Windows 7 post/pre SP1 4KB/512B writes alignment

[Coup27]

Sorry for the garbled topic but I did my best.

In the Seagate XT Gen 2 review, Anand wrote:

Our Heavy Storage Bench workload is even more write intensive. Furthermore, having been recorded on a Windows 7 pre-SP1 install, we see some of the potential penalties from moving to a 4KB sector drive. Most writes are 4KB aligned in Windows 7, however pre-SP1 there were still some significant cases where alignment could be an issue. Here we see the 750GB/4KB Momentus XT actually fall behind the 500GB drive with 512B sectors because of this difference.

​

I included these results because if you formatted your drive with Windows 7 and later applied SP1 to the install, you may see this sort of performance regression when moving to a 4KB sector drive. The only way to avoid this is to reformat your drive using Windows 7 SP1 and install from a Windows 7 SP1 DVD/image. In place upgrades won't avoid the alignment issues that are exhibited here. For a greater understanding of why 4KB sectors are necessary and why alignment can be problematic on these drives, have a look at our coverage here.

Could somebody please explain this a little better? I have always used my retail Windows 7 DVD and installed SP1 either through update or downloaded the whole .exe and installed it locally after my last build.

I have always used an SSD as my primary drive since getting 7 but would like to know exactly what's being referred to here.

Cheers

 

[Cyberia123]

Ditto that

 

[RU482]

I thought 4K alignment was a given if you did a fresh install, starting back with Windows Vista. This is the first I have heard of misalignment in Win7. Is it only isolated to HDD, or would it also be on SSD?

Could it be due to 512B emulation?

I would enjoy further explanation as well/

 

[VirtualLarry]

Even though "Gold" Windows 7 supposedly supports 4K sector drives, in practice, there are bits of code here and there that didn't really enforce that.

SP1 fixed most of those bits of code.

 

[bryanW1995]

I thought 4K alignment was a given if you did a fresh install, starting back with Windows Vista. This is the first I have heard of misalignment in Win7. Is it only isolated to HDD, or would it also be on SSD?

Could it be due to 512B emulation?

I would enjoy further explanation as well/

It's only an issue on a HD.

 

[Mark R]

There is no difference in the alignment between Win7 Pre SP1 and Win 7 SP1. Both start the partition on a 4k boundary, and both use a default 4k cluster size. This is the optimal format for 4k sector drives.

The difference in SP1 is that the OS has a number of changes that optimize the way in which the OS accesses 4k sector drives (especially those with 4k sectors that emulate 512 byte sectors). Updating a Win7 original system to Win7 SP1 will make available all the enhancements.

When a small file (< 1 sector e.g. 512 bytes) is written, is has to be "rounded up" to 512 bytes. When a drive claims to have 512 byte sectors (but is actually 4k sectors), the rounding to 512 bytes still occurs, and these writes have to be converted to "read-modify-write" operations in the drive, causing low performance.
In SP1, the drivers have been rewritten to detect 4k drives (with 512 byte emulation), and small files will be rounded up to 4k bytes when sent to the drive.

There has also been a change to the windows database service (which provides high-speed access to database files, for searches, queries, etc. This service is used by many core components of windows, such as windows update, windows mail, etc.). For speed, the database service directly accesses the raw sectors on the hard drive, bypassing the file system.

In SP1, the database service has been changed, so that writes are "rounded up" to 4k.

 

[abekl]

Thanks for that explanation Mark