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Microsoft has given a strong, though qualified, endorsement for running Windows 7 on PCs equipped with solid-state disk (SSD) drives, saying it has tuned the upcoming operating system to run faster on the still-emerging storage technology.

At the same time, Microsoft admitted that it has not solved two lingering problems that can cause SSDs - mostly lower-end, older ones - to perform sluggishly or even worse than conventional hard drives.

Out of the box, Windows 7 should install and "operate efficiently on SSDs without requiring any customer intervention," Microsoft distinguished engineer Michael Fortin wrote in a posting at the Engineering Windows 7 blog.

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Users of Windows 7 - the Release Candidate 1 became available for public download today - will experience the full benefit of SSDs in areas where the storage technology shines.

Small chunks of data can be read about 100 times faster from an SSD than a hard drive, since an SSD doesn't require a rotating disk head to be physically repositioned, Fortin wrote. SSDs will also read large files such as videos up to twice as fast as a hard drive, wrote Fortin. Many SSDs will also write large files more quickly than a hard drive, especially when the SSD is new or empty.

The first generation of SSDs introduced mostly via netbooks two years ago were largely a disappointment, as they were slower and pricier than expected. But performance gains, as well as falling prices, have many PC makers excited anew about SSDs.

Asus has debuted its S121 netbook with a 512GB SSD that will run Windows 7 when it becomes available.

However, Fortin said that Windows 7 users could experience freeze-ups while writing small files and see overall performance slow down over time, depending on the quality and age of the SSD they're using. The freezing problem is caused by the "complex arrangement" of memory cells in flash chips, he said, as well as the fact that data must be erased from cells before new data can be written to them.

And few SSDs today include RAM caches that can speed up performance, as most hard drives do. As a result, "We see the worst of the SSDs producing very long I/O times as well, as much as one half to one full second to complete individual random write and flush requests," Fortin wrote.

"This is abysmal for many workloads and can make the entire system feel choppy, unresponsive and sluggish."

That is despite improvements Microsoft made in Windows 7 such as resizing partitions to better fit SSDs and "reducing the frequency of writes and flushes," wrote Fortin.

Even features such as ReadyBoost, which was created by Microsoft to take advantage of USB flash drives using solid-state memory to accelerate performance of Windows Vista or 7, will actually slow down when run with most SSDs, wrote Fortin. As a result, Windows 7 will turn off ReadyBoost for SSDs.

Meanwhile, performance degradation over time is caused, again, by the need to erase data before it can be written, and the increasing fragmentation of data on SSDs as they fill up.

Some vendors such as Intel say they have mitigated the problem on their SSDs, but none claim to have solved it.

Unlike with hard drives, automatically defragmenting SSDs is not recommended because it can prematurely wear them out. Windows 7 turns off defragging by default.

Fortin said the performance degradation is not as serious as the freeze-ups. "We do not consider this to be a show stopper," he wrote. "We don't expect users to notice the drop during normal use."

Disk compression is also not recommended for heavily-written data such as web browser caches or email files, Fortin said, because of the potential for a slowdown on SSDs, though it is fine for non-heavily written data. However, some features, such as Windows Search and Bitlocker encryption, should work identically well or better on SSDs, Fortin said.

Computerworld's Lucas Mearian contributed to this story.