Prefetch and SSDs

[essenbe]

Intel dosen't explain anything.

 

[carwiz]

Microsoft did some extensive testing on the then current SSDs for the Windows 7 release. They determined that the gain from Superfetch and Prefetch was, at best, a slight positive to neutral improvement when using SSDs. The thought was to disable these features and free the processor of "un-necessary" overhead and thus increase the NET gain of a PC as a whole. This applies to the older SSDs. The newer SSDs are substantially faster with higher transfer rates so if you do use a SSD (or more), you will improve throughput by disabling Superfetch and Prefetch. This is a general statement as there are always exceptions. For instance, some motherboard controllers will reach near maximum throughput on a SATAII (3GB/s) interface much less the SATAIII (6Gb/s) standard. (To wit: Marvel controllers.) In cases like this or with sub-par SSDs, it's better to leave Superfetch and Prefetch enabled.

Disabling De-fragmentation is a different reason all together. That is done for longevity of the SSD.

 

[GeneO]

Carwiz,

You wont increase throughput by disabling prefetch or superfetch. The only time superfetch or prefetch affects your systems are

1. When it keeps track of what programs you open and what files you use to boot. This amounts to nothing.
2. Briefly when you log in for superfetch as it loads frequently accessed programs into memory.

It has nothing to with nor affects your SSD throughput.I don;t understand what you are trying to say about SATA 2.0 vs 3.0 and what that has to do with prefetch and superfetch or what the Marvell 6Gb/s has to do with anything.

 

[cluberti]

True. Disabling prefetching and defragmentation passes on an SSD are in an attempt for longevity. They're not performance gains (they're net neutrals, really).

 

[essenbe]

Part of what carwiz was saying came from an interview with a Msft engineer on how windows 7 dealt with SSDs. They origionally designed windows to turn off Superfetch and prefetch on 1st gen. SSDs. They found that the SSD degraded and slowed down until they turned superfetch and prefetch back on. So, they left it that way. We are now in the 3rd gen. of SSDs but windows still handles them like 1st gen. Still, on most SSD websites they say turn off superfetch. I know Intel and OCZ do. With superfetch on, Intel's toolbox configuration section will turn red and tell you your system is not properly configured.

 

[stereo]

What do you mean about write caching on the SSD ?
I mean the option " Turn off Windows write-cache buffer flashing on the device"
Should be enabled or not ?

 

[GeneO]

What do you mean about write caching on the SSD ?
I mean the option " Turn off Windows write-cache buffer flashing on the device"
Should be enabled or not ?

Windows needs to keep its filesystem in a consistent state - that means it needs to know that if it writes critical data to a disk drive, it needs to know it was actually write on the disk. When the drive has an on-board memory buffer, it will return a success to windows when it has the data in the buffer, and will write it to disk later, when the buffer gets full. If the power drops or the before the data in the disk drive's buffer gets written to disk, it will leave windows in an inconsistent state and you would have to recover or reinstall.

With write cache flushing, when windows is writing data that is critical to maintaining the consistency of the operating system, it tells tells the disk drive to write it from cache t disk immediately.

So, you should not turn this off unless your disk system has a battery backup (unlikely) or you are on a UPS that can hold power until you can shutdown cleanly. Otherwise you run the risk of corrupting the OS.

 

[GeneO]

Part of what carwiz was saying came from an interview with a Msft engineer on how windows 7 dealt with SSDs. They origionally designed windows to turn off Superfetch and prefetch on 1st gen. SSDs. They found that the SSD degraded and slowed down until they turned superfetch and prefetch back on. So, they left it that way. We are now in the 3rd gen. of SSDs but windows still handles them like 1st gen. Still, on most SSD websites they say turn off superfetch. I know Intel and OCZ do. With superfetch on, Intel's toolbox configuration section will turn red and tell you your system is not properly configured.

I think you might have that backwards. I think you meant that they found that the SSD degraded and slowed down until they turned superfetch and prefetch back off, not on.

I have been running my M4 SSD for a month with both fetches on. I don't even have TRIM as I am using the Marvell controller, and I have had no performance degradation whatsoever.

Cheers

 

[essenbe]

source- SSD and Windows 7 From 2009

 

[GeneO]

Ah yes, in the context of that article what you said makes sense. The reason the turned it on for low performing first generation SSD was the same reason it is used for disk - a marked perceived improved app load time by pre-loading often used apps to memory.

