AHCI slower than IDE :-P

Actually on my system AHCI was gobs faster than IDE because, they do use different drivers. My IDE driver would not allow multithreaded reads/writes and the AHCI driver did. Speeding up the multithreaded 4k tests by /at least/ 10x.

But it is mainly about the additional features for new drives. If you had a good driver for a good sata chip in IDE mode then yes the differences would be minimal (and on a multi run test may average out to the same). But in some cases, due to using completely different drivers, the difference /can/ be night and day.

I,ve posted this in other discussions, but since you do have a sincere question about speed variances, I'll post again.

I am giving three ATTO tests of my Patriot Pyro 60 GB SSD which I purchased for $109.xx.

This is a direct COPY of my system which was transferred from a regular HDD to the SSD. I am running WIN 7 PRO which enabled me to do the initialize and quick format of a partition which via 'disk management'.

The Built in WIN 7 disk management correctly aligns the first partion and all succeeding ones automatically for a SSD. I used my 'emergency restore' copy of acronis to simply copy my previously 'backed up' installation from the HDD to the SSD.

first, SSD as IDE no windows cache:


Second as AHCI no windows cache:



Finally as AHCI using windows cache:

Whats is that "using windows cache" option?

That is the regular windows read and write cache. To get an exact representation of how a hard drive physically works, benchmarks are normally run with both the read and write cache disabled which forces the drive to do only actual physical reads/writes to the hard drive or SSD.

In the case of ATTO this accomplished by clicking on the direct I/O box in the atto control panel to enable or disable the windows cache during the test.

The reason for presenting the caches figures is that there has been several people asking how much difference it makes by enabling the write cache on the hard drives.

I was simply trying to present as much info as possible for others to consider in regards to the advantages of using the regular windows cache. Even though SSDs are very fast they still can not match the speed of regular on board Randon Access Memory (RAM) and if one is repeatedly reading the same blocks from memory, their entire system can be sped up by enlarging the cache if it currently is not large enough to keep the repeat reads in RAM.

In some older early versions of windows, it was also possible to enable/disable the read cache but that function is no longer available. The most one can do regarding the 'read' cache is to modify the vcache setting to change the amount of memory to dedicate to the cache.

DGLang said:

Windows does have both read and write caches, actually a SINGLE cache used for both purposes.


Hard disk drive manufacturers began adding their own on board cache to their drives once they saw how much windows cache helped..

Various manufacturers then began marketing hard drive controllers which contained on board memory to further improve windows performance. The additional cache on the controller GREATLY improved performance. I used to own such a controller which I used on my early DOS and OS/2 machines. Even today some controllers have their own on-board cache to help them achieve their own best performance.

The system buffering or cache mentioned above IS the normal windows caching that I was talking about.

The reason for NOT disabling the hard drive's on-board cache during benchmarking is that if that cache was disabled, then any benchmark results would not accurately reflect the hard drive's performance under normal operating conditions.

Well you did say disk write cache, not the system cache.

I am afraid even though you say a lot you do not understand the difference. The Windows 7 file cache caches files in RAM. It completely manages this cache and knows if any of it needs flushed to disk when a file is modified. When it flushes a file out of cache to disk or writes directly to disk, it expects that the data it wrote to the disk to actually be stored on the disk. When the disk driver returns from a write, the windows 7 system assumes it is on disk. This is very important to understand.

The file systems cache is there to improve performance to frequently accessed files and to be able to delay disk accesses so that they can be intelligently ordered for optimal performance. File performance for applications, even for writes, can appear enhanced as with your ATTO application. It has a completely different role than the disk's on-board buffer - it truly acts as a cache.

The on-board "write cache" on a disk drive is used to buffer data to reduce the latency of the disk. It is more properly referred to as a disk buffer, since it does not cache anything. It is truly a buffer between RAM and disk.

When a disk with a write buffer receives data from windows, it immediately returns a success to Windows once it is in the buffer, even though it is not on disk yet. At that point windows thinks it is safely on disk, but it isn't. If the power drops, then the data in the write buffer is lost and is never written to disk; even though Windows thinks it is there, it is not. This results in file corruption and is a reason you should not disable write-cache buffer flushing for a disk device unless it is backed up by a battery, like higher end raid controller's write buffer's are. Corruption like this can corrupt the whole file system, not just a file.

When write-cache buffer flushing is enabled for a disk (default), Windows, at critical points, instructs the disk drive to flush the data from the disk buffer to the disk immediately. It does this at critical points to ensure file system integrity. The best possible performance options without a battery backup is to enable "write cache" for the device, but leave write cache flushing enabled.

A disk's on-board buffer is also used as a read buffer and helps with read performance by reading ahead from the disk. The OS has no control over it (it is always enabled).

http://en.wikipedia.org/wiki/Disk_bu....2Fread-behind

If you use the AHCI driver for SATA disks, the drive can buffer writes and reads for optimal performance, but does not tell the operating system a queued write is successful until it is actually written to disk.

In short the operating system cache and disk buffer serve different purposes. The hard drive manufacturers didn't add on-board cache to emulate the operating system file cache.

BTW I misspoke in a couple of posts above. I said Windows has write caching turned off by default. It is on by default. What is off by default is disabling write cache flushing.