The Glitch - A Friend in Computing

The Glitch was conceived during 2004, and started out simply as a site to assist people with computer problems. Today the site has grown considerably and has become a one-stop shop for many aspects of modern computing and communication


Solid State Drive (SSD)

Q. What is a Solid State Drive?


A. A Solid State Drive or SSD is a type of storage device (like a hard drive), but with no moving parts. They are manufactured using a type of flash memory (NAND), similar to that found in devices such as USB memory sticks.  

Q. What are the benefits of an SSD over a conventional hard drive?


A. There are many benefits to using a Solid State Device rather than a than conventional hard drive:

1. Performance - SSDs are much faster at both reading and writing
2. Power - An SSD uses a lot less power
3. Heat - SSDs only generate a very small amount of heat
4. Noise - SSDs don't make any noise
5. Weight - SSDs are generally much lighter
6. Reliability - High reliability because they have no moving parts

Q. Are there any disadvantages to using an SSD?


A. You could be forgiven for thinking SSDs are perfect, (almost but not quite), they do have a couple of problems:

1. Cost - Currently SSDs are comparatively expensive
2. Capacity - Currently SSDs come in relatively small sizes (32GB-256GB)
3. Wear Rate - Little 'real world' information exists on their longevity

To be fair, all of the above disadvantages should dissipate over the next couple of years and I can see a time when SSDs will completely replace conventional hard drives.

Q. What is TRIM all about?


A. TRIM is what is called a 'Garbage Collection' command that can be utilised by an operating system to directly communicate with an SSD to signal when blocks of data are no longer in use and thus require wiping/reclaiming.

The TRIM command is important as it stops 'progressive performance degradation' of SSDs where write operations can slow considerably over time, due to significant overheads when overwriting previously used (not empty) data blocks. 

The TRIM command must be supported at the operating system level and as such only Windows 7 currently supports TRIM, an important consideration when moving to a Solid State Drive.

A way to check to see if your Windows 7 operating system has TRIM support activated is by using the following command.

fsutil behavior query disabledeletenotify

If you get back 'DisableDeleteNotify = 0' then TRIM is active
If you get back 'DisableDeleteNotify = 1' then TRIM is disabled

Important note: The above command does not give any indication whether your physical SSD supports TRIM or is indeed using these commands, it only shows that Windows 7 is actively able to send TRIM commands or not!

Q. What is wear leveling?


A. ALL storage devices have a limit to the number of times any area of it's storage can be written to before the likely hood of failure is significantly increased. This is one of the reasons why storage devices may error when used very heavily over an expended period of time or just because they have been used for several years without being replaced.

Because the storage chips inside an SSD can also error/fail for the same sorts of reasons, wear leveling techniques are employed to spread the data evenly across all of it's cells to help extend the life of the storage.

This is an important consideration when purchasing an SSD because every time you double their size you effectively double their life expectancy, simply because it is able to use many more cells to spread your data across.

Q. What is the difference beween an SLC based SSD and an MLC based SSD?


A. MLC and SLC are both types of SSD NAND flash memory:

MLC stands for Multi-Level Cell - so called because they are able to store two bits of information per cell, thus they make SSDs cheaper to make due to their ability to store more information per cell (they allow denser storage).

SLC stands for Single-Level Cell - so called because they only store a single bit of information per cell. This makes SLC more expensive as you need more cells per megabyte of storage than MLC, but it has the advantage of higher cell endurance, and hence reliability, due to the same cells being written to less often. (See also 'Wear Leveling' above). 

Q. What is the expected lifetime of an SSD?


A. Some people look at the MTBF figures (Mean Time Between Failures), but these numbers are both an estimate and an average and can be misleading. Others look at the maximum defined 'write cycles' per cell, and although this is more scientific way of estimating the life of an SSD it still does not replicate 'real world' usage.

So with little 'real world' or indeed historical information currently available it is difficult to give an clear answer, but many people give a raw estimate of around 10 years as a starting point, (for an MLC based device), but even this figure is a 'worst case scenario' and pessimistic to say the least. Saying this, not many people keep the same hard drive for more than 10 years!

An SLC based SSD's expected lifetime jumps to 10x that of an MLC based device, with 100+ years being estimated, but unfortunately SLC based devices are very expensive!

Saying this, SSD's have a percentage of additional (auxiliary) cells (up to 20% of it's total capacity is not unheard of) that can be utilised should cells fail (similar to the way a conventional hard drive has some supplementary disk space to remap failed blocks to), this gives SSD's a degree of additional reliability that is worth being aware of.

