Trimming it down

Trim will tell the device which blocks and sectors aren't being used, allowing it to use areas that have already been erased and pre-erasing areas where files have been deleted. This will also maximise unused space, which the SSD can use for wear levelling, to stop some areas of flash wearing out before the rest of the drive, which is cheaper than adding extra capacity.

The completion time for Trim command must stay under 20ms to qualify for a Windows 7 logo. "The goal of Trim is to improve performance," says Senior Program Manager Frank Shu; "We don't want to create a performance bottleneck - if we send a lot of Trim data to the device and the device is busy doing garbage collection we could have a long delay, hurting system performance and responsiveness."

For the logo an SSD will also have to report that it's not a spinning hard drive (which works with ATA and SCSI interfaces), so Windows can turn off scheduled defragmentation. SSDs connected via SATA must use the faster SATA-IO connection, prioritise data reads over slower data write and not slow down as you approach the drive capacity.

ATA and SCSI interfaces also let a drive report its size as an offset, avoiding problems in aligning the NTFS partition to the 24K page size of most SSDs. However, this is only a problem from machines upgraded from Windows XP where the first partition is in the middle of an SSD page, which can degrade performance by 50 per cent because modified files have to be moved more often.

Remove the bloat

Defragmentation places files around a drive platter so they will load more quickly to deal with latency issues that you don't have with SSD. You also don't need disk optimisation algorithms like SuperFetch, which Grimsrud found caused an SSD system to make 25 per cent more disk accesses than with a hard drive (it also used an additional 400mW of power).

Grimsrud also suggested that SSD lifetime isn't a real problem: "All this concern about drives wearing out: the reality is so much different from user perception, it's almost laughable. But the mystique of having a technology that has an internal wear out makes people get excited about that."

John Loveall, Director of Program Management for the Windows Storage platform, hinted that commands like Trim could evolve in future versions of Windows. "This is the start of us looking at richer commands being sent down to devices. We are creeping on the edge of some architectural changes and next time around we may be talking about fundamental changes, but this time it's incremental."

Bytes versus blocks

Vlad Sadovsky went into more details, suggesting that Windows could move to addressing individual bytes on an SSD rather than blocks and sectors. "What about byte addressing: do we need another layer? It would definitely be very useful and we know where the OS would benefit. We also definitely know the OS would have to be rewritten at kernel level to take account of it - we would have to do [things like] security differently." And he confirmed "we have started looking at this."

In the short term, if you're installing the Windows 7 M3 pre-beta build on a netbook yourself, the most common problem is not getting drivers but getting them installed; driver installers often look for hard-coded version numbers and many won't run under Windows 7. However, if you copy the driver files onto the PC and let Device Manager look for an updated driver, you can get most devices working.

Now read TechRadar's hands on: pre-beta Windows 7 review