But what I think they do is use the WEI to determine if the SSD is a low performer or not. If it is, a poor performer they turn superfetch and prefetch on, if not, they turn it off. So they aren't really treating all SSD like they are first generation.

However I don't think that article has any bearing on whether one should turn superfetch and prefetch on for the current generation. I still think it is better having it on for the reasons I have stated.

Cheers

 

[carwiz]

I doubt that WEI is used to control the setting. WEI is operator controlled--It may or may not be run. My disk scores 7.9 and SF and PF were both set on. One way to tell if SF is doing any good is turn it off for a few days then go check the boot log. You can compare the times. Every boot and shutdown is logged down to the ms. If you look through the history you'll find variations in the boot times as high as a few seconds with no changes at all. In other words, I wouldn't jump to a conclusion one way or the other on 2-3 second changes. A lot will depend on how much your system loads on startup. (Services, desktop and programs)

I forgot to mention: You would think that with a stable system the boot time would be at or near the same or at least within a 100ms. That's not the case. Why is that? Certainly if SF was doing it's job, startup times would be constant. Don't you think?

 

[GeneO]

I believe it is WEI, but MS may determine it some other way. Well, perhaps MS has changed the way they do this, realizing it is just as well on.

SF has nothing to do with boot time, unless you are including pre-loading the programs - which can vary. That is prefetch. Even so, I believe Windows does not always run the same tasks at every boot (maintenance etc) and the boot time is dependent on hardware responses that can vary (disk spin up, wireless network startup, etc). My boot times vary little.

 

[cluberti]

The WEI score is determined from the winsat tests, which is the same way the SSD's characteristics are determined. Keep in mind that the winsat tests != WEI score - they're performance tests, and the WEI score is *derived* from your winsat test scores.

 

[carwiz]

I think there's a bit of confusion on everyone's part. Winsat is what WEI is derived from. You can see the tests in the log. Therefore, WEI = Winsat. Also, Superfetch AND Prefetch BOTH can affect boot (startup). The options for BOTH are exactly the same. They're located in the same Registry section(s).

0 = Do nothing (disable)
1 = Cache applications only
2 = Cache boot files only
3 = Cache everything (default)

 

[GeneO]

OK, tell me how superfetch affects boot startup pleas (sincerely - I would really like to know).

Cluberti is right. Winsat is a command line utility that WEI uses tto create a performance score based on the various results from winsat. They aren't the same thing, but that is splitting hairs.

 

[carwiz]

Superfetch loads memory based on applications you use NOW, Prefetch builds indexes and maintains databases of frequently called modules. Both are useful for speeding startup or boot as many call it. The options for both/either can be set different or the same.

 

[cluberti]

What specifically do you want to know? Unless you have a 1st gen SSD, you won't notice the difference on or off. If, however, you do have an older SSD with 1st gen performance, you might. Given that, reducing the fetch hit on disk might be beneficial from a longevity standpoint (and given it makes no difference, setting these to 0 in the registry is a pretty good idea), but if it's on it's not exactly the end of the world either.

 

[carwiz]

There's no wear overhead for reads from a SSD. But the fact that Prefetch has to maintain several databases, it adds processor overhead and disk writes. Superfetch uses memory-based indexes and they're NOT carried over to the next boot. They are built for every Windows startup. (And therein lies the processor overhead.)

 

[GeneO]

What specifically do you want to know? Unless you have a 1st gen SSD, you won't notice the difference on or off. If, however, you do have an older SSD with 1st gen performance, you might. Given that, reducing the fetch hit on disk might be beneficial from a longevity standpoint (and given it makes no difference, setting these to 0 in the registry is a pretty good idea), but if it's on it's not exactly the end of the world either.

I am just curious how things work.

 

[GeneO]

There's no wear overhead for reads from a SSD. But the fact that Prefetch has to maintain several databases, it adds processor overhead and disk writes. Superfetch uses memory-based indexes and they're NOT carried over to the next boot. They are built for every Windows startup. (And therein lies the processor overhead.)

Not much to add tio today's processors I expect.

I am confused. I thought superfetch loaded frequently used programs into memory when you login, so would save that across boots. Otherwise, what does it do? What do you mean by it uses memory based indices. Indices for what? That does not explain how it works but only, perhaps, one of the mechanisms it uses (for what?)