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SSD, TRIM and IOMeter
by Marc Prieur
Published on June 22, 2010

TRIM in Windows 7, in practice
To be able to use TRIM in Windows 7, you of course need an SSD that supports TRIM but also an IDE controller or AHCI drivers that are TRIM compatible and thus allow the TRIM command from Windows to reach the SSD.

This is the case for generic Windows 7 IDE/AHCI drivers as well as Intel RST drivers from version 9.6 onwards, which allows TRIM on an SSD even if you also have a RAID of hard drives beside it. Marvell and AMD drivers are not however TRIM compatible for the moment and you’re better off using the generic Microsoft drivers.(Update : latests AMD and Marvell drivers are now TRIM compliant !)

When is the TRIM command active? We measured performances obtained with random 4 KB writes in I/O Meter under various conditions. To recap, on the unused SSD we obtained 11.8 MB/s in this test, and 2.5 MB/s when fully written via a sequential write.

If the drive is fully written with a sequential write and the resulting file is kept and then the partition is deleted, performance levels remain at 2.5 MBs.

If the SSD is fully written via a sequential write and the resulting file is deleted, performance levels are re-established at 11.8 MB/s.

If the drive is fully written via a sequential write and the partition is deleted and then quick formatted to recreate it, performance levels are re-established at 11.8 MB/s.

Sequential write performance after random writes are also re-established after formatting or deletion of the file:

As you can see, Windows 7 sends the TRIM command to the SSD in two instances:

- When the partition is formatted, even if quick formatted
- When a file is deleted

Of course in the first instance, the whole SSD is formatted, whereas in the second only the NAND pages used by files are deleted. Note that quick formatting therefore takes a little longer because of communication of the TRIM command; it takes around 30 seconds.

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