originating device has 
passed on all of the data to the buffer, it can then move onto other things 
while the buffer completes the job of sending the remaining data to the 
drive's platters (or to a blank disk in the case of optical drives).

    It's important to note that the term "Buffer" is used improperly by 
nearly all storage device marketing departments to describe an area of an 
optical or hard drive's electronics that is used for caching, making all of 
this rather confusing to folks who don't catch the irony.  And no, the 
completion of a 'write' (the emptying of the buffer onto the drive platters) 
does not automatically make a drive safe for removal.  All of the data that 
has been added to the drive may now be safely tucked away from harm, but at 
least part of its metadata may still be sitting in the cache.  Until that 
additional info is properly added to the allocation table (FAT, FAT32, NTFS, 
etc.), the drive is not yet in full possession of the map to those newly 
written areas.  If you remove the drive in this state, the data that still 
has metadata in the cache will be lost since the drive will not specifically 
know where it resides.  While it's possible to retrieve this lost data, it 
should go without saying that it's not exactly a stress free procedure and 
should be avoided.

    Again, your best approach will always be to unmount a drive before 
removal and protect the overall system from blackouts with a good UPS.

Peace,
G

"The only dumb questions are the ones that are never asked"

----- Original Message ----- 
From: <dsw32952@xxxxxxxxx>
To: <pctechtalk@xxxxxxxxxxxxx>
Sent: Thursday, May 08, 2008 9:03 AM
Subject: -=PCTechTalk=- More external drive questions


> If I understand the use of a write cache correctly, everything that gets 
> written passes through the cache and it will eventually and probably 
> rather quickly write all its data to the drive in question and thus empty 
> itself.
> Correct?  If yes, how quickly? (probably microseconds?)
>
> Does the completion of the write make the drive safe to remove or is there 
> another step in the process?
>
> What's the difference between a buffer and a cache?  Does a cache suffer 
> overflow and underflow problems like a buffer?
>
> Don 


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