-=PCTechTalk=- Re: More external drive questions
- From: "Don101" <don101@xxxxxxxxxx>
- To: <pctechtalk@xxxxxxxxxxxxx>
- Date: Thu, 8 May 2008 22:16:39 -0400
In theory then, if the drive is set to NOT cache, it could be removed after
the activity light quits blinking?
You used the term "hard drive". Was that meant to be generic and include
internal and external hard drives and thumb drives, memory cards and other
external "mass storage" devices?
A hard drive has a Level 1 cache and a Level 2 cache. What's up with that?
Those caches are built into the drive. Where is the cache (s) for non-hard
drive external mass storage devices?
In the absence of a reboot, shutdown or make safe process will a cache
eventually write all that meta data to the drive?
This stuff is getting really interesting.
:-))
My next round of questions will cover removing software and partitions on
thumbdrives that were put there by the manufacturer. Also formatting and
choosing a file system for them.
Don
----- Original Message -----
From: "GuitarMan" <gman.pctt@xxxxxxxxx>
To: <pctechtalk@xxxxxxxxxxxxx>
Sent: Thursday, May 08, 2008 1:40 PM
Subject: -=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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