One last thing to consider is the fact that most if not all manufacturers now give at least a 3 year warranty on their SSD based products, not quite the 5 years that some hard drive manufacturers give but confidence is growing in the technology year-on-year!

Q. I was told not to use any Defragmentation software on my SSD, is this right?


A. Yes, this is perfectly correct, using any defragmentation software on your SSD is pointless and can actually be detrimental to the drive.

Accessing information on an SSD is very different to that of a conventional hard drive, where it can take a relatively long time for fragmented information to be accessed, not so for an SSD, who's access times are practically instant all the time no matter where the information is stored on the drive - thus defragmenting your SSD is almost completely irrelevant!

Something else to consider is 'Wear Leveling' (covered in one of my other questions above); when you try to defragment an SSD it interferes with the drives wear leveling algorithms which help spread the data evenly across all of it's cells to help extend the life of the storage. All that moving of data creates many more additional writes that can only have a negative effect on your drives longevity.

NOTE: Microsoft's Windows 7 may well automatically turn off the defragmentation facility for your SSD for you, should Windows 7 not recognise your SSD or you just want to check that Defragmentation is indeed turned off:

Click the START button
Click the ' Computer' link (right-hand side of the start menu)
Right-Click your SSD device and select properties
Select the 'Tools' Tab
Now select the 'Defragmentation' button (middle option)

If you see the text 'Scheduled Defragmentation is turned off' then you have nothing to worry about, but if you don't see this then click the 'Configure Schedule' button and deselect the option labelled 'Run on a schedule (recommended)' click OK and then Close.

Q. Is there anything else like Defragmentation that I should switch off when using an SSD?


A. Yes, most people also switch off Windows 7's 'Superfetch' service and disk indexing. Both these functions help speed up the access to files, both have considerably less impact when faster storage devices are used, and so are irrelevant when using an SSD, besides they can both significantly increase the frequency your device is written to!

Use the utility called 'Tweak Prefetch' within my 'System Tools' section in my Software area to disable Windows 7's 'Superfetch' service.

To turn off the disk index:
Click the START button
Click the 'Computer' link (right-hand side of the start menu)
Right-Click your SSD device and select properties
Select the 'General' Tab (if you're not already on it)
Deselect the option 'Allow files on this drive to have contents indexed'
Now Click Apply and OK

You can even disable Windows Search altogether:
Click the START button
Right-Click the 'Computer' link (right-hand side of the start menu)
Select 'Manage' from the menu
Double-click 'Services and Applications'
Double-click 'Services'
Scroll down and Right-Click 'Windows Search'
Right-Click and select 'Properties'
Click the Startup type and select 'Disabled' from the drop down menu
Click 'Stop' and then 'OK'

Some sites tell you to disable 'Virtual Memory' and although this is good idea in theory, in practice it might stop certain applications from working will have to experiment and see - I would personally leave 'Virtual Memory' turned on!

Q. Is it true that I can't recover deleted files from an SSD that uses TRIM?


A. Yes, it seems you are far less likely to be able to recover usable information from files that have been accidentally delete from an SSD that uses TRIM, simply because TRIM is altering 'no longer used' cells to clear them of data to maintain writing speeds (See TRIM above).

This makes if very unlikely that file recovery software will be able to retrieve intact information on deleted data, simply because it has, more than likely, already partly or completely been over-written by TRIM.

This is an important consideration, and SSD owners who's devices support TRIM should take extra measures to safeguard their data, with extra backups or maybe consider the use of Windows 7's 'previous versions' option to automatically keep older copies of files should you accidentally delete an important file! 

Q. My SSD manufacturer mentions AHCI mode what is this and where do I set this?

New Item

A. AHCI stands for 'Advanced Host Controller Interface' it enables advanced Serial ATA features such as NCQ (Native Command Queuing), HP (Hot Plug) and staggered spin up of multiple hard drives on compatible storage controller chipsets. AHCI enabled on a controller that is used by an Solid State Drive can improve it's performance.

AHCI may need to be enabled in your motherboard's BIOS:
WARNING - Enabling AHCI mode (or indeed RAID which also uses AHCI) in your BIOS after installing your operating system is not recommended when a Serial ATA device is your boot drive, as it can cause an immediate blue screen error:

To check if your Windows installation already has AHCI enabled look at the following registry entries:




In both cases, if the item 'Start' has a value 0 then AHCI is already enabled.

If these values are not 0 the changing them should be safe, but you must immediately restart your system afterwards, enter your BIOS and ensure the AHCI is enabled, this should stop the blue screen issue from occurring - should you get anything wrong, just enter your BIOS again and disable AHCI to gain access to your boot device !


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